Advancements in gelatin-based hydrogel systems for biomedical applications: A state-of-the-art review

A gelatin-based hydrogel system is a stimulus-responsive, biocompatible, and biodegradable polymeric system with solid-like rheology that entangles moisture in its porous network that gradually protrudes to assemble a hierarchical crosslinked arrangement. The hydrolysis of collagen directs gelatin construction, which retains arginyl glycyl aspartic acid and matrix metalloproteinase-sensitive degeneration sites, further confining access to chemicals entangled within the gel (e.g., cell encapsulation), modulating the release of encapsulated payloads and providing mechanical signals to the adjoining cells. The utilization of various types of functional tunable biopolymers as scaffold materials in hydrogels has become highly attractive due to their higher porosity and mechanical ability; thus, higher loading of proteins, peptides, therapeutic molecules, etc., can be further modulated. Furthermore, a stimulus-mediated gelatin-based hydrogel with an impaired concentration of gellan demonstrated great shear thinning and self-recovering characteristics in biomedical and tissue engineering applications. Therefore, this contemporary review presents a concise version of the gelatin-based hydrogel as a conceivable biomaterial for various biomedical applications. In addition, the article has recapped the multiple sources of gelatin and their structural characteristics concerning stimulating hydrogel development and delivery approaches of therapeutic molecules (e.g., proteins, peptides, genes, drugs, etc.), existing challenges, and overcoming designs, particularly from drug delivery perspectives.

A self-gelling hydrogel based on thiolated hyaluronic acid for three-dimensional culture of ovine preantral follicles

Three-dimensional (3D) ovarian follicle culture offers a promising option for fertility preservation in patients who cannot receive ovarian tissue transplantation. Our research evaluated the potential of a hydrogel composed of thiolated hyaluronic acid (HA-SH) for ovine preantral follicle development compared to routinely used alginate hydrogel (ALG). Synthesized via a carbodiimide reaction, HA-SH facilitated a self-crosslinking hydrogel through disulfide bond formation. Ovine preantral follicles (200-300 μm) retrieved through mechanical and enzymatic methods were encapsulated individually in either ALG or HA-SH hydrogels. Although both hydrogels adequately supported follicle survival, 3D integrity, and antrum formation over a 17-day in vitro culture, follicle growth was significantly higher within the HA-SH hydrogel. Gene expression analysis underscored that some folliculogenesis-related genes (ZP3, BMP7, and GJA1) and a steroidogenic gene (CYP19A1) demonstrated higher expression levels in HA-SH encapsulated follicles versus ALG. Collectively, our findings advocate for HA-SH hydrogel as a potent biomaterial for in vitro follicle cultures, attributing its efficacy to facile gelation, bio-responsiveness, and superior support for follicle growth.

Mechanobiology of cancer cell responsiveness to chemotherapy and immunotherapy: Mechanistic insights and biomaterial platforms

Mechanical forces are central to how cancer treatments such as chemotherapeutics and immunotherapies interact with cells and tissues. At the simplest level, electrostatic forces underlie the binding events that are critical to therapeutic function. However, a growing body of literature points to mechanical factors that also affect whether a drug or an immune cell can reach a target, and to interactions between a cell and its environment affecting therapeutic efficacy. These factors affect cell processes ranging from cytoskeletal and extracellular matrix remodeling to transduction of signals by the nucleus to metastasis of cells. This review presents and critiques the state of the art of our understanding of how mechanobiology impacts drug and immunotherapy resistance and responsiveness, and of the in vitro systems that have been of value in the discovery of these effects.

Strain-Stiffening Hydrogels with Dynamic, Secondary Cross-Linking

Hydrogels are water-swollen, typically soft networks useful as biomaterials and in other fields of biotechnology. Hydrogel networks capable of sensing and responding to external perturbations, such as light, temperature, pH, or force, are useful across a wide range of applications requiring on-demand cross-linking or dynamic changes. Thus far, although mechanophores have been described as strain-sensitive reactive groups, embedding this type of force-responsiveness into hydrogels is unproven. Here, we synthesized multifunctional polymers that combine a hydrophilic zwitterion with permanently cross-linking alkenes, and dynamically cross-linking disulfides. From these polymers, we created hydrogels that contain irreversible and strong thiol-ene cross-links and reversible disulfide cross-links, and they stiffened in response to strain, increasing hundreds of kPa in modulus under compression. We examined variations in polymer composition and used a constitutive model to determine how to balance the number of thiol-ene vs disulfide cross-links to create maximally force-responsive networks. These strain-stiffening hydrogels represent potential biomaterials that benefit from the mechanoresponsive behavior needed for emerging applications in areas such as tissue engineering.

Substrate stiffness controls proinflammatory responses in human gingival fibroblasts

Soft gingiva is often compromised in gingival health; however, the underlying biological mechanisms remain unknown. Extracellular matrix (ECM) stiffness is involved in the progression of various fibroblast-related inflammatory disorders via cellular mechanotransduction. Gingival stiffness might regulate cellular mechanotransduction-mediated proinflammatory responses in gingival fibroblasts. This in vitro study aims to investigate the effects of substrate stiffness on proinflammatory responses in human gingival fibroblasts (hGFs). The hGFs isolated from two healthy donors cultured on type I collagen-coated polydimethylsiloxane substrates with different stiffnesses, representing soft (5 kPa) or hard (25 kPa) gingiva. Expression levels of proinflammatory mediators, prostaglandin E2 or interleukin-1β, in hGFs were significantly higher with the soft substrate than with the hard substrate, even without and with lipopolysaccharide (LPS) to induce inflammation. Expression levels of gingival ECM and collagen cross-linking agents in hGFs were downregulated more with the soft substrate than with the hard substrate through 14 days of culture. The soft substrate suppressed the expression of mechanotransduction-related transcriptional factors and activated the expression of inflammation-related factors, whereas the hard substrate demonstrated the opposite effects. Soft substrate induced proinflammatory responses and inhibition of ECM synthesis in hGFs by inactivating cellular mechanotransduction. This supports the importance of ECM stiffness in gingival health.

