Facile preparation of photodegradable hydrogels by photopolymerization

Modular Multiwell Viscoelastic Hydrogel Platform for Two- and Three-Dimensional Cell Culture Applications

Hydrogels have gained significant popularity as model platforms to study reciprocal interactions between cells and their microenvironment. While hydrogel tools to probe many characteristics of the extracellular space have been developed, fabrication approaches remain challenging and time-consuming, limiting multiplexing or widespread adoption. Thus, we have developed a modular fabrication approach to generate distinct hydrogel microenvironments within the same 96-well plate for increased throughput of fabrication as well as integration with existing high-throughput assay technologies. This approach enables in situ hydrogel mechanical characterization and is used to generate both elastic and viscoelastic hydrogels across a range of stiffnesses. Additionally, this fabrication method enabled a 3-fold reduction in polymer and up to an 8-fold reduction in fabrication time required per hydrogel replicate. The feasibility of this platform for two-dimensional (2D) cell culture applications was demonstrated by measuring both population-level and single-cell-level metrics via microplate reader and high-content imaging. Finally, a 96-well hydrogel array was utilized for three-dimensional (3D) cell culture, demonstrating the ability to support high cell viability. Together, this work demonstrates a versatile and easily adaptable fabrication approach that can support the ever-expanding tool kit of hydrogel technologies for cell culture applications.

Orthogonally Crosslinked Gelatin-Norbornene Hydrogels for Biomedical Applications

The thiol-norbornene photo-click reaction has exceptionally fast crosslinking efficiency compared with chain-growth polymerization at equivalent macromer contents. The orthogonal reactivity between norbornene and thiol/tetrazine permits crosslinking of synthetic and naturally derived macromolecules with modularity, including poly(ethylene glycol) (PEG)-norbornene (PEGNB), gelatin-norbornene (GelNB), among others. For example, collagen-derived gelatin contains both cell adhesive motifs (e.g., Arg-Gly-Asp or RGD) and protease-labile sequences, making it an ideal macromer for forming cell-laden hydrogels. First reported in 2014, GelNB is increasingly used in orthogonal crosslinking of biomimetic matrices in various applications. GelNB can be crosslinked into hydrogels using multi-functional thiol linkers (e.g., dithiothreitol (DTT) or PEG-tetra-thiol (PEG4SH) via visible light or longwave ultraviolet (UV) light step-growth thiol-norbornene reaction or through an enzyme-mediated crosslinking (i.e., horseradish peroxidase, HRP). GelNB-based hydrogels can also be modularly crosslinked with tetrazine-bearing macromers via inverse electron-demand Diels-Alder (iEDDA) click reaction. This review surveys the various methods for preparing GelNB macromers, the crosslinking mechanisms of GelNB-based hydrogels, and their applications in cell and tissue engineering, including crosslinking of dynamic matrices, disease modeling, and tissue regeneration, delivery of therapeutics, as well as bioprinting and biofabrication.

Modular multiwell viscoelastic hydrogel platform for two- and three-dimensional cell culture applications

Hydrogels have gained significant popularity as model platforms to study the reciprocal interactions between cells and their microenvironment. While hydrogel tools to probe many characteristics of the extracellular space have been developed, fabrication approaches remain challenging and time-consuming, limiting multiplexing or widespread adoption. Thus, we have developed a modular fabrication approach to generate distinct hydrogel microenvironments within 96-well plates for increased throughput of fabrication as well as integration with existing high-throughput assay technologies. This approach enables in situ hydrogel mechanical characterization and was used to generate both elastic and viscoelastic hydrogels across a range of stiffnesses. Additionally, this fabrication method enabled a 3-fold reduction in polymer and up to an 8-fold reduction in fabrication time required per hydrogel replicate. The feasibility of this platform for cell culture applications was demonstrated by measuring both population-level and single cell-level metrics via microplate reader and high-content imaging. Finally, the 96-well hydrogel array was utilized for 3D cell culture, demonstrating the ability to support high cell viability. Together, this work demonstrates a versatile and easily adoptable fabrication approach that can support the ever-expanding tool kit of hydrogel technologies for cell culture applications.

Green Chemistry for Crosslinking Biopolymers: Recent Advances in Riboflavin-Mediated Photochemistry

Riboflavin (RF), which is also known as vitamin B2, is a water-soluble vitamin. RF is a nontoxic and biocompatible natural substance. It absorbs light (at wavelengths of 380 and 450 nm) in the presence of oxygen to form reactive singlet oxygen (¹O₂). The generated singlet oxygen acts as a photoinitiator to induce the oxidation of biomolecules, such as amino acids, proteins, and nucleotides, or to initiate chemical reactions, such as the thiol-ene reaction and crosslinking of tyramine and furfuryl groups. In this review, we focus on the chemical mechanism and utilization of the photochemistry of RF, such as protein crosslinking and hydrogel formation. Currently, the crosslinking method using RF as a photoinitiator is actively employed in ophthalmic clinics. However, a significant broadening is expected in its range of applications, such as in tissue engineering and drug delivery.

