Footwear is the most commonly worn apparel in daily life, and its design features must prioritise anatomy, comfort and hygiene. For this reason, it is important to develop sustainable improvements to footwear’s functional properties.

Footwear that carries the body’s weight during the day can affect foot health physically, chemically and microbiologically. Continuous contact with the external environment exposes footwear to microorganisms during normal use. All sorts of footwear play a role in the transport, spread and contamination of pathogenic or non-pathogenic microorganisms (1).

There are different microorganisms in every part of the human body. Sweat is regularly secreted from the body under normal conditions. It contains 98% water and urea, uric acid, fatty acid, lactic acid and sulfates (2). The feet have more sweat glands than other parts of the body. Sweat secreted from feet during the usage of footwear is decomposed by means of foot microbiota; as a result, bad odours emerge in footwear. Brevibacterium linens, Staphylococcus epidermidis, Staphylococcus aureus and Escherichia coli are some microorganisms that make footwear unhygienic. As a result of the breakdown of amino acids in sweat and skin by these microorganisms, bad odour arises in feet, socks and footwear (3, 4).

Nowadays, in addition to individual foot care and hygiene for odour prevention in shoes, commercial materials with various deodorising and antimicrobial effects are also employed (5). Footwear insock is a thin layer of materials put into the shoe after manufacture to cover the insole. It directly contacts the sole of the wearer’s foot and can provide a more sanitary environment when specially treated for antimicrobial purposes (6).

Spray drying is an advantageous way to encapsulate active substances and essential oils. Spray drying is a common and accepted encapsulation method for industrial applications. With this method, it is possible to mass produce capsules. The distribution of particles is uniform (7–10).

Microencapsulation technology has been used for the application of orange oil to textiles and leathers, being an economically viable, fast and efficient method by combining core and shell materials, desirable perceptual and functional characteristics, and also allowing functional substances to be released in a controlled manner. This technique has also been used to microencapsulate a wide range of active, functional, sensitive or volatile substances (11–14). Tea tree oil containing melamine formaldehyde microcapsules, essential oils (eucalyptus, lavender or oregano), polyurethane dispersions containing photoactive antimicrobial agents, zinc oxide and silver nanoparticles are some substances that protect upper leathers from the harmful propagation of microorganisms (3, 15). In addition, aromas confined to microcapsules are also used to prevent bad odours in footwear (16, 17). Application of antibacterial and aromatic materials onto footwear insocks to control bad odours is good for foot hygiene and desired shoe comfort.

The use of orange oil presents as an ecological alternative to synthetic chemicals, attracting the attention of the scientific community to the development of eco-friendly antimicrobials. In this study, microparticles were produced by a spray drying method after the emulsions with orange oil and chitosan were prepared in different ratios. Microparticles manufactured in this way were then transferred to the surface of the footwear insock leathers using a finishing process. Afterward, some tests and analyses were performed on microparticle coated footwear insock leather samples to evaluate the effectiveness of the microparticles, their presence on the leather surface and their antimicrobial properties.

Pharmaceutical grade cold pressed orange oil was donated from Ephesus, Turkey. Chitosan was purchased from Acros Organics^TM (Belgium). Analytical grade chemicals were used in the analyses. Insock leathers, without dye and ready for experimental application, were donated from Ata Dilek Leather (Izmir, Turkey).

For the microparticle production, chitosan (shell material) was added to 1% w/w aqueous acetic acid for preparing the chitosan solution. This solution was stirred at 45°C by using a magnetic stirrer until wholly dissolved. During the pre-emulsion preparation, orange oil (core material) was gradually mixed into the chitosan solution and stirred for 1 min at 10,000 rpm. The surfactants as compound emulsifiers used for pre-emulsion preparations were Tween 40 with Span 20 at the ratio of 8:2 w/w. Then, the microparticles were prepared by using an SD Basic spray dryer (Labplant, UK) with nozzle diameter of 0.5 mm. The orange oil to chitosan ratios in the four encapsulating compounds came to 1:1, 1:1.33, 1:1.67 and 1:2 w/w. The ingredients of the formulations in the spray-drying process are shown in Table I. Homogeneous emulsions were fed to the spray dryer under the following conditions: pump speed 12 ml min⁻¹, outlet air temperature 114°C and inlet air temperature 175°C.

The microparticles’ morphology was examined by a Quanta 250 FEG scanning electron microscope (FEI, USA) at 2 kV accelerating voltage. Before coating in an argon atmosphere with gold-palladium by a K550X sputter coating machine (Quorum Emitech, UK), the samples were mounted onto an aluminium stub. The grain side of leathers coated with microparticles was examined by a TM1000 tabletop scanning electron microscope (Hitachi, Japan) after coating with gold-palladium.

The FTIR spectra of the spray-dried microparticles and leathers with microparticles were procured by a Spectrum 100 FTIR attenuated total reflectance (ATR) spectrometer (PerkinElmer, USA). The measurements were made using four scans with a resolution of 4 cm⁻¹ between 4000 cm⁻¹ to 650 cm⁻¹ wavenumber ranges at room temperature.

Encapsulation efficiencies of microparticles were calculated as the amount of orange oil (core material) encapsulated in the microparticles. The encapsulation efficiency was calculated using Equation (i) (18).

(i)

A solvent extraction method was used to determine total oil content. A 0.1 g measurement of orange oil loaded microparticles was dissolved in 10 ml of 1% acetic acid solution at room temperature for 45 min. Released orange oil which was obtained from the completely dissolved microparticles was placed in a beaker containing 50 ml n -hexane for extraction 45 min. So as to determine the total amount of orange oil in the microparticles, this extract was filtered through a syringe filter (0.22 μm). Orange oil content in the filtrate was measured using a UV-1800 UV-vis spectrophotometer (Shimadzu, Japan) at 202 nm in triplicate. Surface oil content was also determined by the same solvent extraction method described above, except for a dissolving process in 1% acetic acid solution (11).

In vitro release studies of microparticles and microparticle loaded leathers were carried out at a speed of 100 rpm in phosphate-buffered saline (PBS) and methanol at 37°C. 1 mg orange oil loaded microparticles was suspended in beakers containing 4 ml methanol and 16 ml of PBS. Insock leathers with 2.5 cm² area were placed in beakers containing 16 ml methanol and 16 ml of PBS for in vitro release studies of microparticle loaded leathers. At suitable time intervals, the medium in the beakers was filtered through a 0.22 μm syringe filter. Sink conditions were maintained in the receptor compartment during in vitro release studies. The released amount of orange oil was analysed by UV method, as previously described, for 5 h. Experiments were performed five times.

A spraying pistol with nozzle diameter of 0.5 mm was used to apply microparticles to the insock leathers during the finishing process. Spray-dried microparticles were added to the finishing recipe as 20 g m⁻² (19). The basic finishing recipe for the insock leathers is given in Table II (20).

The efficacy of microparticle coated insock leathers against test microorganisms Staphylococcus aureus ATCC^® 6538^TM, Escherichia coli ATCC^® 25922^TM, Candida albicans ATCC^® 10231^TM, Klebsiella pneumoniae ATCC^® 4352^TM and Bacillus subtilis ATCC^® 6633^TM was examined by agar disc diffusion method (21–24). Test microorganisms were placed into an incubator for incubation at 37°C for 18 h in the Mueller Hinton broth (MHB) medium. Then, microorganisms were inoculated in petri dishes containing 10⁵ colony forming unit (CFU) ml⁻¹ of Mueller Hinton agar (MHA) medium. Next, microparticle coated insock leather samples with 12.7 mm diameter were placed into the petri dishes (20, 25). All petri dishes were placed into an incubator for incubation at 37°C for 24 h, and inhibition zones were measured to determine antibacterial activity.

In this study, orange oil microparticles were successfully prepared by spray drying method. This method is a simple, viable method to obtain microparticles, suitable to prevent active substance biological activity loss, avoiding exposure to elevated heating and to organic solvents.

3.1 Surface Appearance of Microparticles and Microparticle Coated Insock Leathers

3.2 Fourier Transform Infrared Spectroscopy Studies

3.3 Encapsulation Efficiency

3.4 In Vitro Release Studies of Microparticles and Microparticle Coated Insock Leathers

3.5 Microbiologic Studies on Microparticle Coated Insock Leathers

During the usage of footwear, perspiration and bacterial activity negatively impact foot health and generate bad odours from both the feet and footwear. Shoe production using natural and non-toxic materials that prevent or inhibit bad odours and bacterial activity is one solution to this hygienic problem. Likewise, the successful application of microparticles that release for a long time on footwear insock leather is an important alternative to existing toxic products.

