BACKGROUND Hydrogels are formed through the cross-linking of hydrophilic polymer chains within an aqueous microenvironment. The gelation can be achieved through a variety of mechanisms, spanning physical entanglement of polymer chains, electrostatic interactions, and covalent chemical cross-linking. The water-rich nature of hydrogels makes them broadly applicable to many areas, including tissue engineering, drug delivery, soft electronics, and actuators. Conventional hydrogels usually possess limited mechanical strength and are prone to permanent breakage. The lack of desired dynamic cues and structural complexity within the hydrogels has further limited their functions. Broadened applications of hydrogels, however, require advanced engineering of parameters such as mechanics and spatiotemporal presentation of active or bioactive moieties, as well as manipulation of multiscale shape, structure, and architecture. ADVANCES Hydrogels with substantially improved physicochemical properties have been enabled by rational design at the molecular level and control over multiscale architecture. For example, formulations that combine permanent polymer networks with reversibly bonding chains for energy dissipation show strong toughness and stretchability. Similar strategies may also substantially enhance the bonding affinity of hydrogels at interfaces with solids by covalently anchoring the polymer networks of tough hydrogels onto solid surfaces. Shear-thinning hydrogels that feature reversible bonds impart a fluidic nature upon application of shear forces and return back to their gel states once the forces are released. Self-healing hydrogels based on nanomaterial hybridization, electrostatic interactions, and slide-ring configurations exhibit excellent abilities in spontaneously healing themselves after damages. Additionally, harnessing techniques that can dynamically and precisely configure hydrogels have resulted in flexibility to regulate their architecture, activity, and functionality. Dynamic modulations of polymer chain physics and chemistry can lead to temporal alteration of hydrogel structures in a programmed manner. Three-dimensional printing enables architectural control of hydrogels at high precision, with a potential to further integrate elements that enable change of hydrogel configurations along prescribed paths. OUTLOOK We envision the continuation of innovation in new bioorthogonal chemistries for making hydrogels, enabling their fabrication in the presence of biological species without impairing cellular or biomolecule functions. We also foresee opportunities in the further development of more sophisticated fabrication methods that allow better-controlled hydrogel architecture across multiple length scales. In addition, technologies that precisely regulate the physicochemical properties of hydrogels in spatiotemporally controlled manners are crucial in controlling their dynamics, such as degradation and dynamic presentation of biomolecules. We believe that the fabrication of hydrogels should be coupled with end applications in a feedback loop in order to achieve optimal designs through iterations. In the end, it is the combination of multiscale constituents and complementary strategies that will enable new applications of this important class of materials.

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Advances in engineering hydrogels

Over the past few years, there has been a great deal of interest in the development of hydrogel materials with tunable structural, mechanical, and rheological properties, which exhibit rapid and autonomous self-healing and self-recovery for utilization in a broad range of applications, from soft robotics to tissue engineering. However, self-healing hydrogels generally either possess mechanically robust or rapid self-healing properties but not both. Hence, the development of a mechanically robust hydrogel material with autonomous self-healing on the time scale of seconds is yet to be fully realized. Here, the current advances in the development of autonomous self-healing hydrogels are reviewed. Specifically, methods to test self-healing efficiencies and recoveries, mechanisms of autonomous self-healing, and mechanically robust hydrogels are presented. The trends indicate that hydrogels that self-heal better also achieve self-healing faster, as compared to gels that only partially self-heal. Recommendations to guide future development of self-healing hydrogels are offered and the potential relevance of self-healing hydrogels to the exciting research areas of 3D/4D printing, soft robotics, and assisted health technologies is highlighted.

Self-Healing Hydrogels

Alleviating the access abyss in palliative care and pain relief—an imperative of universal health coverage: the Lancet Commission report

Advanced Therapeutic Dressings for Effective Wound Healing—A Review

Recent advances on antimicrobial wound dressing: A review.

https://centaur.reading.ac.uk/38544/10/1-s2.0-S0014305714004091-main%20%281%29.pdf

Biomedical applications of hydrogels: A review of patents and commercial products

Novel antimicrobial hydrogels with good mechanical and physical properties were synthesized by autoclaving aqueous mixtures of poly(vinyl alcohol) and poly(methyl vinyl ether-alt-maleic anhydride). The structure of these materials was studied by infrared spectroscopy, scanning electron microscopy and solid state nuclear magnetic resonance. The swelling behavior, mechanical properties and adhesion of the hydrogels to porcine skin were evaluated. It was established that these hydrogels exhibited antimicrobial properties and inhibited bacteria growth against Staphylococcus aureus. The biocompatibility of the hydrogels was confirmed using an MTT assay (indirect cytotoxicity) and by monitoring cell proliferation in contact with the gels (direct cytotoxicity).

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Antimicrobial hydrogels based on autoclaved poly(vinyl alcohol) and poly(methyl vinyl ether-alt-maleic anhydride) mixtures for wound care applications

Cryotropic gelation is a low cost, well-known technique that has been used for decades for the preparation of cryogels based on poly(vinyl alcohol) This technique does not require addition of any cross-linkers or initiators because a physical cross-linking takes place during the cryotropic gelation Poly(vinyl alcohol)–Gantrez® AN cryogels with highly porous structure were successfully produced from the aqueous polymer mixtures with addition of 04, 06 and 08% w/v NaOH, which were frozen at −18 °C for approximately 9 hours and then thawed at room temperature (∼25 °C) These cryogels exhibited excellent mechanical properties (with 158–218% elongation to break) and high swelling ability in water (∼40 g of deionized water could be absorbed per 1 g of initial gel within 48 h) They exhibited intrinsic antimicrobial properties against Staphylococcus aureus without the addition of any antimicrobial agent due to the acidic nature of Gantrez® AN and related low pH of these cryogels The in vitro biocompatibility of these cryogels was assessed using human dermal fibroblasts with very encouraging results These cryogels are promising for applications in wound care

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Poly(vinyl alcohol)–Gantrez® AN cryogels for wound care applications

One of the most prevalent applications of hydrogels is wound management. Thanks to their high water content and unique physical properties, hydrogels could potentially resemble biological tissues including human skin. (Peppas et al. 2000, Gupta et al. 2010, Calo & Khutoryanskiy 2014, Jones et al. 2006) There is active interest in the development of new and advanced hydrogel-based products from both an academic and industrial perspective. In fact, hydrogels exhibit many characteristics of the ‘ideal’ wound dressing. These include: the capability of maintaining a moist environment at the wound site allowing gas exchange (moisture vapour transmission), biocompatibility, fast absorption of wound exudate, protection of newly formed or delicate skin and easy and relatively painless dressing removal. (Thomas 1990, Gupta et al. 2010, Vowden & Vowden 2014, Boateng & Catanzano 2015) In this Chapter we will provide the reader with an overview of the most recent hydrogel materials designed for wound management.

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Enrica Caló

Papers

Biomedical applications of hydrogels: A review of patents and commercial products

Antimicrobial hydrogels based on autoclaved poly(vinyl alcohol) and poly(methyl vinyl ether-alt-maleic anhydride) mixtures for wound care applications

Poly(vinyl alcohol)–Gantrez® AN cryogels for wound care applications

Hydrogels in wound management