Hydrogels get Magical

Dr Peter Harrop
Hydrogels get Magical
Just jelly-like materials? Well, there is a new one that supports an elephant but goes squishy when it jumps off. The new IDTechEx reports, "Self-Healing Materials Markets 2022-2042" and "Hydrogel Markets in Agriculture, Industry, Energy, Electronics, Electrical, Consumer 2022-2042" have the big picture.
 
In December 2021, Cambridge University UK researchers announced this a jelly-like material that can withstand the equivalent of an elephant standing on it, without breaking and then it recovers its normal shape. The soft-yet-strong self-healing material acts like an ultra-hard, shatterproof glass when compressed, despite its 80% water content. They believe it can be optimized for soft robotics, bioelectronics, and cartilage replacement for biomedical use. They also envisage a pressure sensor for real-time monitoring of human motion, including standing, walking, and jumping. Barrel-shaped molecules called cucurbiturils led to the breakthrough.
 
The IDTechEx report points out that anti-fouling coatings are sought for everything from ships to food aquaculture and human implants. In 2021 work was reported on the fabrication of bio-based amphiphilic hydrogel coating with excellent antifouling and mechanical properties using a biocidal hydrogel-based approach.
 
All gels will creep back after damage but special hydrogels get optimized for human wound healing, tissue engineering, and drug delivery. Ultrasound-triggered drug delivery has shown to improve chemotherapy when ionically cross-linked self-healing hydrogel is employed. In wound healing and tissue engineering, research eliminates problem aspects such as adhesion and it adds useful features such as being antibacterial. Some hydrogels are injectable for controlled release of encapsulated therapeutics (cell therapy and drug delivery) - delivered as a liquid and gelated in situ to accommodate irregular defects of the desired position.
 
Certain hydrogels heal chemical as well as mechanical damage incurred. For example, polyvinyl alcohol is a biocompatible and non-toxic synthetic polymer having a crystalline nature that has been used for the synthesis of hydrogel via a freezing/thawing method. This can exhibit a highly self-healing ability without any peripheral stimulus at room temperature.
 
Metallo-polymeric hydrogels are proving useful for recoverable properties, self-healing, and other functions. The subject of self-healing polymeric hydrogels is progressing rapidly. It is primarily driven by needs in healthcare because they are exceptionally biocompatible: no surprise because they greatly resemble biological systems. Both physical diffusion of molecules and chemical recombination of the cleaved bonds assist the process of self-healing in hydrogels.
 
Extrinsic self-healing employs artifacts such as embedded microcapsules that release reagents when the material is cracked. For extrinsic self-healing capability in polymers, one option is to copy the surface structure of animals. A vascular (piped liquid) network is present throughout the part so, when a crack occurs, fast-acting polymerization agents can be pushed through this system. Advantages are that it is easy to replenish, can be linked with self-healing hydrogel, and can carry out repeated repairs. Not quite a sea slug repeatedly regrowing its head or even a lizard growing a new tail but useful nonetheless. Disadvantages include being slower to do the healing and it requires an external pumping network.
 
As we start to learn from nature, we make stretchable and creeping polymers such as hydrogels that inherently "intrinsically" creep into damage but often artifacts like microcapsules are needed.
 
Popular materials used for self-healing. Source IDTechEx - "Self-Healing Materials Markets 2022-2042"
 
As the IDTechEx report, "Advanced Wound Care Technologies 2020-2030" explains, tissue engineering seeks to develop tissue and organ substitutes for maintaining, restoring, or augmenting functions of their injured or diseased counterparts in vivo. Increasing demand for biomaterials for regeneration or replacement of damaged tissue drives the development of new tissue engineering structures. Demand for engineered tissues is rapidly growing due to the limited availability of donor tissues and organs for transplantation. Tailored multifunctional hydrogels can be excellent cell delivery vehicles for therapeutic healing and tissue regeneration because of their high water content, as well as their responsiveness to various environmental stimuli such as temperature, pH, and enzymes.
 
The properties of these hydrogels derive from their molecular structure: namely their highly swollen, hydrophilic 3D cross-linked polymer network that may be either chemically or physically cross-linked to form a material that mimics advantageous properties of the highly hydrated extracellular matrix (ECM) and facilitates nutrient and oxygen transport due to its porous structure. Hydrogels have a long history as tools for tissue regeneration and 3D cell culture as they may be engineered to mimic the desired aspects of the native local ECM depending on their intended usage.
 
Supramolecular hydrogels are now emerging as a promising tool for tissue regeneration. They can be made biocompatible and recapitulate the viscoelastic nature of the ECM better than their elastic, covalently cross-linked counterparts due to the presence of dynamic linkages. The resulting viscoelastic and dynamic behavior of these linkages are responsible for other advantages such as self-healing and injectability. Supramolecular hydrogels can self-heal after damage either spontaneously or in the presence of a physiological stimulus. This characteristic extends the lifetime of materials and makes them ideal candidates for applications involving repeated mechanical stress or injection.
 
Ideally, hydrogels for tissue engineering should enable cell infiltration as well as encapsulate and deliver cells and biologics and be able to autonomously, rapidly, and repeatedly heal in situ at physiological conditions. Adhesives that are naturally self-healing are an important part of most tissue engineering procedures. Commonly used tissue adhesives include hydrogels because of exceptional adhesion and non-toxicity.
 
Routes to self-healing hydrogels are vast in number. Some based on natural polymers include supramolecular interactions or reversible covalent bonds. That includes chitosans, alginates, and celluloses. Alternatives are synthesized inorganic, organic, or composite materials. It can be complicated but worthwhile. For instance, amphiphilic diblock copolypeptide hydrogels DCHs are synthetic materials exhibiting tunable composition, structure, and properties. The addition of polyelectrolyte provides self-healing, shear-thinning, and biocompatibility that have been widely applied as biomaterials. Self-healing hydrogel based on metal-ligand assembly has been successfully used in bone regeneration. We could talk about the new self-healing hydrogel electrolyte in supercapacitors but let us settle by saying that hydrogels are truly becoming the gymnasts of chemistry.
 
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