Our research found that emulsions with orange oil and chitosan have natural antibacterial activity. These emulsions, when successfully converted into encapsulated powders by a spray drying method, produce a core-shell material. SEM images showed how an effective finishing process was used to apply laboratory produced microparticles to the surface of footwear insock leather. Microbiological tests performed on microparticle coated leathers proved that footwear insock leathers were fortified with antibacterial properties.

These findings demonstrate that application of orange oil-chitosan microparticles onto footwear insock leather surfaces is an alternative natural method to control hygiene and eliminate bad odours. Non-toxic, functional, leather shoes can incorporate such natural materials in their manufacture and maintenance. This production improvement would thus contribute to people’s foot health, hygiene and comfort.

Metalloenzymes have been prominent in the field of enzyme engineering since its emergence some 40 years ago, at the birth of protein and enzyme engineering (1, 2). Metal ions or cofactors in solution have an intrinsic chemistry that can be catalytic and these are accessible to detailed mechanistic study. These properties mean that co-localisation of substrate and metal within a peptidic scaffold can be sufficient in forming an ArM, without further influence from the protein on the catalytic mechanism. With the advent of modern protein engineering and design technologies, ArMs were developed by incorporating metal binding sites in or adjacent to hydrophobic pockets. While the resulting ArMs were active, they often displayed low efficiency and specificity. Therefore, directed evolution (i.e. iterative rounds of mutagenesis and selection for activity, Figure 1) has become a key step in creating enzymes with new and useful properties. The choice of starting point for such a forced evolution campaign, in this case the metal-protein complex formed initially, is of great importance. Since any particular enzyme follows a unique evolutionary trajectory as new mutations move it along the fitness landscape towards (potentially local) maxima, choice of the starting point may directly predetermine the result. By nature of the selection process, it is further possible, that trajectories leading to the global maximum fitness fall beneath the cut-off limit for further evolution, becoming inaccessible. For instance, a mutation introduced in the first round of mutagenesis may lead to a destabilisation of the protein at assay conditions, causing that initial variant to be discarded through selection. However, a compensating mutation to that variant in a subsequent round of mutagenesis could result in an enzyme which is stable, active and closer to a global fitness maximum. Finally, not every method of generating ArMs may be compatible with current methods for directed evolution and therefore limit the extent of evolution that can be achieved.

Fig. 1.

In this perspective, different routes towards ArMs are considered in the context of the starting protein scaffold as well as the type of catalytic centre and reactions involved. Advances in ArMs have recently been reviewed and the reader is referred to these for further details of the strategies used to find new systems (3–5). This article aims to provide an overview of the strengths and weakness of these different approaches and to provide a perspective of some challenges that remain.

One particular area that will greatly impact chemical production on this planet is synthetic biology. Replacing synthetic catalysts, acting on petrochemical feedstocks in non-aqueous solvents, with biocatalytic systems working in water with simple carbon neutral feedstocks (carbon dioxide even?) is clearly highly desirable. But why engineer new enzymes, particularly using expensive and relatively scarce transition metals, when the ability to find new catalysts amongst gene products from all corners of the biological world has developed at staggering pace (6–8)? As a consequence of the latter, any target chemical can conceivably be obtained by recombining pre-existing metabolic pathways (9). What will new and unnatural metalloenzymes provide?

One clear feature is orthogonality: the objective of introducing functionality into a cell that has no counterpart in the natural world could provide chemistry that biology cannot currently catalyse, alkene metathesis for example. As there is a limit to the number of additional transformations a viable cell will perform, these orthogonal reactions may allow access to much shorter, and therefore more efficient, pathways. If not for a synthetic purpose, one could also imagine orthogonal catalytic chemistry providing a diagnostic or reporter output without interference from the host endogenous processes. For it to be truly orthogonal, it is difficult to imagine evolving a new enzyme based around metals already abundant in nature and already used as catalysts in biology. The transition metals used by nature are very carefully controlled by acquisition and regulatory networks that ensure catalytic metal ions are not free to operate outside the endogenous metabolism. Therefore, there is significant advantage in trying to introduce metals that biology currently has no evolved means of metabolising. This work therefore focuses primarily on non-biological transition metal cofactors as a route to introducing novel orthogonal activity into a biologically viable system.

Natural evolution has provided numerous examples of metal ions used by enzymes for a plethora of different catalytic purposes. Rigorous mechanistic and structural biochemistry has advanced understanding of the mechanistic detail of metalloenzyme activity significantly, to the point that a few underpinning principles can be identified, linking protein structure and thermodynamics to catalytic activity of metal centres. Together with the knowledge garnered from extensive research on transition metal catalysts, it is possible to establish key properties desirable for novel ArMs.

3.1 Considerations on Protein-Substrate Interactions

3.2 Considerations on Metal Chemistry in Proteins

3.3 The Optimal Method of ArM Formation

ArMs are generated either from the combination of an unnatural transition metal cofactor being introduced into a protein scaffold or a natural metalloprotein being evolved in a laboratory to enhance or alter its natural catalytic reactivity. A detailed review of the field of the directed evolution of natural metalloproteins is out of the scope of this perspective. However, the engineering and evolutionary approaches developed by Frances Arnold and applied to haem metalloproteins (for example, cytochrome P450) are particularly noteworthy and applicable when evolving unnatural metal-protein hybrid catalysts (27–29). Four successful strategies have been employed to localise an unnatural metal to a well-defined location within a protein matrix.

4.1 Metal Ion Substitution in Natural Enzymes

4.2 Supramolecular, Non-Covalent Binding of Tagged Complexes

4.3 Covalent Anchoring Through Metal Ligands

4.4 Direct Activation by Metal Coordination to Protein Side Chains

Significant advances in the incorporation of organometallic complexes into proteins in order to generate ArMs have been made. The studies highlighted above reliably create hybrid molecules where the stability and turnover number of the metal centre is higher than the comparable small molecule organometallic complex in aqueous solution. Maybe unsurprisingly, the propensity for side reactions and catalyst decomposition is lowered once the complex is in a hydrophobic protein environment, already showcasing the usefulness of these hybrid systems. However, the question remains, as to whether or not these strategies make full use of the protein component. The unique and numerous demands of ArMs call for a highly integrated approach. To date, most of the work described in the literature attempts to exploit the chemistry of metal ions and their complexes in a protein scaffold but with limited influence from the protein on any catalytic activity because metal-protein coordination is largely indirect and so cooperativity is limited.

The potential for synthetic organometallic chemistry to deliver cofactors which utilise ligand chemistry not available to naturally evolved systems can vastly expand the orthogonal catalysis available in synthetic biological applications. Using such molecules to embed novel metal-peptide hybrid complexes in protein scaffolds allows for three-dimensional and electronic control around the metal centre that reduces the need for intricate synthetic catalyst generation. Instead, control of the steric and electronic environment around the metal ion can be delivered via the protein coordination sphere, particularly where a direct coordination bond is used to anchor the metal ion to the protein. When combined with the ability to efficiently evolve enzymes, a sophisticated organometallic precursor complex together with a suitable apoprotein could potentially give rise to a number of diverse reactivities. Therefore, new problems in catalysis could be addressed in a faster and more specific manner than with small molecule catalysts. Together with non-covalent contributions to catalysis and the intermolecular interactions that pre-organise substrates and stabilise transition states, such a system contains many readily evolvable components.

The majority of protein scaffolds selected for ArM construction have been chosen because of their apparent engineerability. However, in most cases the focus seems to lie solely on the peptidic component with little consideration for evolution of the metal complex. Although methods of selection and directed evolution have been applied, these are often operating on already well-defined protein scaffolds that carry an abiotic cofactor but not a direct protein-metal complex, which inevitably limits the scope for evolution. Arguably it is desirable, therefore, to select for a promiscuous and versatile protein starting point which is not constrained by one energy minima but instead can potentially offer numerous distinct metal-binding environments, both in terms of direct coordination and through secondary, intramolecular spheres of influence, ultimately generating differential catalytic ArM activity.

Performing catalysis with exogenous metal complexes within cellular environments has enormous potential applications in medicinal chemistry and synthetic biology. Given the potential difficulties associated with cell-uptake, minimising deactivation, overcoming toxicity of exogenous metal ions and precise localisation of metal cofactors in cells, the idea of using traditionally inert organometallic complexes has obvious advantages in that reactive promiscuity is reduced. As pointed out above, such complexes would be designed to have a latent catalytic activity which emerges once the metal complex is bound to a protein. The design challenges raised by this approach are not just as a result of a need to control the electronic and three-dimensional steric coordination sphere of the metal ion, but also to limit ligand exchange processes, restricting lability of a precursor complex (in the cellular milieu) until it reaches a specific protein target. Since the metal-ligand exchange processes for 4d and 5d metal complexes are typically slow, they are particularly attractive from this point of view but are hard to predict ab initio.

