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Given the importance of the extracellular medium during tissue formation, it was wise to develop an artificial structure that mimics the extracellular matrix while having improved physico-chemical properties. That is why the choice was focused on gelatin methacryloyl (GelMA), an inexpensive biocompatible hydrogel. Physicochemical and mechanical properties were improved by the incorporation of nanoparticles developed from two innovative fabrication processes: High shear fluid and low frequencies/high frequencies ultrasounds. Both rapeseed nanoliposomes and nanodroplets were successfully incorporated in the GelMA networks during the photo polymerization process. The impact on polymer microstructure was investigated by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and enzymatic degradation investigations. Mechanical stability and viscoelastic tests were conducted to demonstrate the beneficial effect of the functionalization on GelMA hydrogels. Adding nanoparticles to GelMA improved the surface properties (porosity), tuned swelling, and degradability properties. In addition, we observed that nanoemulsion didn’t change significantly the mechanical properties to shear and compression solicitations, whereas nanoliposome addition decreased Young’s modulus under compression solicitations. Thus, these ways of functionalization allow controlling the design of the material by choosing the type of nanoparticle (nanoliposome or nanoemulsion) in function of the application.

Many features of extracellular matrices, e.g., self-healing, adhesiveness, viscoelasticity, and conductivity, are associated with the intricate networks composed of many different covalent and non-covalent chemical bonds. Whereas a reductionism approach would have the limitation to fully recapitulate various biological properties with simple chemical structures, mimicking such sophisticated networks by incorporating many different functional groups in a macromolecular system is synthetically challenging. Herein, we propose a strategy of convergent synthesis of complex polymer networks to produce biomimetic electroconductive liquid metal hydrogels. Four precursors could be individually synthesized in one to two reaction steps and characterized, then assembled to form hydrogel adhesives. The convergent synthesis allows us to combine materials of different natures to generate matrices with high adhesive strength, enhanced electroconductivity, good cytocompatibility in vitro and high biocompatibility in vivo. The reversible networks exhibit self-healing and shear-thinning properties, thus allowing for 3D printing and minimally invasive injection for in vivo experiments. The need for multifunctional materials for tissue engineering applications requires the development of multicomponent systems. Here, the authors report on the creation of a liquid metal-containing hydrogel with multiple covalent and noncovalent bonds to produce a tailorable, biocompatible biomaterial.

Load-bearing tissues, such as muscle and cartilage, exhibit high elasticity, high toughness and fast recovery, but have different stiffness (with cartilage being significantly stiffer than muscle) 1 – 8 . Muscle achieves its toughness through finely controlled forced domain unfolding–refolding in the muscle protein titin, whereas articular cartilage achieves its high stiffness and toughness through an entangled network comprising collagen and proteoglycans. Advancements in protein mechanics and engineering have made it possible to engineer titin-mimetic elastomeric proteins and soft protein biomaterials thereof to mimic the passive elasticity of muscle 9 – 11 . However, it is more challenging to engineer highly stiff and tough protein biomaterials to mimic stiff tissues such as cartilage, or develop stiff synthetic matrices for cartilage stem and progenitor cell differentiation 12 . Here we report the use of chain entanglements to significantly stiffen protein-based hydrogels without compromising their toughness. By introducing chain entanglements 13 into the hydrogel network made of folded elastomeric proteins, we are able to engineer highly stiff and tough protein hydrogels, which seamlessly combine mutually incompatible mechanical properties, including high stiffness, high toughness, fast recovery and ultrahigh compressive strength, effectively converting soft protein biomaterials into stiff and tough materials exhibiting mechanical properties close to those of cartilage. Our study provides a general route towards engineering protein-based, stiff and tough biomaterials, which will find applications in biomedical engineering, such as osteochondral defect repair, and material sciences and engineering. The introduction of chain entanglements into protein-based hydrogels yields hydrogels with high stiffness, high toughness, fast recovery and ultrahigh compressive strength, with mechanical properties close to those of cartilage.

New trends in biomedical applications of the hybrid polymeric hydrogels, obtained by combining natural polymers with synthetic ones, have been reviewed. Homopolysaccharides, heteropolysaccharides, as well as polypeptides, proteins and nucleic acids, are presented from the point of view of their ability to form hydrogels with synthetic polymers, the preparation procedures for polymeric organic hybrid hydrogels, general physico-chemical properties and main biomedical applications (i.e., tissue engineering, wound dressing, drug delivery, etc.).