Photoinduced Dithiolane Crosslinking for Multiresponsive Dynamic Hydrogels

While many hydrogels are elastic networks crosslinked by covalent bonds, viscoelastic hydrogels with adaptable crosslinks are increasingly being developed to better recapitulate time and position-dependent processes found in many tissues. In this work, 1,2-dithiolanes are presented as dynamic covalent photocrosslinkers of hydrogels, resulting in disulfide bonds throughout the hydrogel that respond to multiple stimuli. Using lipoic acid as a model dithiolane, disulfide crosslinks are formed under physiological conditions, enabling cell encapsulation via an initiator-free light-induced dithiolane ring-opening photopolymerization. The resulting hydrogels allow for multiple photoinduced dynamic responses including stress relaxation, stiffening, softening, and network functionalization using a single chemistry, which can be supplemented by permanent reaction with alkenes to further control network properties and connectivity using irreversible thioether crosslinks. Moreover, complementary photochemical approaches are used to achieve rapid and complete sample degradation via radical scission and post-gelation network stiffening when irradiated in the presence of reactive gel precursor. The results herein demonstrate the versatility of this material chemistry to study and direct 2D and 3D cell-material interactions. This work highlights dithiolane-based hydrogel photocrosslinking as a robust method for generating adaptable hydrogels with a range of biologically relevant mechanical and chemical properties that are varied on demand.

On-demand removable hydrogels based on photolabile cross-linkings as wound dressing materials

A novel strategy based on photocleavable cross-linkings is proposed and demonstrated to develop hydrogels that can be removed in a noninvasive, on-demand, and controllable way.

Fundamentals of Laser-Based Hydrogel Degradation and Applications in Cell and Tissue Engineering

The cell and tissue engineering fields have profited immensely through the implementation of highly structured biomaterials. The development and implementation of advanced biofabrication techniques have established new avenues for generating biomimetic scaffolds for a multitude of cell and tissue engineering applications. Among these, laser-based degradation of biomaterials is implemented to achieve user-directed features and functionalities within biomimetic scaffolds. This review offers an overview of the physical mechanisms that govern laser-material interactions and specifically, laser-hydrogel interactions. The influences of both laser and material properties on efficient, high-resolution hydrogel degradation are discussed and the current application space in cell and tissue engineering is reviewed. This review aims to acquaint readers with the capability and uses of laser-based degradation of biomaterials, so that it may be easily and widely adopted.

Spatiotemporal hydrogel biomaterials for regenerative medicine

Hydrogels mimic many of the physical properties of soft tissue and are widely used biomaterials for tissue engineering and regenerative medicine. Synthetic hydrogels have been developed to recapitulate many of the healthy and diseased states of native tissues and can be used as a cell scaffold to study the effect of matricellular interactions in vitro. However, these matrices often fail to capture the dynamic and heterogenous nature of the in vivo environment, which varies spatially and during events such as development and disease. To address this deficiency, a variety of manufacturing and processing techniques are being adapted to the biomaterials setting. Among these, photochemistry is particularly well suited because these reactions can be performed in precise three-dimensional space and at specific moments in time. This spatiotemporal control over chemical reactions can also be performed over a range of cell- and tissue-relevant length scales with reactions that proceed efficiently and harmlessly at ambient conditions. This review will focus on the use of photochemical reactions to create dynamic hydrogel environments, and how these dynamic environments are being used to investigate and direct cell behavior.

Modular and Adaptable Tumor Niche Prepared from Visible Light Initiated Thiol-Norbornene Photopolymerization

Photopolymerized biomimetic hydrogels with adaptable properties have been widely used for cell and tissue engineering applications. As a widely adopted gel cross-linking method, photopolymerization provides experimenters on-demand and spatial-temporal controls in gelation kinetics. Long wavelength ultraviolet (UV) light initiated photopolymerization is among the most popular methods in the fabrication of cell-laden hydrogels owing to its rapid and relatively mild gelation conditions. The use of UV light, however, still causes concerns regarding its potential negative impacts on cells. Alternatively, visible light based photopolymerization can be used to cross-link cell-laden hydrogels. The majority of visible light based gelation schemes involve photoinitiator, co-initiator, and comonomer. This multicomponent initiation system creates added challenges for optimizing hydrogel formulations. Here, we report a co-initiator/comonomer-free visible light initiated thiol-norbornene photopolymerization scheme to prepare modular biomimetic hydrogels suitable for in situ cell encapsulation. Eosin-Y was used as the sole initiator to initiate modular gelation between synthetic macromers (e.g., thiolated poly(vinyl alcohol) or poly(ethylene glycol)) and functionalized extracellular matrices (ECMs) including norbornene-functionalized gelatin (GelNB) or thiolated hyaluronic acid (THA). These components are modularly cross-linked to afford bioinert (i.e., purely synthetic), bioactive (i.e., using gelatin), and biomimetic (i.e., using gelatin and hyaluronic acid) hydrogels. The stiffness of the hydrogels can be easily tuned without affecting the contents of the bioactive components. Furthermore, the use of naturally derived biomacromolecules (e.g., gelatin and HA) renders these hydrogels susceptible to enzyme-mediated degradation. In addition to demonstrating efficient and tunable visible light mediated gelation, we also utilized this biomimetic modular gelation system to formulate artificial tumor niche and to study the effects of cell density and gel modulus on the formation of pancreatic ductal adenocarcinoma (PDAC) spheroids.