In conclusion, in order to optimise the chemistry and biochemistry of ArMs, directed evolutionary campaigns coupled with high throughput screening methods rather than individually-designed synthetic strategies are much more likely to generate optimised orthogonal catalysts for new and efficient metabolic processes. Direct coordination between metal ions and enzymes is essential in order to deliver truly interdependent systems, ideally where entatic states deliver enhanced reactivity, efficiency and selectivity that cannot easily be replicated in conventional, synthetic metal catalysis. Going forward, methods of generating ArMs should be evaluated and developed for both their ability to be used in directed evolution procedures and the extent to which the protein scaffold participates in the activity of the metal complex.

A cooling tower is a heat dissipation unit which cools bulk water in industrial systems. Cooling towers provide cooling by spraying the heated water coming from the system onto a fill material and rejecting the heat to the open atmosphere (1). The cooled water returns to a basin to recirculate again through the system. Common uses of wet cooling towers include air conditioner systems, manufacturing facilities, telecommunication devices and power plants. Such man-made installations provide an ideal environment for bacterial growth similar to an incubator, supported by water temperatures ranging between 24°C and 38°C (2–4). The heated water comes from the source to the heat exchanger that allows the exchange of heat between two liquids at different temperatures by indirect contact inside water jacketed tubes (5).

Wet cooling towers providing cool water for heating, ventilating and air conditioning (HVAC) systems are known to be subject to contamination. Organic and inorganic substances in bulk water are deposited on the water contact surfaces, reducing the heat transfer significantly and threatening the operating stability of the whole system. Established biofilms offer cleaning challenges because they are resistant to most chemical and physical cleaning protocols and they also reduce the heat transfer efficiency (6, 7). HVAC systems are responsible for about half of the energy consumed in modern buildings and industrial facilities. Therefore, biofouling is always a significant issue for heat exchangers and should be taken into account during heat exchanger design and production. As a solution, altering the surface properties could be an effective approach to reduce biofouling in such hard-to-reach articulated systems (2, 8, 9).

A biofilm layer is a community formed by bacterial cells living in a polymeric matrix that they produce; a functional partnership adhered to a living or inanimate surface organised by microorganisms in a dense exopolymer matrix. The capability of microbes to stick to a substratum and to produce a biofilm layer has great significance in a diversity of cooling towers, where fouling can act as a perpetual source of contamination. Biofilm layer must be kept to a minimum in order to prolong the operating life of man-made water systems and facilitate control of pathogens. Disinfectants may be used for this purpose (4, 9).

Industrial cooling towers can be manufactured from different materials. Generally, towers are made of reinforced concrete or fibreglass, stainless steel, wood or reinforced plastic sheets. The fill material is generally made of plastic sheets (polypropylene, polyethylene or polyvinylchloride) where heat dissipation occurs. For corrosion resistance, towers are specially treated, painted and covered with a protective film layer (7). In the case of corrosive water or atmospheric conditions, the use of plastic towers is recommended. But heat exchanger units are made of stainless steel or copper for better thermal conduction (5). The critical issue that affects cooling is the aggregation of deposits over the heat exchanger surfaces which includes biofouling. Conventional steel heat exchangers may have corrosion or deposits may have formed on the heat exchanger tubes. Both of these factors reduce the heat transfer rate (10). To solve this problem, novel anti-fouling coatings are considered. Nano-silica can be used in the form of liquid composites in many matrices as coating materials. Nano-silica is used in the textile and automotive industries because of its self-cleaning, abrasion resistant, hydrophobic and oleophobic features. It is known that nano-silica is able to create low-cost, hard and tough coatings which are resistant to wear and weathering (11).

Although biofilm formation on plastic fill surfaces in wet cooling towers has been studied widely, no studies were found on biofilm formation on steel heat exchangers in cooling towers. As coating of heat exchangers is not common, the aim of the current work was to limit tenacious biofouling on heat exchangers using a nano-silica coating, which will lead to longer material life, better cooling of water and less clogging in closed-loop systems.

A brand new fabricated closed-loop cooling tower was monitored for six months. A real size, fully working closed-loop cooling tower system was kept in operation by the manufacturer during the experimental period at the factory test laboratory. The system was filled with distributed network water. Regular blowdown was implemented to limit the concentration of dissolved solids. In a circulation rig, hot process water was kept separate from the cooling water in a closed-loop system (Figure 1). For the experiments, a half portion of the stainless steel (316 SS) heat exchanger tubes were coated with nano-silica and the other part was left without coating. Coating was done by coaxial electrospraying before assembly and left to cure in air for 24 hours. Coaxial electrospraying has several implicit advantages such as high encapsulation efficiency and uniform particle distribution. The coating thickness was between 4–6 μm. Before coating, the heat exchanger tubes were sprayed with 96% ethanol to remove any dirt, oil or grime. This application made the bonding of the coating stronger.

Fig. 1.

Silica in powder form is hydrophilic. To produce hydrophobic nano-silica, the silica particles were transformed by fluorination to confer hydrophobicity. The final particle size was about 40 nm. The aqueous form of the nano-silica coating contains ethanol as solvent to keep it in liquid form before use. The final nano-silica product was supplied by a local company. After curing, the coating was solid on the surfaces, and no colour change, shedding or weight loss were observed on any of the coated test surfaces after the experimental period. The stability of the coating was tested in a different study by the present author (9) and the mean overall adhesion capability of the coating was recorded as 1.6 using a pull-off adhesion tester, which matches very well with the general rating of adhesion. Water was circulated over the stainless steel (316 SS) heat exchanger tubes, where natural biofilm formation was allowed to occur. Sampling of the biofilm required dismantling the outer shell of the heat exchanger unit every month. The system temperature water was kept constant at 37°C using an electrical heating unit to eliminate temperature fluctuation which might influence biofilm formation over time.

Pipe segments were cut monthly from the heat exchanger using an angle grinder, kept in a container filled with system water and brought quickly to the laboratory for analysis. LIVE/DEAD^® Bac Light^TM Bacterial Viability Kit (Invitrogen^TM, Thermo Fisher Scientific, USA) dye was added immediately to cover the surfaces completely to stain the actively respiring and dead bacteria. After 15 min, the surfaces were rinsed with sterile bi-distilled water to remove unattached cells, air dried, covered with immersion oil and cover slip, then examined in the dark. This was repeated every month until the study finished at the sixth month. An epifluorescence microscope (Eclipse 80i, Nikon Instruments Inc, Japan) was used to visualise the biofilm cells in situ. The camera enables counting and taking images of bacteria on solid surfaces, with the signals displayed on the computer monitor. Counting and recording were carried out using special software (NIS-Elements, Nikon Instruments Inc, Japan). Signals obtained from 20 randomly selected regions were recorded. Images were saved for later analysis.

The LIVE/DEAD^® kit stains dead cells red and live cells green in colour. The LIVE/DEAD^® test kit contains two DNA-binding dyes, propidium iodide and SYTO^® 9. These dyes differ in their spectral properties and their ability to enter the living bacterial cell. The first dye in the kit is SYTO^® 9, which can pass through the membrane of all bacteria and stain the cells green. Propidium iodide only enters into cells with a damaged cell membrane, allowing them to appear red under fluorescent light. The number of viable and dead bacteria on surfaces can be determined in a single step using a dual emission filter cube (Chroma Technology GmbH, Germany).

For both parameters over the six-month duration of the experiment, the difference between the average bacterial numbers were compared by two-way analysis of variance. A follow-up post-hoc analysis was done in order to determine differences. The difference was considered significant when p < 0.05. SPSS^® Version 18.0 software (IBM Corp, USA) was used for the statistical analyses.

The bacterial numbers from the LIVE/DEAD^® test kit were analysed in situ on the surfaces using the manufacturer’s software during the experimental period for six months. The results are given in Table I. The number of signals per cm² were calculated using the magnification factor. Since the raw data were too scattered, the values are given in the logarithmic (log₁₀) base for better comparison. The logarithmic reduction was clearly significant starting from the first sampling.

The total bacterial numbers on coated tubes were recorded as 49,090 cell cm⁻² after the initial month, and 13,016,957 cell cm⁻² on uncoated surfaces after the first month. The results distinctly showed that this type of coating reduces biofouling formation on heat exchanger surfaces from the start of the experimental set-up. The numbers of surface associated bacteria on uncoated control tubes gradually increased and reached 1.28 × 10¹² cell cm⁻² after the sixth month, at which time the biomass on nano-silica coated tubes was 6.3 × 10⁴ cell cm⁻². No significant rise (p < 0.05) of bacterial numbers on nano-coated heat exchanger tubes was recorded during the six-month period in terms of total biofilm counts. This outcome demonstrates that a nano-silica coating can clearly reduce the bacterial biofilm layers on coated heat exchanger surfaces.

