A thermoplastic form of chitosan retains biological activity, enabling molded biodegradable plastics that store enzymes or living microbes and break down pollutants while maintaining strength and stability.
(Nanowerk Spotlight) Materials that maintain biological activity are usually soft, wet, and fragile. They exist as hydrogels, thin membranes, or porous scaffolds where enzymes can diffuse and cells can survive. Their strength depends on water and when that water is removed, they dry, crack, or collapse. Engineering plastics occupy the opposite end of the spectrum. They are dense and tightly packed at the molecular level, able to resist biological interaction. Enzymes denature on their surfaces, and microbial cells cannot penetrate their networks. The distance between these two classes of matter has defined the limits of biomaterials.
A material that retains biological function while existing as a solid part must do more than support life in solution. It has to withstand the process of being shaped into a dense object, the same way conventional polymers are formed into housings, casings, or structural components. That means remaining intact when compacted and dried, holding together under load, and resisting the weakening effects of water.
Most natural polymers fail at one of these steps. Proteins unfold when they leave hydrated environments. Cells rupture when squeezed or dehydrated. Even polymers that tolerate processing often survive only as thin films, never as thick structures that can carry real mechanical demands.
Work reported in Advanced Materials (“Thermoplastic Molding of Chitosan to Form Living Plastic Materials”) shows that this barrier is not absolute. The study uses chitosan, a sugar-based polymer obtained from crustacean shells and fungal cell walls, as the backbone of a solid material that retains biological function.
Overview of chitin/chitosan processing. a) Chitin naturally sourced and chitosan as a chemical derivative after deacetylation; b) Chitosan (medium molecular weight (≈200 KDa) cross-linking with citric acid, green ball with exposed carboxylic groups; c) Preparation of chitosan-based dense plastics: step 1 – dialysis of chitosan-citric acid solution against deionized (DI) water with dialysis membrane Mw cut off: 3.5 kDa; step 2 – freeze drying to generate Cs-CA powder; step 3 – thermomechanical compression for the densification of the powder into plastics. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
Chitosan already has useful traits. It is biodegradable and compatible with human cells. But it is difficult to shape as a solid, because its chains form strong hydrogen-bond networks that refuse to flow or melt. Conventional strategies dissolve it in acid and cast it into films or hydrogels, which weaken in water and often require chemical cross linkers.
This work instead adjusts chitosan’s molecular structure so it can be processed directly as a powder and compressed into robust parts.
Citric acid is the key. It is a small, naturally occurring molecule with three carboxyl groups. When mixed with chitosan in acidic solution, its negatively charged carboxyl groups bind to positively charged amino groups on the polymer. Some of those interactions are temporary ionic attractions, while others become covalent amide bonds. That dual linking reorganizes how the chains pack together. Instead of a network defined by hydrogen bonding alone, the polymer gains connections strong enough to withstand handling and mechanical pressure.
Excess citric acid was removed through dialysis. As the pH rose, the linked chitosan chains aggregated into insoluble clusters. Freeze drying produced a powder called Cs–CA. The chemistry of that powder was verified before any mechanical tests. Nitrogen measurements showed that about 29 % of nitrogen atoms existed in amide form, a clear indication of covalent linkage.
X-ray diffraction revealed tighter packing, with average interchain spacing reduced from about 8.8 Å to about 7.75 Å. The structural shift helped explain why the modified polymer behaved differently in bulk.
Thermal behavior showed the same trend. Untreated chitosan degraded most rapidly around 314 °C. Cs–CA reached peak degradation around 345 °C. The increase indicates a stronger network that resists breakdown because chains are anchored chemically rather than held together only by hydrogen bonding.
The decisive tests occurred when Cs–CA was shaped into solid forms. The powder was pressed at 632 MPa for 20 min at temperatures from 60 °C to 125 °C. Under these conditions, it fused into dense, uniform bars and discs. Untreated chitosan subjected to identical conditions fractured easily, forming only loosely compacted bodies. The molded Cs–CA behaved like a usable solid. Flexural strength reached about 90 MPa at 60 °C and about 125 MPa at 125 °C, performance typical of practical engineering polymers. Stiffness increased in parallel, roughly doubling relative to unmodified chitosan.
Water exposure, which normally ruins chitosan materials, became predictable instead of catastrophic. Immersed in buffered saline at 37 °C, samples molded at 60 °C swelled by about 60 %. Samples molded at 90–125 °C swelled by about 30 %. Their effective water diffusion rate dropped accordingly, and estimated porosity fell from about 0.67 in low-temperature samples to about 0.40 in tighter, hotter-processed ones. The material did not dissolve or soften like chitosan films. It stayed intact.
Despite its stability, the plastic remained biodegradable. Lysozyme, an enzyme that cleaves the chitosan backbone, broke down molded parts. At 10 wt % lysozyme, samples degraded fully over about 25 d. At 5 wt %, mass loss reached about half. This slower rate compared with hydrogels reflects restricted enzyme access, not chemical resistance. The polymer stayed digestible but required time.
The powder-based approach also allowed inclusion of biological agents. When tetracycline hydrochloride was mixed into Cs–CA before molding at 125 °C, the resulting plastic inhibited Bacillus pasteurii. The antibiotic survived fabrication and diffused outward.
Embedded enzymes performed as expected: protease retained about 62 % of its activity when molded at 60 °C and about 20 % at 125 °C. Plastics compressed without heat preserved nearly full activity and stayed stable in humid storage. These results show that the material can act as a reservoir for biological function.
The most ambitious demonstration involved living microbes. Because highly charged chitosan can damage cells, the team confirmed viability before and after dialysis. Dialyzed mixtures supported bacterial cultures. Lyophilized Pseudomonas putida cells were then mixed with Cs–CA powder and molded at 60 °C. Microscopy showed intact rod-shaped cells distributed throughout the matrix. Viability assays confirmed that the bacteria survived compression.
These living plastics degraded phenol, a toxic pollutant. In minimal medium containing phenol at 500 mg L⁻¹, free bacteria began degradation after about 24 hours. Plastics containing bacteria required about 96 hours to adapt, then phenol levels declined while cell counts increased.
When bacterial loading was doubled, the lag shortened to about 24 hours and phenol was removed within about 196 h. At 100 mg L⁻¹, removal was faster. The microbes did not consume the plastic, indicating that phenol served as the carbon source. After rinsing, the same molded piece completed a second degradation cycle more quickly than the first.
This study demonstrates that chitosan can be reorganized into a thermoplastic that merges durability with biological function. Instead of dissolving natural polymers into thin films, this work treats them as candidates for molded parts that remain stable in water, break down under enzymatic pressure, and store active agents or living microbes. The resulting material blurs boundaries between plastics and biology, raising the possibility of engineered objects that do not merely resist their environment but interact with it in precise, useful ways.
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