Hydrogel-based scaffolds have been commonly used in regenerative engineering research to replace defective, degenerated or damaged tissues.1, 2
Hydrogels are crosslinked 3D networks that are composed of highly hydrophilic polymers. The ability to generate 3D flexible matrices allows for studying cell-cell and cell-biomaterial interactions in a controlled manner. For this reason, synthesizing hydrogels from materials that are derived from native extracellular matrix (ECM) molecules is a popular approach to synthesize biomimetic materials. Hydrogels can potentially mimic the native ECM environment by their soft and flexible structures and high water content. Therefore, they are widely used for both surface seeding and 3D cell encapsulation to form biomimetic constructs. Cell-laden hydrogel systems have been used to study a number of different biological outcomes, such as cellular differentiation, vascularization, or angiogenesis.3, 4
These hydrogels can be formed by ultraviolet (UV) photocrosslinking of prepolymer solutions that contains the cells.
Photocrosslinking is a simple approach to induce the formation of 3D hydrogel networks. Photocrosslinkable hydrogels demonstrate a number of advantages compared to other stimuli. For instance, photocrosslinking is a cost-effective, rapid and simple way of fabricating 3D hydrogels with controlled shape, size, and spatial resolution.2
Photocrosslinked cell-laden hydrogels have been successfully used for a number of applications, such as growth factor/drug delivery, regenerative medicine, and tissue engineering to study behavior of cells, for example proliferation, endothelialization, and stem cell differentiation.5–7
A variety of cell-laden gels have been created by methacrylate functionalization of different polymers such as gelatin and HA and subsequent UV crosslinking of resultant polymer precursors.
HA is a non-adhesive8–11
and non-immunogenic polymer. This anionic biopolymer consists of D-N-acetylglucosamine and D-glucuronic acid repeating units.15
HA is a viscoelastic biomaterial and can be degraded by hyaluronidase enzyme.1, 2, 16–19
HA is well-recognized as a major ECM component in a variety of tissues9
such as central nervous system, connective, epithelial, cardiovascular tissues, cartilage as well as synovial and vitreous fluids. In addition, HA is an essential component in the formation of cardiac jelly while heart morphogenesis take place.20
This polymer has been reported to play significant roles in wound healing, cellular proliferation, angiogenesis and cell-receptor interactions.1
For instance, adhesion receptors, such as receptor for HA mediated motility (RHAMM), cluster of differentiation marker 44 (CD44) and intracellular adhesion molecule-1 (ICAM-1) possess binding affinities against HA.21, 22
The carboxylate functional groups of HA can be chemically modified or methacrylated to facilitate crosslinking upon exposure to UV light.23
Following this strategy, HA methacrylate (HAMA) can be synthesized at different methacrylation degrees to fabricate hydrogels with tunable physical properties including degradation, stiffness, and pore architecture.20
Although HAMA is a promising hydrogel for biological applications, the nonadhesive nature prevents its use in applications where cell spreading is involved. The addition of gelatin with cell-interactive functional groups to the HA hydrogel matrix can improve cell adhesion properties of the resulting hybrid hydrogels.
Gelatin is traditionally obtained by partially hydrolyzing collagen and is composed of a heterogeneous mixture of proteins.23
Collagen is the most substantial protein constituent of the tissues throughout the human body.24, 25
For example, collagen is abundantly present in cartilage, bone, skin, ligament, tendon, heart, blood vessels, cornea, and epithelium.24
Gelatin is a biocompatible material and has been used for coating of standard tissue culture dishes to promote cell adhesion for different cell types.26
Furthermore, gelatin has been utilized for a number of small molecule delivery and tissue engineering applications.23, 27–35
Gelatin degrades due to its matrix metalloproteinase (MMP) sensitive protein sequences, which is usually a desirable biomaterial property for in vivo
implanted hydrogels. Degradation of tissue engineered constructs is essential for many applications in regenerative medicine to allow for the deposition of newly formed ECM by the cells.36
Cellular behavior (e.g. spreading, migration, differentiation) is strongly influenced by degradation properties of the scaffold, since scaffold degradation enables deposition and formation of new tissue. In some applications, scaffold degradation may also assist with controlled release of small molecules from the scaffold. The lysine functional groups on gelatin structure can be chemically modified or methacrylated to induce crosslinking upon exposure to UV light. Methacrylated gelatin (GelMA) is biaoactive and it interacts with various cell lines.37
Furthermore, GelMA allows the spreading of encapsulated cells due to its cell adhesive functional groups.37
However, similar to collagen gels, UV-crosslinked gelatin hydrogels are mechanically weak.
Fabrication of hybrid hydrogels has been a popular approach to improve material and/or biological properties of biomaterials.1
Although HA-gelatin hybrid hydrogels are promising biomimetic substrates38
, their material properties have not been thoroughly characterized. In this study, we have used different compositions of HAMA and GelMA to generate tunable hybrid hydrogels and characterized their biological and mechanical properties. The physical properties of the resulting hydrogels, such as swelling, degradation and compressive moduli were controlled by varying prepolymer compositions prior to UV crosslinking. In addition, biological responses of human umbilical cord vein endothelial cells (HUVECs) to HAMA-GelMA hybrids were characterized by seeding cells on the hydrogel surfaces or encapsulating them within 3D structures of hybrids formed by using different compositions of HAMA and GelMA. Due to their abundance in the native ECM, HA and collagen/gelatin hybrids have great potential to be used for different tissue engineering applications (e.g. neural, bone, vascular, cardiac, skin) and regenerative medicine research.