Adjusting Degree of Modification and Composition of gelAGE-Based Hydrogels Improves Long-Term Survival and Function of Primary Human Fibroblasts and Endothelial Cells in 3D Cultures

Pristine Photopolymerizable Gelatin Hydrogels: A Low-Cost and Easily Modifiable Platform for Biomedical Applications

The study addresses the challenge of temperature sensitivity in pristine gelatin hydrogels, widely used in biomedical applications due to their biocompatibility, low cost, and cell adhesion properties. Traditional gelatin hydrogels dissolve at physiological temperatures, limiting their utility. Here, we introduce a novel method for creating stable hydrogels at 37 °C using pristine gelatin through photopolymerization without requiring chemical modifications. This approach enhances consistency and simplifies production and functionalization of the gelatin with bioactive molecules. The stabilization mechanism involves the partial retention of the triple-helix structure of gelatin below 25 °C, which provides specific crosslinking sites. Upon activation by visible light, ruthenium (Ru) acts as a photosensitizer that generates sulphate radicals from sodium persulphate (SPS), inducing covalent bonding between tyrosine residues and "locking" the triple-helix conformation. The primary focus of this work is the characterization of the mechanical properties, swelling ratio, and biocompatibility of the photopolymerized gelatin hydrogels. Notably, these hydrogels supported better cell viability and elongation in normal human dermal fibroblasts (NHDFs) compared to GelMA, and similar performance was observed for human pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). As a proof of concept for functionalization, gelatin was modified with selenous acid (GelSe), which demonstrated antioxidant and antimicrobial capacities, particularly against E. coli and S. aureus. These results suggest that pristine gelatin hydrogels, enhanced through this new photopolymerization method and functionalized with bioactive molecules, hold potential for advancing regenerative medicine and tissue engineering by providing robust, biocompatible scaffolds for cell culture and therapeutic applications.

Optical Fiber-Assisted Printing: A Platform Technology for Straightforward Photopolymer Resins Patterning and Freeform 3D Printing

Light-based 3D printing techniques represent powerful tools, enabling the precise fabrication of intricate objects with high resolution and control. An innovative addition to this set of printing techniques is Optical Fiber-Assisted Printing (OFAP) introduced in this article. OFAP is a platform utilizing an LED-coupled optical fiber (LOF) that selectively crosslinks photopolymer resins. It allows change of parameters like light intensity and LOF velocity during fabrication, facilitating the creation of structures with progressive features and multi-material constructs layer-by-layer. An optimized formulation based on allyl-modified gelatin (gelAGE) with food dyes as photoabsorbers is introduced. Additionally, a novel gelatin-based biomaterial, alkyne-modified gelatin (gelGPE), featuring alkyne moieties, demonstrates near-visible light absorption thus fitting OFAP needs, paving the way for multifunctional hydrogels through thiol-yne click chemistry. Besides 2D patterning, OFAP is transferred to embedded 3D printing within a resin bath demonstrating the proof-of-concept as a novel printing technology with potential applications in tissue engineering and biomimetic scaffold fabrication, offering rapid and precise freeform printing capabilities.

Evaluation of allylated gelatin as a bioink supporting spontaneous spheroid formation of HepG2 cells

The spheroid culture system has gained significant attention as an effective in vitro model to mimic the in vivo microenvironment. Even though numerous studies were focused on developing spheroids, the structural organization of encapsulated cells within hydrogels remains a challenge. Allylated gelatin or GelAGE is used as a bioink due to its excellent physicochemical properties. In this study, GelAGE was evaluated for its capacity to induce spontaneous spheroid formation in encapsulated HepG2 cells. GelAGE was synthesized and characterized using 1HNMR spectroscopy and ninhydrin assay. Then the physicochemical and biological attributes of GelAGE hydrogel was examined. The results demonstrate that GelAGE has remarkable ability to induce the encapsulated cells to self-organize into spheroids.

Metal-Phenolic Network Directed Coating of Single Probiotic Cell Followed by Photoinitiated Thiol-Ene Click Fortification to Enhance Oral Therapy

Probiotics-based oral therapy has become a promising way to prevent and treat various diseases, while the application of probiotics is primarily restricted by loss of viability due to adverse conditions in the gastrointestinal (GI) tract during oral delivery. Layer-by-layer (LbL) single-cell encapsulation approaches are widely employed to improve the bioavailability of probiotics. However, they are generally time- and labor-intensive owing to multistep operation. Herein, a simple yet efficient LbL technique is developed to coat a model probiotic named Escherichia coli Nissle 1917 (EcN) through polyphenol-Ca2+ network directed allyl-modified gelatin (GelAGE) adsorption followed by cross-linking of GelAGE via photoinitiated thiol-ene click reaction to protect EcN from harsh microenvironments of GI tract. LbL single-cell encapsulation can be performed within 1 h through simple operation. It is revealed that coated EcN exhibits significantly improved viability against acidic gastric fluid and bile salts, and enhanced colonization in the intestinal tract without loss of proliferation capabilities. Furthermore, oral therapy of coated EcN remarkably relieves the pathological symptoms associated with colitis in mice including down-regulating inflammation, repairing epithelial barriers, scavenging reactive oxygen species (ROS), and restoring the homeostasis of gut microbiota. This simplified LbL coating strategy has great potential for various probiotics-mediated biomedical and nutraceutical applications.