Renewable resources are used increasingly in the production of polymers. In particular, monomers such as carbon dioxide, terpenes, vegetable oils and carbohydrates can be used as feedstocks for the manufacture of a variety of sustainable materials and products, including elastomers, plastics, hydrogels, flexible electronics, resins, engineering polymers and composites. Efficient catalysis is required to produce monomers, to facilitate selective polymerizations and to enable recycling or upcycling of waste materials. There are opportunities to use such sustainable polymers in both high-value areas and in basic applications such as packaging. Life-cycle assessment can be used to quantify the environmental benefits of sustainable polymers.

Self-healing hydrogels may mimic the behavior of living tissues, which can autonomously repair minor damages, and therefore have a high potential for application in biomedicine. So far, such hydrogels have been processed only via extrusion-based additive manufacturing technology, limited in freedom of design and resolution. Herein, we present 3D-printed hydrogel with self-healing ability, fabricated using only commercially available materials and a commercial Digital Light Processing printer. These hydrogels are based on a semi-interpenetrated polymeric network, enabling self-repair of the printed objects. The autonomous restoration occurs rapidly, at room temperature, and without any external trigger. After rejoining, the samples can withstand deformation and recovered 72% of their initial strength after 12 hours. The proposed approach enables 3D printing of self-healing hydrogels objects with complex architecture, paving the way for future applications in diverse fields, ranging from soft robotics to energy storage. Self-healing hydrogels can mimic the damage repair behaviour of living tissues, but such hydrogels have only been processed via extrusion-based additive manufacturing technology. Here, the authors demonstrate a rapidly self-healing hydrogel which can be processed by DLP printing.

Conductive and stretchable materials that match the elastic moduli of biological tissue (0.5–500 kPa) are desired for enhanced interfacial and mechanical stability. Compared with inorganic and dry polymeric conductors, hydrogels made with conducting polymers are promising soft electrode materials due to their high water content. Nevertheless, most conducting polymer-based hydrogels sacrifice electronic performance to obtain useful mechanical properties. Here we report a method that overcomes this limitation using two interpenetrating hydrogel networks, one of which is formed by the gelation of the conducting polymer PEDOT:PSS. Due to the connectivity of the PEDOT:PSS network, conductivities up to 23 S m −1 are achieved, a record for stretchable PEDOT:PSS-based hydrogels. Meanwhile, the low concentration of PEDOT:PSS enables orthogonal control over the composite mechanical properties using a secondary polymer network. We demonstrate tunability of the elastic modulus over three biologically relevant orders of magnitude without compromising stretchability ( > 100%) or conductivity ( > 10 S m − 1 ). Conductive and stretchable materials that match the elastic moduli of biological tissue are desired for enhanced interfacial and mechanical stability. Here the authors show a method for fabricating highly conductive hydrogels comprising two interpenetrating networks.