As expected, nano-silica coating slowed down the adhesion and colonisation of bacteria on the substrata thanks to its strong hydrophobic properties. The pH, dissolved oxygen, total dissolved matter and temperature values of the water in the system during the six-month test period were recorded and are given in Table II. The values in Table II were important to monitor circulating water due to the blowdown regime.

It is known that even with conventional cleaning and disinfection regimens, there is a problem fighting against biofilm formation and development of microbial resistance (12). Based on previous studies conducted in this field (13, 14), it is impossible to eliminate the formation of biofilm layers on surfaces, but biofilm formation can be reduced (9, 15, 16). For this purpose, it is possible to modify surfaces with different coatings. The nano-hydrophobic coating changes the surface properties of the material and supports less biofilm formation (16–18). Hydrophobic coatings limit the wettability of the surface, making it difficult for organic and inorganic matter or microorganisms to adhere; and even if they do, they can easily be detached from the surface by physical factors such as laminar or turbulent water shear stress (19).

The issue of antimicrobial coatings has been extensively studied (20–24). The problem with these products is development of bacterial resistance against the agent (11, 25). Even antibiotic-containing coatings have been reported to promote biofilm formation (26). Silver compounds combined with silica, silane and titanium coatings in particular gave antimicrobial properties but the problem of toxicity in medical devices was mentioned (27). In industrial use, the resistance of microorganisms is at the top of the list as a disadvantage (28). In addition, silver compounds in water systems will reach the aquatic environment and appear as a separate environmental problem.

It is also emphasised that anti-biofilm coatings are very important for preventing the formation of a biofilm layer at an early stage (29, 30). However, studies conducted to date are mostly aimed at solving clinical problems and have been done in vitro with pure cultures (15, 17, 18, 31–33). Using monospecies biofilms is a sterile approach and cannot represent mixed cultures in the natural environment and their interaction with each other. Sol-gel products and superhydrophobic coatings which are more strongly water repellent (31, 34) have also been tried. It was observed that the life of these coatings was not as long as hydrophobic coatings. On the other hand, the high cost of superhydrophobic coatings was a drawback. Contrary to hydrophobic coatings, some hydrophilic coatings were also found to be effective against biofouling. Holberg et al. (8) reported that biocide-free silicone coatings showed promising real-life performance on fresh water-cooled heat exchangers and also performed well in laboratory tests.

Ding et al. (35) tested an environmentally friendly antifouling coating product, butenolide, which was designed for controlled release from biodegradable polyurethane. The anti-fouling effect was shown by in situ tests. The main target was marine biofouling, especially larval settlement on surfaces. Since the adhesion of fouling organisms relies on a microbial biofilm layer, inhibition of primer settlement is crucial. Hu et al. (36) sprayed bacterial-anti-adhesive modified polystyrene microspheres to construct bacterially-anti-adhesive surfaces. It can be used on any surface thanks to the lotus effect. It was reported as robust and durable on surfaces. Similar surface engineering strategies focus on altering the physicochemical properties of the material surface. In general, reduced efficacy of regular disinfectants leads to progress in development of antimicrobial surfaces and coatings (37, 38).

This is the first report of a nano-silica coating on a stainless steel cooling tower heat exchanger. The study showed that the nano-silica coating significantly reduced bacterial fouling on surfaces. There are many similar surfaces with biofouling problems which have contact with water and require a solution. Nano-silica has proven to be effective at reducing the formation of biofilms on surfaces and can be applied as a cost-effective, effortless, non-toxic, readily available material. Due to growing restraints on environmental release of biocidal agents and the growing restrictions on the use of disinfectants in man-made water systems, as well as demand to decrease the cost of system maintenance, different ways to limit biofilms in man-made water systems hold much expectation.

SMEs refers to the ability of the material to memorise a shape and materials that possess these properties have a multitude of exciting technical and medical applications (1–14). For materials such as alloys this is commonly in a one-way SME (7, 15), however, there are a variety of materials that are capable of reverting to their permanent shape or original state upon exposure to a stimulus (such as a temperature change) or indeed multiple stimuli (16). SMP-based materials have been widely investigated since the 1980s because of the abundance of potential applications imparted by their interesting properties (for instance, stimuli-responsiveness and ability to change shape), which can lead to technological innovation and the generation of new high value products for technical and medical applications (1, 17–19).

The reversible transformation of SMPs functions by primary crosslinking net points (hard segments) memorising and determining the permanent shape, and secondary switching segments (soft segments) with a transition (T_trans) to reduce strain stress and hold the temporary shape. Below the T_trans, the material will be in its permanent shape and be stiffer than when T_trans is reached and the SMPs are more malleable and can be deformed into a desired shape (usually through application of an external force). The deformed state is maintained once the external force has been removed and the system is no longer at or above T_trans. SMPs revert to their original state once the T_trans conditions are met. This process describes the SME pathway of SMP-based materials that are thermally-induced (albeit not for some light or chemical-induced systems).

While most SMP-based materials hold a single permanent shape and a single temporary shape, recent advances in SMP technology have allowed the generation of multiple-shaped-memory materials with different stimuli responses (light or chemical) (16, 20, 21). An interesting example of this is a triple shaped-memory material generated by combining two dual SMPs with different glass transition temperatures (T_g) (22, 23), where the SMPs switch from one temporary shape to another at the first T_trans, and then back to the permanent shape at another, higher activation temperature (22).

SMPs have a large range of properties from stable to biodegradable and transient, elastic to rigid or soft to hard, depending on the structural units that constitute the SMP. Consequently, SMPs not only respond to temperature (24) and magnetism (25) like shape-memory alloys (SMAs) (26), but also to moisture (27), electricity (28), light (29) and chemical stimuli (such as a pH change) (30). Moreover, there are other principles of SME; for instance, a thermal-responsive SMP can proceed via a Diels-Alder reaction (chemical crosslinking/reversible covalent bonds) (31). SMPs tend to have much milder processing conditions than SMAs (<200°C, low pressure), have a greater extent of deformation (strain more than 200% for most materials) and tend to be based on cheap starting materials with simple synthetic procedures (12, 32). After the term ‘shape-memory’ was first proposed by Vernon in 1941 (32), the significance of SMPs was not fully realised until the 1960s, when crosslinked polyethylene (PE) was used to make heat-shrinkable tubes and films (33). Significant investment in the development of SMPs began in the 1980s (34) with rapid progress realised in the last decade, particularly with a view to the generation of shape-memory materials with exciting and versatile features.

Two important quantities used to describe SMEs are the strain recovery rate (R_r) and the strain fixity rate (R_f). R_r describes the ability of a material to memorise its permanent shape, while R_f describes the ability of switching segments to fix the mechanical deformation. R_r is calculated using Equation (i):

(i)

where N is the cycle number, ε_m is the maximum strain imposed on the material and ε_p is the strain of the sample after recovery. R_f is calculated using Equation (ii):

(ii)

where ε_u is the strain in the fixed temporary shape. SMPs respond to specific stimuli through changes in their macroscopic properties (for example, shape) (26). The polymer network underlying active movement involves a dual system, one that is highly elastic and another that can reduce the stiffness upon application of a certain stimulus. The latter system incorporates either molecular switches or stimulus sensitive domains (35). Their shape-memory feature is a result of the combination of the polymer’s architecture, and a programming procedure that enables the formation of a temporary shape. Net points consist of covalent bonds or intermolecular interactions and the SMP’s hard segments form the net points that link the soft segments (acting as a fixed phase), whereas the soft segments work as the molecular switches (acting as a reversible phase). The fixed phase prevents free flow of the surrounding polymer chains upon the application of stress. The reversible phase, on the other hand, undergoes deformation in a shape-memory cycle and is responsible for elasticity. For example, if the T_trans is T_g, the micro-Brownian motion of the network chains is fixed at low temperature (below T_g) and will be switched back on at high temperature (above T_g), recovering its original state. When T_trans is the crystal melting temperature (T_m), the switching segments crystallise at low temperature (below T_m), and then recover their original state at high temperature (above T_m). In addition, T_g normally extends over a broader temperature range compared to T_m, which tends to have relatively sharper transitions in most cases (26). Moreover, after the exposure to a specific stimulus and the T_trans is achieved, the strain energy in the deformed state is released, resulting in the shape recovery phenomenon. The general process of this SME for SMPs is depicted in Figure 1, wherein the polymer network structure is either chemically or physically crosslinked and the switching units are made from a semi-crystalline or amorphous phase.

Fig. 1.

Shape-memory behaviour can be demonstrated in various polymer systems that are significantly different in molecular structure and morphology. SME mechanisms differ according to the specific SMP(s); for instance, the SME mechanism of the chemically crosslinked semi-crystalline PE SMP. The crystalline phase, with a T_trans being T_m, is used as the molecular switching unit providing shape fixity. The chemically crosslinked PE network memorises the permanent shape after deformation upon heating (12, 36, 37), and the mechanism of the thermally-induced shape-memory PE (SMPE) is depicted in Figure 2.