Flexible Allyl-Modified Gelatin Photoclick Resin Tailored for Volumetric Bioprinting of Matrices for Soft Tissue Engineering

Volumetric bioprinting (VBP) is a light-based 3D printing platform, which recently prompted a paradigm shift for additive manufacturing (AM) techniques considering its capability to enable the fabrication of complex cell-laden geometries in tens of seconds with high spatiotemporal control and pattern accuracy. A flexible allyl-modified gelatin (gelAGE)-based photoclick resin is developed in this study to fabricate matrices with exceptionally soft polymer networks (0.2-1.0 kPa). The gelAGE-based resin formulations are designed to exploit the fast thiol-ene crosslinking in combination with a four-arm thiolated polyethylene glycol (PEG4SH) in the presence of a photoinitiator. The flexibility of the gelAGE biomaterial platform allows one to tailor its concentration spanning from 2.75% to 6% and to vary the allyl to thiol ratio without hampering the photocrosslinking efficiency. The thiol-ene crosslinking enables the production of viable cell-material constructs with a high throughput in tens of seconds. The suitability of the gelAGE-based resins is demonstrated by adipogenic differentiation of adipose-derived stromal cells (ASC) after VBP and by the printing of more fragile adipocytes as a proof-of-concept. Taken together, this study introduces a soft photoclick resin which paves the way for volumetric printing applications toward soft tissue engineering.

Direct ink writing of multifunctional nanocellulose and allyl-modified gelatin biomaterial inks for the fabrication of mechanically and functionally graded constructs

Recreating the intricate mechanical and functional gradients found in natural tissues through additive manufacturing poses significant challenges, including the need for precise control over time and space and the availability of versatile biomaterial inks. In this proof-of-concept study, we developed a new biomaterial ink for direct ink writing, allowing the creation of 3D structures with tailorable functional and mechanical gradients. Our ink formulation combined multifunctional cellulose nanofibrils (CNFs), allyl-functionalized gelatin (0.8-2.0 wt%), and polyethylene glycol dithiol (3.0-7.5 wt%). The CNF served as a rheology modifier, whereas a concentration of 1.8 w/v % in the inks was chosen for optimal printability and shape fidelity. In addition, CNFs were functionalized with azido groups, enabling the spatial distribution of functional moieties within a 3D structure. These functional groups were further modified using a spontaneous click chemistry reaction. Through additive manufacturing and a readily available static mixer, we successfully demonstrated the fabrication of mechanical gradients - ranging from 3 to 6 kPa in indentation strength - and functional gradients. Additionally, we introduced dual gradients by combining gradient printing with an anisotropic photocrosslinking step. The developed biomaterial ink opens up possibilities for printing intricate multigradient structures, resembling the complex hierarchical organization seen in living tissues.

Tannic acid-based dual-network homogeneous hydrogel with antimicrobial and pro-healing properties for infected wound healing

The clinical treatment of infected skin injuries caused by exogenous bacteria faces great challenges. Conventional therapeutic approaches are difficult to achieve synergistic effects of infection control and induction of skin regeneration. In this study, a novel tannic acid-based physically cross-linked double network hydrogel (PDH gel) was prepared on demand by covalent cross-linking of tannic acid (TA) with polyvinyl alcohol (PVA) and chelating ligand of TA with Fe³⁺. The homogeneity of the hydrogel was achieved by the action of glycol dispersant. With the anti-inflammatory and antioxidant properties of Fe³⁺ and TA, this hydrogel exhibited excellent antibacterial properties by achieving 99.69% and 99.36% bacterial inhibition against E.coli and S. aureus, respectively. Moreover, the PDH gel exhibits good biocompatibility, stretchability (up to 200%) and skin-friendliness. After 14 days of PDH-1 gel implantation in a rat model infected by S. aureus, the wound healing rate was as high as 95.21%. PDH gel-1 showed more granulation tissue, more pronounced blood vessels, higher collagen fiber density and good collagen deposition, and its recovery effect was better than that of PSH gel and PDH gel-2 in vivo. Hence, this study provides a novel avenue for the design of future clinical infected wound healing dressings.