Synthesis, properties, and biomedical applications of gelatin methacryloyl (GelMA) hydrogels

Abstract

Gelatin methacryloyl (GelMA) hydrogels have been widely used for various biomedical applications due to their suitable biological properties and tunable physical characteristics. GelMA hydrogels closely resemble some essential properties of native extracellular matrix (ECM) due to the presence of cell-attaching and matrix metalloproteinase responsive peptide motifs, which allow cells to proliferate and spread in GelMA-based scaffolds. GelMA is also versatile from a processing perspective. It crosslinks when exposed to light irradiation to form hydrogels with tunable mechanical properties. It can also be microfabricated using different methodologies including micromolding, photomasking, bioprinting, self-assembly, and microfluidic techniques to generate constructs with controlled architectures. Hybrid hydrogel systems can also be formed by mixing GelMA with nanoparticles such as carbon nanotubes and graphene oxide, and other polymers to form networks with desired combined properties and characteristics for specific biological applications. Recent research has demonstrated the proficiency of GelMA-based hydrogels in a wide range of tissue engineering applications including engineering of bone, cartilage, cardiac, and vascular tissues, among others. Other applications of GelMA hydrogels, besides tissue engineering, include fundamental cell research, cell signaling, drug and gene delivery, and bio-sensing.

Keywords: Biomedical; GelMA; Gelatin; Hydrogel; Methacryloyl; Tissue engineering.

Figure 1

Synthesis and characterization of GelMA hydrogels. (A) Scheme for preparation of photocrosslinked GelMA hydrogel. (i) Reaction of gelatin and methacrylic anhydride for grafting of methacryloyl substitution groups. The modification occurs at primary amine and hydroxyl groups. The RGD domains are illustrated as red segments along the GelMA chains, and their chemical structure is depicted within the inset. (ii) Representative reactions during the photocrosslinking of GelMA to form hydrogel networks. Free radicals are generated from photoinitiators, which initiate the chain polymerization of the methacryloyl substitutions. Propagation occurs between methacryloyl groups located on the same chain and on different chains. Termination occurs between two propagating chains or between one propagating chain and a second radical. Chain transfers and many other minor reactions are not shown, for clarity. (B) The compressive modulus reported by several studies on GelMA hydrogels [6, 70, 71, 74, 85, 90]. (C) SEM images of GelMA hydrogels, showing the effect of the degree of methacryloyl substitution on the pore sizes of GelMA hydrogels. Adapted from Chen et al. [9], with permission from Wiley, copyright 2012.

Figure 2

Microfabrication techniques used to produce GelMA hydrogels constructs. (A) Schematic representation of photopatterning of GelMA using a pre-patterned photomask (B) Stacked layers of patterned GelMA hydrogels fabricated using a micro-mirror projection stereolithography system. Adapted from Gauvin et al. [41], with permission from Elsevier, copyright 2012. (C) Schematic representation of a fiber-assisted micromolding technique for production of parallel microgrooved surfaces that serve as a template for micropatterning GelMA. Adapted from Hosseini et al. [1], with permission from Wiley, copyright 2014. (D) Schematic representation of the self-assembly of microgels fabricated by photopatterning. Adapted from Zamanian et al. [53], with permission from Wiley, copyright 2010. (E) Examples of the microfluidics of GelMA hydrogels microfabrication. (i) Schematic representation of the method for coating microchannels with GelMA hydrogel. Adapted from Annabi et al. [60], with permission from The Royal Society of Chemistry, copyright 2013. (ii) Fabrication of spherical GelMA microhydrogels using a microfluidic flow-focusing device. Adapted from Cha et al. [80], with permission from the American Chemical Society, copyright 2014. (F) Schematic representation of the bioprinting method for fabricating microchannels inside a GelMA hydrogel using an agarose template. Adapted from Bertassoni et al. [63], with permission from The Royal Society of Chemistry, copyright 2014. (G) Biotextile techniques as applied to the microfabrication of hybrid alginate and GelMA fibers and their assembly; (i) a representative SEM image of fabricated fiber; (ii) picture and micrograph of a typical woven fabric; (iii) a braided construct from NIH 3T3 fibroblasts, HUVECs, and hepatocytes (HepG2) as a liver model, with permission from the Wiley, copyright 2015.