Fig. 2.

The associated modulus of elasticity is dictated by configurational entropy reduction that occurs with deformation of the constituent chains and is therefore often termed entropy elasticity. For T>T_trans (T_g, T_m or other), polymer networks exhibit super-elasticity wherein the polymer chain segments between crosslink points can deform quite freely and are prone to being twisted randomly via rotations about backbone bonds, maintaining a maximum entropy and minimum internal energy as macroscopic deformation occurs (12). The classic prediction from rubber elastic theory is that the resulting elastic shear modulus (G) is proportional to both crosslink density and temperature (Equation (iii)):

(iii)

where ν is the number density of network chains, p the mass density, R the universal gas constant and M_C the molecular weight between crosslinks. From a macroscopic viewpoint, the SME in SMPs can be graphically represented in three-dimensions (3D). Tensile strain vs. temperature and tensile stress (for example, elongation) is depicted in Figure 3.

Fig. 3.

Using the shape-memory strain-temperature-stress relationship description in Figure 3, the features of SMPs that allow for good shape-memory behaviour include: a sharp transition that can be used to quickly fix the temporary shape at low temperature, and the ability to trigger shape recovery at high temperature; super-elasticity above T_trans that leads to the eventual shape recovery and avoids residual strain (permanent deformation); and complete and rapid fixing of the temporary shape by immobilising the polymeric chains without creep thereafter (12, 37). Thus far, the SME models describing how SMPs recover their original state prominently involve thermo-responsive SMPs. However, careful design of the polymers allows the opportunity for SMPs to possess different stimuli responses and applications.

A multitude of different triggers for SMEs and SMPs exist. However, an in-depth review is outside the scope of this mini-review, and therefore a few examples are highlighted below.

It is possible to generate thermally-induced SMEs in a variety of materials (18–20, 38–40), however a comprehensive overview is outside the scope of this mini-review. As previously discussed, the SME of SMPs can be thermally-induced, and these SMPs are the most common (26). Figure 1 depicts a general overview of the SME mechanism of SMPs, with a schematic of the SME mechanism for thermally-induced SMPs with T_g (amorphous cases) and T_m (crystalline cases). Figure 2 presents a specific example of the SME mechanism for SMPE with the T_trans being T_m. In addition, advanced thermomechanical constitutive models have been used to study the materials’ behaviour (for example strain-temperature-stress development with time) in a very accurate way (41). By applying these models to SME mechanistic studies and the detailed characterisation of the SMPs (crosslinks, intermolecular and intramolecular interactions involving the SMPs) (12), a deeper understanding of the SME of SMPs can be achieved, which has proven beneficial for the development of new SMPs and their proposed applications (31). For example, poly(ε-caprolactone) (PCL), typically a biodegradable polymer, has been reported to possess high shape fixity and recovery. This was achieved by integrating reversible bonds within the PCL polymer network via the Diels-Alder addition of 1,2,4-triazoline-3,5-dione (TAD)-anthracene and Alder-ene addition of TAD-indole (42). These PCL SMPs were reported to attain recovery ratios greater than 99% (43). Furthermore, a dual-functional (self-healing and shape-memory) polymer network was achieved by crosslinking a polydimethylsiloxane (PDMS) polymer containing dense carboxylate groups (100% mol) (PDMS-COOH) with small amount of poly(ethylene glycol) diglycidyl ether (PEGDGE) (44). This SMP (PDMS‐COO-E) actuates at body temperature (37°C) with possible strain ca. 200% and shape recovery ratios at 98.06%. In addition, a 25 mm × 4 mm × 1 mm sample cut into two separate pieces healed (the two pieces become one whole piece with no evidence of a cut) when the two cut surfaces were brought into contact after 6 h at 25°C. Thus, the unique material, PDMS‐COO-E, may have a wide range of applications in many fields, including wearable electronics, biomedical devices and four‐dimensional (4D) printing (1, 19). Interestingly, the material was also reported to possess a greater than 85% light transmittance (425 nm to 700 nm) (44), therefore PDMS-COO-E has potential applications in transparent electronic devices. Figure 4 illustrates the possible SME mechanism of PDMS-COO-E. The short PDMS linear chains are crosslinked by chemical covalent interactions and abundant hydrogen bonds into a 3D network. The covalent crosslinked networks of PDMS-COO-E maintain the permanent shape and resilience, whereas, at ca. 37°C the weak hydrogen bonds are broken, and the dynamics of polymer chains increase, resulting in recovering the permanent shape. Meanwhile, a large number of hydrogen bonds enable the samples to heal at temperature without external stimulus (44).

Fig. 4.

It is possible to generate light-induced SMEs in a variety of materials (18–20, 38, 40, 45), however a comprehensive overview is outside the scope of this mini-review. Light-activated SMPs (LASMPs) (46) typically use photothermal or photochemical (photocrosslinking or photocleavage) triggers for SMEs. For instance, photothermal LASMPs typically employ photo-absorber molecules and particles that convert light to heat, thereby increasing the temperature at the desired region within the LASMP. Photochemical LASMPs incorporate photosensitive molecules to create or cleave bonds during irradiation with light, imparting potentially very swift SMEs (47, 48). It is possible to improve the response time of SMPs by increasing the thermal conductivity with various conductive additives (49). However, the heating and cooling of materials with substantial thickness takes time, which can be minimised by using light to trigger transitions in LASMPs (46). It is also possible to generate multistimuli-responsive materials using components of the materials that respond to different wavelengths of light (for example, one wavelength of light to induce photocrosslinking, while a second wavelength of light cleaves bonds). It is possible to produce materials that can be reversibly switched between an elastomer and a rigid polymer employing polymers containing cinnamic groups (48) that can be fixed into pre‐determined shapes utilising ultraviolet (UV) light illumination (>260 nm), and then recovered their original state when exposed to UV light at a different wavelength (<260 nm) (49). Figure 5 depicts one example of the process of LASMPs shape recoverability.

Fig. 5.

It is possible to generate electrically-induced SMEs in a variety of materials (18, 20, 50–55), however a comprehensive overview is outside the scope of this mini-review. A variety of electrically conductive materials including organic electronic materials (including conductive polymers such as polypyrrole (PPy) (28, 56–58) and carbon nanotubes (CNTs) (59, 60)) and inorganic electronic materials (such as alloys, metals (61) and silver nanowires (NWs)), have been incorporated in materials displaying SMEs to impart swift triggers to the SMEs, enabling a variety of interesting applications.

Highlighting some of the potential of electrically-induced SMEs, electrically-induced SMP composites incorporating shape-memory polyurethane (SMPU) and Ag NWs in a bilayer structure exhibits flexibility and electrical conductivity (62–64), which may find applications as capacitive sensors, healable transparent conductors and wearable electronics (65). In such materials the Ag NWs are randomly distributed on the surface layer of the composite to form a conductive percolating network that retains conductivity (200 Ω sq⁻¹) after a 12% elongation. However, continual increase in elongation causes a dramatic increase to the composites’ resistance value and the eventual loss of electrical conductivity (66). When the material (deformed or in its original state), is connected to a typical circuit, a low voltage of 1.5 V was enough to activate a light-emitting diode (LED) (65). The composites possessing a higher Ag NW content exhibited a higher recovery ratio and reached the maximum recovery speed quicker (66). It was assumed that all the heat from electrical (Joule) heating was absorbed by the sample, i.e. no convective loss (67). Therefore, the composites with higher Ag NW content had a lower resistance value and the heating effectiveness was promoted. Heat initiates the thermal T_trans of the SMPU leading to an improved shape recovery, and voltages as low as 5 V reverted bent composites to their original state within 3 s (66). This represents a good example of a multifunctional SMP and demonstrates the potential of SMP designs driving technological innovation. A schematic of the composite is shown in Figure 6.

Fig. 6.