Tuning Hydrogels by Mixing Dynamic Cross-Linkers: Enabling Cell-Instructive Hydrogels and Advanced Bioinks

Rational design of hydrogels that balance processability and extracellular matrix (ECM) biomimicry remains a challenge for tissue engineering and biofabrication. Hydrogels suitable for biofabrication techniques, yet tuneable to match the mechanical (static and dynamic) properties of native tissues remain elusive. Dynamic covalent hydrogels possessing shear-thinning/self-healing (processability) and time-dependent cross-links (mechanical properties) provide a potential solution, yet can be difficult to rationally control. Here, the straightforward modular mixing of dynamic cross-links with different timescales (hydrazone and oxime) is explored using rheology, self-healing tests, extrusion printing, and culture of primary human dermal fibroblasts. Maintaining a constant polymer content and cross-linker concentration, the stiffness and stress relaxation can be tuned across two orders of magnitude. All formulations demonstrate a similar flow profile after network rupture, allowing the separation of initial mechanical properties from flow behavior during printing. Furthermore, the self-healing nature of hydrogels with high hydrazone content enables recyclability of printed structures. Last, a distinct threshold for cell spreading and morphology is observed within this hydrogel series, even in multi-material constructs. Simple cross-linker mixing enables fine control and is of general interest for bioink development, targeting viscoelastic properties of specific cellular niches, and as an accessible and flexible platform for designing dynamic networks.

Designing Hydrogels for 3D Cell Culture Using Dynamic Covalent Crosslinking

Designing simple biomaterials to replicate the biochemical and mechanical properties of tissues is an ongoing challenge in tissue engineering. For several decades, new biomaterials have been engineered using cytocompatible chemical reactions and spontaneous ligations via click chemistries to generate scaffolds and water swollen polymer networks, known as hydrogels, with tunable properties. However, most of these materials are static in nature, providing only macroscopic tunability of the scaffold mechanics, and do not reflect the dynamic environment of natural extracellular microenvironment. For more complex applications such as organoids or co-culture systems, there remain opportunities to investigate cells that locally remodel and change the physicochemical properties within the matrices. In this review, advanced biomaterials where dynamic covalent chemistry is used to produce stable 3D cell culture models and high-resolution constructs for both in vitro and in vivo applications, are discussed. The implications of dynamic covalent chemistry on viscoelastic properties of in vitro models are summarized, case studies in 3D cell culture are critically analyzed, and opportunities to further improve the performance of biomaterials for 3D tissue engineering are discussed.

Hyperbranched polymer hydrogels with large stimuli-responsive changes in storage moduli and peroxide-induced healing

Hydrogels prepared using hyperbranched polymers with dynamic disulfide bonds show larger changes in moduli upon exposure to chemical stimuli for both softening and stiffening responses compared to linear polymers.

Reversibly Softening and Stiffening Organogels Using a Wavelength-Controlled Disulfide-Diselenide Exchange

Wavelength-dependent light-responsive seleno-sulfide dynamic covalent bonds were used to prepare organogels with reversible changes in stiffness. The disulfide cross-link organogels prepared from norbornene-terminated poly(ethylene glycol) (PEG-diNB) and poly(2-hydroxypropyl methacrylate-stat-mercaptoethyl acrylate) (PEG-diNB-poly(HPMA-stat-MEMA)) polymers underwent exchange reactions with 5,5'-diselenide-bis(2-aminobenzoic acid) upon irradiation with UV light. Following irradiation with visible light, the seleno-sulfide bonds were cleaved, reforming disulfide cross-links and the 5,5'-diselenide-bis(2-aminobenzoic acid). Reduction in G' with disulfide-diselenide exchange was consistent with that observed following a thiol-disulfide exchange reaction. Recovery of G' upon disulfide bond formation was 85-95% of the initial value in the as-prepared gel over five cycles of bond cleaving and reformation. This initial study shows the potential of the wavelength-controlled disulfide-diselenide chemistry to develop light-responsive reversible organogels. These organogels have the potential to be used in functional materials such as polymeric actuators or biomimetic soft robotics.

100th Anniversary of Macromolecular Science Viewpoint: Poly(N-isopropylacrylamide)-Based Thermally Responsive Micelles

Poly(N-isopropylacrylamide) (PNIPAAm)-based thermally responsive micelles are of great importance as smart materials for a number of applications such as drug delivery and biosensing, owing to their tunable lower critical solution temperature (LCST). Their design and synthesis in the nanoscale size range have been widely studied, and research interest in their structural and physic-chemical properties is continually growing. In this Viewpoint, representative research on the construction of PNIPAAm-based thermally responsive micelles as well as their applications are highlighted and discussed, which would serve as a good start for newcomers in this field and a positive guide for future research.

Dynamic covalent bonds in self-healing, shape memory, and controllable stiffness hydrogels

A review of hydrogels containing dynamic bonds that are shown to provide benefits for applications including self-healing and stimuli-induced stiffness changes.