Photopatterning of multifunctional hydrogels to direct adult neural precursor cells

Matrix-metalloproteinase and photosensitive peptide units are combined with heparin and poly(ethylene glycol) into a light-sensitive multicomponent hydrogel material. Localized degradation of the hydrogel matrix allows the creation of defined spatial constraints and adhesive patterning for cells grown in culture. Using this matrix system, it is demonstrated that the degree of confinement determines the fate of neural precursor cells in vitro.

Thiol-norbornene photo-click hydrogels for tissue engineering applications

Thiol-norbornene (thiol-ene) photo-click hydrogels have emerged as a diverse material system for tissue engineering applications. These hydrogels are cross-linked through light mediated orthogonal reactions between multi-functional norbornene-modified macromers (e.g., poly(ethylene glycol), hyaluronic acid, gelatin) and sulfhydryl-containing linkers (e.g., dithiothreitol, PEG-dithiol, bis-cysteine peptides) using low concentration of photoinitiator. The gelation of thiol-norbornene hydrogels can be initiated by long-wave UV light or visible light without additional co-initiator or co-monomer. The cross-linking and degradation behaviors of thiol-norbornene hydrogels are controlled through material selections, whereas the biophysical and biochemical properties of the gels are easily and independently tuned owing to the orthogonal reactivity between norbornene and thiol moieties. Uniquely, the cross-linking of step-growth thiol-norbornene hydrogels is not oxygen-inhibited, therefore the gelation is much faster and highly cytocompatible compared with chain-growth polymerized hydrogels using similar gelation conditions. These hydrogels have been prepared as tunable substrates for 2D cell culture, as microgels or bulk gels for affinity-based or protease-sensitive drug delivery, and as scaffolds for 3D cell culture. Reports from different laboratories have demonstrated the broad utility of thiol-norbornene hydrogels in tissue engineering and regenerative medicine applications, including valvular and vascular tissue engineering, liver and pancreas-related tissue engineering, neural regeneration, musculoskeletal (bone and cartilage) tissue regeneration, stem cell culture and differentiation, as well as cancer cell biology. This article provides an up-to-date overview on thiol-norbornene hydrogel cross-linking and degradation mechanisms, tunable material properties, as well as the use of thiol-norbornene hydrogels in drug delivery and tissue engineering applications.

Facile One-step Micropatterning Using Photodegradable Methacrylated Gelatin Hydrogels for Improved Cardiomyocyte Organization and Alignment

Hydrogels are often employed as temporary platforms for cell proliferation and tissue organization in vitro. Researchers have incorporated photodegradable moieties into synthetic polymeric hydrogels as a means of achieving spatiotemporal control over material properties. In this study protein-based photodegradable hydrogels composed of methacrylated gelatin (GelMA) and a crosslinker containing o-nitrobenzyl ester groups have been developed. The hydrogels are able to degrade rapidly and specifically in response to UV light and can be photopatterned to a variety of shapes and dimensions in a one-step process. Micropatterned photodegradable hydrogels are shown to improve cell distribution, alignment and beating regularity of cultured neonatal rat cardiomyocytes. Overall this work introduces a new class of photodegradable hydrogel based on natural and biofunctional polymers as cell culture substrates for improving cellular organization and function.

Novel photolabile crosslinkers based on O-acyloxime moiety

Novel photolabile crosslinkers bearing O-acyloxime moiety are proposed. The crosslinkers were polymerized with methyl acrylate in film state, and the photodegradation of resulting films are demonstrated.

A peptide functionalized poly(ethylene glycol) (PEG) hydrogel for investigating the influence of biochemical and biophysical matrix properties on tumor cell migration

To address the challenges associated with defined control over matrix properties in 3D cell culture systems, we employed a peptide functionalized poly(ethylene glycol) (PEG) hydrogel matrix in which mechanical modulus and adhesive properties were tuned. An HT-1080 human fibrosarcoma cell line was chosen as a model for probing matrix influences on tumor cell migration using the PEG hydrogel platform. HT-1080 speed varied with a complex dependence on both matrix modulus and Cys-Arg-Gly-Asp-Ser (CRGDS) adhesion ligand concentration, with regimes in which motility increased, decreased, or was minimally altered being observed. We further investigated cell motility by forming matrix interfaces that mimic aspects of tissue boundaries that might be encountered during invasion by taking advantage of the spatial control of the thiol-ene photochemistry to form patterned regions of low and high cross-linking densities. HT-1080s in 100 Pa regions of patterned PEG hydrogels tended to reverse direction or aggregate at the interface when they encountered a 360 Pa boundary. In contrast, HT-1080s were apparently unimpeded when migrating from the stiff to the soft regions of PEG peptide hydrogels, which may indicate that cells are capable of "reverse durotaxis" within at least some matrix regimes. Taken together, our results identified matrix regimes in which HT-1080 motility was both positively and negatively influenced by cell adhesion or matrix modulus.