Polymeric blend SMPs can be constructed from two immiscible polymeric matrices. The shape-recovery of these systems can be controlled with relative ease by varying the ratio of the polymer blends (68). However, this process may have adverse effects on shape-memory characteristics and diminish the material’s performance, thereby limiting potential applications. On the other hand, SMP functionality may also be enhanced with other capabilities. For instance, it was recently reported that a new hybrid SMP was developed by combining single-walled CNTs (SWCNT) into a poly(lactic acid) (PLA) and thermoplastic polyurethane (TPU) SMP system, containing poly(ethylene glycol) (PEG) plasticiser (68). By incorporating PEG, the hybrid SMP composite achieved a lower temperature T_g (for example, 10 wt% of PEG lowered T_g of the PLA/TPU sample from 60°C to 40°C), meanwhile enhancing the dispersion of SWCNT (for instance, even at 4 wt% of SWCNT loading, 100% SMP tensile strain was possible, much greater than previously reported electrically-induced SMP studies, i.e. 12% discussed previously). In addition, the presence of the SWCNT can stabilise the SMP system and enhance its shape-fixity after deformations at room temperature conditions (68). Furthermore, the material was capable of a conductivity above 10⁻⁷ S cm⁻¹, which can be considered conductive, as documented (68). The PLA/TPU SMP composite (2 wt% SWCNT and 10 wt% PEG) also achieved shape-recovery, via Joule heating derived from electricity, in 80 s when currents of 125 mA were applied. The high stiffness of SWCNT filler results in decreasing shape-recovery performance because of the hindrance on the polymer chain movements (68). As a result, under room temperature stretching, the R_f and R_r values obtained were ca. 80% and 65%, respectively. Therefore, when its shape-recoverability is compared to other SMPs (shape-recovery ratios being upwards of 98%), the material is lacking. However, the hybrid SMP composite does possess electroactive ability, thus a trade-off relationship between shape-memory/recovery and electroactive ability needs to be carefully considered when designing similar materials.

It is possible to generate water-induced SMEs in a variety of materials (18, 20, 38, 39, 69–72), however a comprehensive overview is outside the scope of this mini-review. Water is an important stimulus due to the fact it is abundant in a multitude of different environments, non-toxic and safe for a variety of applications.

An interesting example highlighting the potential of such materials is based on strong and flexible composite films (73) utilising the combination of a flexible interpenetrating polyol-borate network (74) and electroactive PPy (75, 76) that exchange water with the environment resulting in film expansion or contraction. The free-standing multi-functional SMP films were prepared by electropolymerisation of pyrrole in the presence of the polyol-borate complex (composed of pentaerythritol ethoxylate (PEE) coordinated to boron(III)) (74), wherein the interpenetrating network enables water-gradient-induced displacement, converting chemical potential energy in water gradients to mechanical work (73), and results in adaptation of the architecture in response to an environmental condition change (i.e. sorption and desorption of water which drives the SME process, as depicted in Figure 7). The design of the water-responsive PPy-PEE composites was creatively applied to prepare actuators and generators driven by water gradients. The film actuator can generate contractile stress up to 27 MPa, lift objects 380 times heavier than itself and transport cargo 10 times heavier than itself (73). An assembled generator associating the actuator with a piezoelectric element driven by water gradients, outputs alternating electricity at ca. 0.3 Hz, with a peak voltage of ca. 1 V (73). The electrical energy can be stored in capacitors that could power micro and nanoelectronic devices (73). The SME mechanism for this SMP differs to that of Figure 1 and Figure 2, utilising water as the shape-memory trigger for T_trans, and the original and deformed state interchange automatically via water sorption and desorption states. However, the shape-memory phenomenon remains the same, further demonstrating the potential of SMP designs driving technological innovation.

Fig. 7.

It is possible to generate pH-induced SMEs in a variety of materials (18, 20, 38, 77–80), however a comprehensive overview is outside the scope of this mini-review. An example of the interesting properties of such pH-responsive SMPs and their composites is produced by blending poly(ethylene glycol)-poly(ε-caprolactone)-based polyurethane (PECU) with functionalised cellulose nanocrystals (CNCs) displaying pH responsive pyridine moieties (CNC-C₆H₄NO₂) (81, 82). At high pH values the pyridine is deprotonated, facilitating hydrogen bonding interactions between the pyridine groups and hydroxyl moieties on the cellulose, whereas at low pH values, the protonation of the pyridine moieties diminishes these interactions. By comparison, carboxylic acid functionalised cellulose nanocrystals (CNC-CO₂H) responded to pH variation in the opposite manner (83–85). When the functionalised CNCs were combined with PECU polymer matrix to form a nanocomposite network, the mechanical properties of PECU were improved along with the pH-responsiveness of CNCs (85). The percolated network of pH-sensitive CNC in the polymer matrix served as the switching units for the shape-memory composite, the SME process of this material is depicted in Figure 8 (81, 82). The CNC serves as the switching unit of the SMP composite within the matrix of PECU which is physically crosslinked and microphase separated to yield the net points. Such pH-responsive shape-memory nanocomposites have promise in the design of biomaterials for biomedical applications (for example, SMP-based drug delivery systems triggered by transition along the digestive tract) (83).

Fig. 8.

It is possible to generate magnetically-induced SMEs in a variety of materials (18, 20, 38, 86–88), however a comprehensive overview is outside the scope of this mini-review. The SMP devices discussed thus far are being researched with potential application into wearable electronics, nanoelectronics (such as actuators), biomaterials and biomedical devices (1, 18, 19). However, in some instances (such as medical devices) a key challenge is the design and implementation of a safe and effective method of actuating a variety of device geometries in vivo. As previously discussed, a pH‐triggered SMP design can be potentially effective when utilised as drug delivery devices, when the target environment has a substantial pH difference (for instance, the digestive system) (83). However, the development of electrically and thermally-triggered devices that safely operate in vivo is difficult due to the (generally) high temperatures these SMPs can reach (relative to biological systems). For instance, the electroactive PLA-TPU SMP composite (2 wt% SWCNT and 10 wt% PEG) reaches temperatures greater than 70°C in 80 s as shape-recovery is achieved (68).

An alternative method of achieving actuation is inductive heating by loading ferromagnetic particles into an SMP system and exposing the doped device to an alternating electromagnetic field (89), benefiting from the innate thermoregulation offered by a ferromagnetic material’s Curie temperature (T_c, at which a ferromagnetic material becomes paramagnetic, losing its ability to generate heat via a hysteresis loss mechanism) (90). By using particle sizes and materials that will heat mainly via a magnetic hysteresis loss mechanism over an eddy current mechanism, it is possible to have an innate thermoregulation mechanism that limits the maximum achievable temperature to T_c (89). Therefore, by selecting ferromagnetic particle materials with a T_c within safe medical limits, Curie thermoregulation eliminates the danger of overheating and the need for a feedback system to monitor implanted device temperatures (89). However, this technology is not only useful when applied to medical devices. Other useful applications include remote activation in which wires or connections to SMP devices could be eliminated, simplifying the design and reducing possible points of failure. An example of this method of actuation involves the incorporation of 10% by volume nickel zinc ferrites (for example C2050 (Ceramic Magnetics Inc, USA) and CMD5005 (Ceramic Magnetics Inc), particle sizes ca. 50 μm with spherical shapes) with an ester-based thermoset polyurethane (PU) SMP, MP5510 (SMP Technologies Inc, Japan) (T_g of 55°C) (89). The magnetic field utilised to achieve shape-recovery was a copper-wound solenoid coil with a 2.54 cm diameter, 7.62 cm length and with a total of 7.5 turns. The unit possessed an adjustable power setting capable of outputting 27 W to 1500 W at between 10 MHz and 15 MHz frequency (note: this high frequency may induce eddy currents in the tissue, causing undesirable direct heating of the human body in medical applications) (91). However, an alternating magnetic field of 12.2 MHz and approx. 400 A m⁻¹ (centre of the inductive coil) at room temperature was used for actuation to demonstrate proof of concept for the device. It was also reported that clinically useable frequencies (50 kHz to 100 kHz) (92) should still be effective (89), albeit this could result at a different quantitative level (i.e. shape-recovery and memory performance may be reduced). Furthermore, C2050 and CMD5005 possess a T_c of 340°C and 130°C, respectively. These temperatures exceed physiological limits and are therefore not practical for medical devices currently, however, these doped SMP composites did not exceed temperatures above the respective Ni Zn particle T_c values, signifying a thermoregulation characteristic. In addition, it was stated that the 10% volume of Ni Zn particles did not impact the SMPs shape-memory properties significantly (89). The T_g increased from 55°C to 61.4°C and the shape-recovery of a flower and foam-based device was achieved within 15 s to 25 s, at a temperature range of 23°C to 78.6°C. The potential applications for this device are illustrated in Figure 9. Optimisation of this device/design is still required before it can be considered clinically viable, however, this SMP composite highlights very interesting characteristics, remote activation (via magnetic fields inducing thermally-triggered actuation) and thermoregulation (via T_c temperature of the material being employed).

Fig. 9.

As highlighted above, SMP materials are diverse and respond to many different external stimuli (including temperature, light, electricity, water, pH and electromagnetic fields) by a variety of mechanisms. Although SMPs can be classified based on their composition and structure, stimulus and shape-memory function, their classification can be difficult, as organising these polymeric smart materials into one or two simple categories is an over-simplification of their abilities and characteristics (93).