Figure 3

Examples GelMA-based hybrid hydrogels. (A) Schematic illustration of the preparation of GelMA–carbon nanotube (CNT) hybrids. GelMA coated carbon nanotubes are mixed with GelMA prepolymer and then cross-linked using UV to develop porous structures with improved mechanical and conductive productivities. Adapted from Shin et al. [87] with permission from The American Chemical Society, copyright 2013. (B) Scheme of the preparation of GelMA-methacrylated graphene oxide (MeGO) hybrid. Red points indicate the photocrosslinkable methacryloyl substitution groups. Adapted from Cha et al. [74] with permission from Wiley, copyright 2014. (C) Cartoon illustration of the GelMA-gold nanoparticles (GNP) hybrid and representation of its application as scaffolds for bone regeneration. The addition of gold nanoparticles enhances bone recovery in an animal model.After 8 weeks, bone defects induced in rabbits recovered sooner when treated with GelMA hydrogels with 0.071 and 0.20 μg/mL of gold nanoparticles (Gel-GNP2 and GelGNP3 respectively) than with pristine GelMA hydrogels (Gel). Adapted from Heo et al. [78] with permission from The Royal Society of Chemistry, copyright 2014. (D) Cytoskeleton and nuclei staining (F-actin/DAPI) for HUVEC-laden HAMA-GelMA hybrid hydrogels in 3D.By changing the concentration of HAMA and GelMA, it is possible to fabricate hybrid hydrogels with different stiffness that allows for tunable cellular response. Cell spreading images are taken at day 7. Scale bars represent 100 μm. Adapted from Camci-Unal et al. [88] with permission from ACS publications, copyright 2013. (E) Hybrid construct made with GelMA added to a framework made with methacrylated poly(hydroxymethylglycolide-co-ε-caprolactone) and poly-ε-caprolactone. Adapted from Boere et al. [99] with permission from Elsevier, copyright 2014. (F) Embryoid bodies encapsulated in GelMA/PEG hybrid microgels. Adapted from Qi et al. [47] with permission from Wiley, copyright 2010.

Figure 4

The specific characteristics and functions of each tissue impose particular tissue engineering requirements. In recent literature reports, GelMA hydrogels have proven to be a flexible and highly tunable platform for diverse tissue engineering applications in the areas of neural, cardiovascular, cartilage, bone, muscle, liver, and kidney engineering.

Figure 5

Representative examples of applications of GelMA hydrogels in tissue engineering. (A) Confocal images of 3D cord formation of HUVEC cells on GelMA micropatterned constructs (scale bar: 100 μm). Adapted from Nikkhah et al. [8] with permission from Elsevier, copyright 2012. (B) Transdermal polymerization of GelMA in vivo, and the subcutaneous scaffold after 7 days showing vascularization (scale bar: 1 cm). Adapted from Lin et al. [13] with permission from Elsevier, copyright 2013. (C) Engineered triangular swimmers made of GelMA-CNT showing spontaneous linear traveling (Each ruler marker is 1 mm). Adapted from Shin et al. [87] with permission from The American Chemical Society, copyright 2013. (D) Fluorescence images of assembly of microgels structures with co-culture of 3T3 fibroblasts and HepG2 cells (scale bar: 100 μm). Adapted from Zamanian et al. [53] with permission from Wiley, copyright 2010. (E) (i) GelMA scaffolds before and after 12 days of mineralization (scale bar: 2 mm); (ii) SEM images of GelMA hydrogels before and (iii) after mineralization (scale bar: 50 μm). Adapted from Zhou et al. [81] with permission from The Royal Society of Chemistry, copyright 2014.

Figure 6

Examples of applications of GelMA hydrogels other than in tissue engineering. (A) A gradient system made with GelMA hydrogels using dielectrophoresis. (i) Example of a device with four different zones of gradients; (ii) representation of a method to evaluate the viability of cells (C2C12 myoblasts) when cultured on GelMA gradient hydrogels containing a drug (6-hydroxydopamine: 6-OHDA) immobilized on gold microparticles. Adapted from Ahadian et al. [77] with permission from Elsevier, copyright 2014. (B) GelMA hydrogel ring as a device for the capture, culture, and study of a single neuron cell. (i) Calcein-AM/DAPI stained image and (ii) merged image of the single neuron cell encapsulated in the assembled GelMA hydrogel loop (scale bar: 150 μm). Adapted from Fan et al. [40] with permission from The Royal Society of Chemistry, copyright 2012. (C) Illustration of the use of GelMA in an electrochemical biosensor to detect DNA hybridization. Adapted from Topkaya et al. [108] with permission from Elsevier, copyright 2015.