Hydrogels with improved mechanical properties, made by combining polymer networks with ionic and covalent crosslinks, should expand the scope of applications, and may serve as model systems to explore mechanisms of deformation and energy dissipation. A long stretch for hydrogels Hydrogels are used in flexible contact lenses, as scaffolds for tissue engineering and in drug delivery. Their poor mechanical properties have so far limited the scope of their applications, but new strong and stretchy materials reported here could take hydrogels into uncharted territories. The new system involves a double-network gel, with one network forming ionic crosslinks and the other forming covalent crosslinks. The fracture energy of these materials is very high: they can stretch to beyond 17 times their own length even when containing defects that usually initiate crack formation in hydrogels. The materials' toughness is attributed to crack bridging by the covalent network accompanied by energy dissipation through unzipping of the ionic crosslinks in the second network. Hydrogels are used as scaffolds for tissue engineering 1 , vehicles for drug delivery 2 , actuators for optics and fluidics 3 , and model extracellular matrices for biological studies 4 . The scope of hydrogel applications, however, is often severely limited by their mechanical behaviour 5 . Most hydrogels do not exhibit high stretchability; for example, an alginate hydrogel ruptures when stretched to about 1.2 times its original length. Some synthetic elastic hydrogels 6 , 7 have achieved stretches in the range 10–20, but these values are markedly reduced in samples containing notches. Most hydrogels are brittle, with fracture energies of about 10 J m −2 (ref. 8 ), as compared with ∼1,000 J m −2 for cartilage 9 and ∼10,000 J m −2 for natural rubbers 10 . Intense efforts are devoted to synthesizing hydrogels with improved mechanical properties 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ; certain synthetic gels have reached fracture energies of 100–1,000 J m −2 (refs 11 , 14 , 17 ). Here we report the synthesis of hydrogels from polymers forming ionically and covalently crosslinked networks. Although such gels contain ∼90% water, they can be stretched beyond 20 times their initial length, and have fracture energies of ∼9,000 J m −2 . Even for samples containing notches, a stretch of 17 is demonstrated. We attribute the gels’ toughness to the synergy of two mechanisms: crack bridging by the network of covalent crosslinks, and hysteresis by unzipping the network of ionic crosslinks. Furthermore, the network of covalent crosslinks preserves the memory of the initial state, so that much of the large deformation is removed on unloading. The unzipped ionic crosslinks cause internal damage, which heals by re-zipping. These gels may serve as model systems to explore mechanisms of deformation and energy dissipation, and expand the scope of hydrogel applications.

Gelatin-methacryloyl (GelMA) is one of the most commonly used photopolymerizable biomaterials in bio-applications. However, GelMA synthesis remains suboptimal, as its reaction parameters have not been fully investigated. The goal of this study is to establish an optimal route for effective and controllable GelMA synthesis by systematically examining reaction parameters including carbonate-bicarbonate (CB) buffer molarity, initial pH adjustment, MAA concentration, gelatin concentration, reaction temperature, and reaction time. We employed several analytical techniques in order to determine the degree of substitution (DS) and conducted detailed structural analysis of the synthesized polymer. The results enabled us to optimize GelMA synthesis, showing the optimal conditions to balance the deprotonation of amino groups with minimizing MAA hydrolysis, which led to nearly complete substitution. The optimized conditions (low feed ratio of MAA to gelatin (0.1 mL/g), 0.25 M CB buffer at pH 9, and a gelatin concentration of 10–20%) enable a simplified reaction scheme that produces GelMA with high substitution with just one-step addition of MAA in one pot. Looking forward, these optimal conditions not only enable facile one-pot GelMA synthesis but can also guide researchers to explore the efficient, high methacrylation of other biomacromolecules.

Natural tissues are composed of diverse cells and extracellular materials whose arrangements across several length scales—from subcellular lengths 1 (micrometre) to the organ scale 2 (centimetre)—regulate biological functions. Tissue-fabrication methods have progressed to large constructs, for example, through stereolithography 3 and nozzle-based bioprinting 4 , 5 , and subcellular resolution through subtractive photoablation 6 , 7 – 8 . However, additive bioprinting struggles with sub-nozzle/voxel features 9 and photoablation is restricted to small volumes by prohibitive heat generation and time 10 . Building across several length scales with temperature-sensitive, water-based soft biological matter has emerged as a critical challenge, leaving large classes of biological motifs—such as multiscalar vascular trees with varying calibres—inaccessible with present technologies 11 , 12 . Here we use gallium-based engineered sacrificial capillary pumps for evacuation (ESCAPE) during moulding to generate multiscalar structures in soft natural hydrogels, achieving both cellular-scale (<10 µm) and millimetre-scale features. Decoupling the biomaterial of interest from the process of constructing the geometry allows non-biocompatible tools to create the initial geometry. As an exemplar, we fabricated branched, cell-laden vascular trees in collagen, spanning approximately 300-µm arterioles down to the microvasculature (roughly ten times smaller). The same approach can micropattern the inner surface of vascular walls with topographical cues to orient cells in 3D and engineer fine structures such as vascular malformations. ESCAPE moulding enables the fabrication of multiscalar forms in soft biomaterials, paving the way for a wide range of tissue architectures that were previously inaccessible in vitro. A patterning method using gallium-based engineered sacrificial capillary pumps for evacuation is described for generating structures in soft natural hydrogels across several length scales and enabling a wide range of tissue architectures to be fabricated.