SMPs are considered to consist of net points and molecular switches or stimuli sensitive domains. These net points can be achieved by covalent bonds (chemically crosslinked) or intermolecular interactions (physically crosslinked). Chemically crosslinked SMPs involve suitable crosslinking chemistry and are referred to as thermosets (94, 95). Physically crosslinked SMPs involve a polymer morphology consisting of at least two segregated domains and are referred to as thermoplastics (96). The network chains of the SMP can be either amorphous or crystalline and therefore, the T_trans is either a T_g or T_m. The network architectures are thought to be constructed through crosslinking net points, with polymer segments connecting adjacent net points. The strongly crosslinked architectures ensure the polymer can maintain a stable shape on the macroscopic level (93). Thermoplastic polymers exhibit a more reversible nature (97), meaning the physical crosslinked net points can be disrupted and reformed with relative ease. The interconnection of the individual polymer chains in a physically crosslinked network is achieved by the formation of crystalline or glassy phases. For thermoset polymers, the individual polymer chains are connected by covalent bonds and are therefore more stable than physically crosslinking networks and show an irreversible nature (98–100).

Regarding thermo-responsive SMPs, they can be classified according to the nature of their permanent net points and the T_trans related to the switching domains into four different categories: (a) physically crosslinked thermoplastics, T_trans = T_g; (b) physically crosslinked thermoplastics, T_trans = T_m; (c) chemically crosslinked amorphous polymers, T_trans = T_g; (d) chemically crosslinked semi-crystalline polymer networks T_trans = T_m (93).

For the physically crosslinked SMPs, the formation of a phase-segregated morphology is the fundamental mechanism behind the thermally-induced SME of these materials (93, 99). One phase provides the physical crosslinks while the other acts as a molecular switch. They can be further classified into linear polymers, branched polymers or a polymer complex. Linear SMPs may consist of block copolymers and high molecular weight polymers, the typical physically crosslinked SMP is linear block copolymers, such as PU. In polyesterurethanes (PEU), oligourethane segments are the hard-elastic segments, while polyester serves as the switching segment (99).

For chemically crosslinked SMPs, two methods are commonly used to synthesise covalently crosslinked networks (36, 41). The first method relies on addition of a multi-functional crosslinker during polymerisation (41), whereas the second method relies on the subsequent crosslinking of a linear or branched polymer (36). The networks are formed based on many different polymer backbones. Covalently crosslinked SMPs possess chemically interconnected structures determining the original macroscopic shape. The switching segments of these materials are generally the network chains between net points, and a T_trans of the polymer segments is used as the shape-memory switch. The chemical, thermal, mechanical and shape-memory properties are determined by the reaction conditions, curing times, the type and length of the network chains and the crosslinking density (35). Comparing physically crosslinked SMPs with chemically crosslinked SMPs, the chemically crosslinked SMPs often show less creep, thus, any irreversible deformation of the polymer during shape recovery is less. This is because covalent crosslinked networks are more stable than physical crosslinked networks. As a result, chemically crosslinked SMPs usually show better chemical, thermal, mechanical and shape-memory properties than physically crosslinked SMPs (96). For example, the shape recovery ratio of thermoplastic SMPU is usually in the range of 90% to 95% within 20 shape recovery cycles, and the elastic modulus is between 0.5 GPa and 2.5 GPa at room temperature (26). Additionally, when exposed to air, it is sensitive to moisture and therefore possesses unstable mechanical properties. In contrast, an epoxy SMP shows better overall performance as a shape-memory material. The shape recovery ratio typically reaches 98–100%, the elastic modulus between 2 GPa and 4.5 GPa, and it is generally stable in the presence of moisture (26). Thermoplastic SMPs (such as SMPU) are mostly researched and used as functional materials at a small scale, such as for biomaterials (30, 97). However, thermosetting SMPs (for example styrene-based SMP (SSMP) and epoxy SMPs) are generally used for structural materials, such as space deployable structures and automobile actuators (97, 98).

The approaches to designing different shape-memory functions become more abundant as scientists and engineers better understand the SME mechanism of SMPs. For instance, discussed thus far are examples of SMPs with polymeric blends, addition of crosslinking species, incorporation of electroactive and ferromagnetic substances. All of which enhances an SMP device functionality, enabling unique and interesting characteristics which can be tailored to a plethora of applications (for example, self-healing and wearable electronics, drug delivery and implantable medical devices) (101–110). Further still, one-way SMEs, two-way SMEs (such as dual shape PPy-PEE, discussed previously), triple SMEs, multiple SMEs and even temperature-memory effects (TMEs) have been widely investigated in SMPs (34). As the types of SMP materials increasingly diversify, two and even three different types of shape-memory functions can be achieved simultaneously in the same SMP material (34, 111). These types of materials can usually be achieved when combining different SMPs possessing different properties. A schematic of one-way, two-way, dual shape and triple shape functionality SMPs is shown in Figure 10, and an integrated insight into the classification of SMPs is shown in Figure 11.

Fig. 10.

Fig. 11.

An example of a selective triple shape multicomposite SMP was documented to incorporate a neat SSMP (112) and two SSMP composites (113). One incorporated iron(II, III) oxide nanoparticles while the other CNT nanoparticles. This unique SMP composite successfully possessed three different regions within the sample: neat SSMP, SSMP-Fe₃O₄ and SSMP-CNT. Because of this, the material also possessed distinct shape-memory capabilities with different triggers. For instance, the material was documented undergoing a three-step shape-memory recovery process, subjected to an alternating magnetic field of 30 kHz, a radio frequency (RF) field of 13.56 MHz and direct oven heating at 130°C (113). Furthermore, the R_f and R_r for the original shape to the first temporary shape (and back to the original shape) was reported at 93% and 93%, respectively. Meanwhile, the R_f and R_r for the first temporary shape to the second temporary shape (and back to the first temporary shape) was at 95% and 99%, respectively (113). The SME mechanism for this multicomposite is represented in Figure 12 and it was concluded that this unique material has promising characteristics to be used in biomimetic materials. Examples of applications of SMP-based materials and their composites are highlighted in Table I.

Fig. 12.

As the understanding of SMPs continually develops among the academic and industrial communities, the generation of new and potentially innovative SMPs will be more rapid while we realise the full potential of these materials. SMPs are one of the most interesting of polymer classes within the field of functional polymers. In addition, SMP composites can enhance the already impressive capabilities of SMPs by imparting new functional characteristics, broadening the potential applications of these materials and enabling a multipurpose material. SMPs and their composites are capable of industrially important applications (examples of which include: self-healing (101–104), generators driven by water gradients (73), sensors (72), task-specific medical devices (18, 105) and wearable electronics (106–110), a few examples of which are highlighted in Table I. The literature published to date de-risks investment from governments and industry to raise the technology readiness levels towards products on the market.

The massive discharge of volatile organic compounds (VOCs) has a great negative impact on the environment (1). Toluene is a common pollutant in VOCs and is produced in a large number of industrial activities, such as chemical refining and dye processing. Toluene stimulates skin and mucosa, and when it reaches a certain high concentration, it also causes paralysis of the human nervous system. Compared with photocatalysis and chemical oxidation, using a biofilter to remove VOCs is more economical and environmentally friendly (2). More important is that it does not produce secondary pollution. The key element to ensure the removal capacity of the biofilter is the preparation of the filler. As a carrier for the transfer of pollutants, the filler can provide a suitable growth environment for microorganisms (3).

Micro-embedding technology is a method which uses physical or chemical methods to keep microorganisms in a defined space, ensuring microorganisms with high activity. The principle of using micro-embedding technology to degrade VOCs is to use a hollow porous membrane to intercept microorganisms inside the filler. The pore size of the hollow porous membrane is smaller than that of microbial cells, so that microorganisms can be embedded. The VOCs can enter the interior of the embedded carrier freely due to the small particle size, and the degradation products can flow out of the carrier through the pore size (4, 5).

A large number of studies have been carried out on different types of fillers. Chen et al. (6) used a two-layer biofilter filled with new mixed packing materials to remove hydrogen sulfide gas. Dumont et al. (7) prepared a nutritional slow-release filler (UP20) to biodegrade H₂S. In the above studies, the fillers were not embedded with microorganisms. The concentration of microorganisms in the filler was small, and the removal efficiency of the biofilter was low in the start-up phase, resulting in a longer start-up period. Zhu et al. (8) used a composite packing material with functional microorganisms to remove H₂S. However, toluene does not biodegrade easily due to the presence of a benzene ring. Zuo et al. (9) found that engineered P. putida could simultaneously degrade organophosphates, pyrethroids and carbamates. Muñoz et al. (10) studied the long-term performance and stability of P. putida in a toluene removal bioreactor. The above studies have found that P. putida is highly effective in degrading organics containing benzene rings.

However, there is a lack of studies on filler micro-embedded P. putida for toluene biodegradation. Existing problems with biofilters packed with fillers include bed clogging, low biomass concentration and pressure drops. These problems become more prominent when the biofilter is operated under high VOC loading rates or long-term operation (11). For example, Ryu et al. (12) found that the benzene removal efficiency of a well-designed biofilter decreased from greater than 90% to approximately 75% after 27 days of operation due to clogging caused by the excess growth of biomass.

The main objective of this study was to evaluate the performance of a self-developed filler micro-embedded with P. putida for toluene removal under various inlet loading rates. The variations in start-up period, pressure drop, biomass concentration and tolerance to transient shock loading were monitored throughout the experiments. Special attention was paid to the analysis of the microbial community attached to these fillers and to monitoring the evolution of the microbial community in various periods.

2.1 Preparation of Filler

2.2 Experimental Setup

2.3 Toluene Concentration Analysis

The determination of toluene concentration was carried out by adsorption of activated carbon and desorption of carbon disulfide, and then the toluene gas was injected into a gas chromatograph (GC-2014, Shimadzu, Japan) equipped with a packed column (free fatty acid phase (FFAP) capillary column, 30 m × 0.25 mm × 0.25 μm) and a flame ionisation detector (FID). The gas chromatography nitrogen was used as the carrier gas with a flow rate of 1 ml min⁻¹. Temperatures of the injection port, column and detection port were set to 150°C, 65°C and 150°C, respectively. Gas samples were collected from the inlet and outlet of the biofilter with a gas-tight syringe and injected into the GC daily (14). Data were obtained from the workstation by automatic comparison of the peak area of the inlet and outlet samples with the baseline of toluene. The performance of the biofilter was evaluated in terms of (%) RE and the elimination capacity (EC) as a function of toluene loading. The RE and EC were calculated as in Equations (i)–(iii):

(i)

(ii)

(iii)

where the C_in and C_out are the inlet and outlet toluene concentration (mg m⁻³), the V is the volume of the whole biofilter (l) and Q is the gas flow rate (l min⁻¹).

2.4 Physical and Chemical Property Analysis

2.5 DNA Extraction and Sequencing

3.1 Physicochemical Properties of the Filler

3.2 Start-up Performance

3.3 Continuous Biodegradation Performance

3.4 Tolerance for Transient Shock Loading

3.5 Biomass Concentration and Pressure Drop in the Biofilter

To explore the bacterial communities in the biomass attached to BF1, genetic sequencing analyses were carried out. Sequencing of 16S rRNA genes amplified from the active bacterial communities during the operational stages revealed 21 phyla, 41 classes, 96 orders, 184 families and 347 genera (28, 29). The community analysis at phylum level of the fillers is shown in Figure 7. The four operational stages were sampled at the 25th day, the 65th day, the 95th day and the 145th day, where the 25th day, the 65th day and the 95th day had a different EBRT and the same inlet toluene concentration, and the 145th day was after two interference-shutdown experiments. The dominant phyla were Firmicutes (63.4 ± 8.7%), followed by Actinobacteria (14.6 ± 3.9%) and Proteobacteria (10.1 ± 4.2%). With decreased EBRT, the abundance of Firmicutes remained high, but the abundance of Actinobacteria decreased, and the abundance of Proteobacteria increased. This is mainly due to a reduction in residence time leading to the inability of microorganisms to fully utilise toluene, and a reduction in the carbon source leading to a change in the proportion of microorganisms (30). After two interference-shutdown experiments, the abundance of Bacteroidetes increased and the normal microecological balance was broken, which indicated that Bacteroidetes is a sensitive biological indicator, similar to the results found by Wolińska (31). Using this indicator (the increase in Bacteroidetes), it can be judged whether the biofilter is in an unstable state, which would provide some guidance for practical engineering applications.

Fig. 7.

In the four operational periods, few Pseudomonas (abundance less than 1%, as shown in Figure S3) were found in the sampling of the above four periods. As the inlet loading rate increased, the abundance of Pseudomonas genus increased from 4.7 × 10⁻⁴ to 1.9 × 10⁻³. After two intervention-shutdown experiments, the abundance of Pseudomonas genus decreased to 8.5 × 10⁻⁵, which indicates that the biofilter was not in a sterile environment and that there are other microorganisms competing with the P. putida added to the filler. When the environmental conditions and the nutrients in the biofilter became unsuitable for the added microorganisms and were suitable for other microorganisms, the other microorganisms were activated and enriched (32). However, in the start-up phase, the biofilter embedded with P. putida started quickly, and the removal efficiency of toluene remained high, which indicated that the added P. putida contributed to the efficient operation of the biofilter (33). These results indicated that the biomass could maintain itself by microbial community changes, and the rapid re-adaptation of the biofilter could contribute to the activity retention of its biomass during the starvation period.

A composite filler micro-embedded with P. putida was prepared and evaluated for the biodegradation of toluene. The biofilter packed with the fillers could start up quickly with 85% RE on the eighth day, and tolerate substantial transient shock loadings. The RE of the biofilter remained above 90% when the EBRT was 18 s and the intake load was not higher than 41.4 g m⁻³ h⁻¹. In the experimental period of 145 days, no filter plugging phenomenon was observed. Moreover, the high removal efficiency and elimination capacity contributed to rich bacterial communities for the efficient biodegradation of toluene. The communities mainly included Firmicutes, Actinobacteria and Proteobacteria, and the abundance of Bacteroidetes increased significantly during the recovery period. The composite filler exhibited favourable physicochemical properties in this experiment and its practicability in industrial engineering should be further investigated.

Acknowledgments

The authors would like to acknowledge the support of the National Natural Science Foundation of China (No. U1304216), the Science and Technology Plan of He’nan Province, China (No. 122102310366), the University Key Research Project of He’nan Province, China (No. 19A610002 and 19A150010), and the China Postdoctoral Science Foundation (No. 2018M632794).

As humans, we seem to desire structure, relationships and laws to understand the universe. Through increased understanding, we can solve the problems and challenges that we perceive. This method and the output are given the label of science. At its best, science provides exquisite understanding, life-changing solutions or sometimes both.

The downside of the structures and rules we impose is that they can create inertia. Because the structure or rule served a purpose in the past, we can be more willing to stand by it blindly than openly seek the understanding or solutions we truly desire; a dynamic seen in the natural and social sciences alike and revealing more about human nature than the universe. One such structure is that of the disciplines within science. We should challenge ourselves to be very clear on the purpose of any structures we adhere to and be ready to remove barriers that get in the way of progress. One such example is uncovering the fertile ground of interdisciplinary research. In recent years interdisciplinary research has been of increasing importance across the sciences. Volume 64 of the Johnson Matthey Technology Review started a celebration of interdisciplinary science by looking at when chemistry collaborates with physics (1) and in this issue, we will celebrate the cross-disciplinary contributions of biology with other fields.

This wide-ranging issue explores topics such as: what we can continue to learn from organisms in unusual environments; how we might leverage biology in artificial situations; and even how we manage the interface between human-made, controlled systems and the outside world. In particular, the diversity of industrial applications is striking. Some are familiar to Johnson Matthey and this journal such as fine chemical synthesis, while others, such as hides and textiles, show that as boundaries within science are removed, previously distant industries will have much to learn from one another.

I have reflected on three themes as this issue has come together. There are numerous examples on each theme and I would challenge the reader to think “what next?” for each:

This third theme is perhaps the most important as it turns niche expertise into something accessible to scientists across fields. Understanding the technology may be beneficial but is not a prerequisite to accessing it. Biology follows favourably in the footsteps of computing in producing such platform technologies and it is an attribute that perhaps we should value and prioritise more in other fields. To expand on this theme, it is exciting to look both backwards and forwards to the contributions made possible by platform technologies from the field of biology. Often these point back to unlocking our understanding of the structure and function of DNA at a molecular level and have resulted in some of the most impactful scientific contributions of the last 50 years or so. Our health has been a significant beneficiary of these advances with cancer drugs providing an illustrative case study. Looking back, we can see recent classes of therapeutics that were significantly enabled by this flow of understanding and platform technologies such as tyrosine kinase inhibitors and antibody-based therapies (2). Most importantly, patient outcomes have improved substantially in part, thanks to these therapies (3).

Looking to the future, gene and cell therapies appear to be following a similar pattern and will hopefully deliver similar patient benefits. Outside of cancer treatments and healthcare, we can see many industries set to benefit from being able to access biological understanding and technologies. This is particularly as we seek to learn from biology and reduce our impact on the planet by using materials and energy in keeping with what Earth can sustain.

As you read through this issue, I hope you enjoy reading something outside of your current field. I would take you back to my earlier challenge and see if you can gain any greater insights by not seeing the separation between your field and those of the authors. Rather, question what you can leverage, what you can learn and what next?