Design properties of hydrogel tissue-engineering scaffolds

Abstract

This article summarizes the recent progress in the design and synthesis of hydrogels as tissue-engineering scaffolds. Hydrogels are attractive scaffolding materials owing to their highly swollen network structure, ability to encapsulate cells and bioactive molecules, and efficient mass transfer. Various polymers, including natural, synthetic and natural/synthetic hybrid polymers, have been used to make hydrogels via chemical or physical crosslinking. Recently, bioactive synthetic hydrogels have emerged as promising scaffolds because they can provide molecularly tailored biofunctions and adjustable mechanical properties, as well as an extracellular matrix-like microenvironment for cell growth and tissue formation. This article addresses various strategies that have been explored to design synthetic hydrogels with extracellular matrix-mimetic bioactive properties, such as cell adhesion, proteolytic degradation and growth factor-binding.

Figure 1. Schematic of hydrogel structure with hydrophilic polymer chains connected through crosslink points or crosslinking polymers

M_c represents the number average molecular weight between two adjacent crosslinks, which is related to the degree of crosslinking. ξ represents the network mesh size and is indicative of the distance between consecutive crosslinking points.

Figure 2. Structures of monomers or macromers (HEMA, AAm, AAc, NIPAm and mPEGMA), and crosslinkers (MBA, EGDA and PEGDA) for preparing nondegradable synthetic hydrogels

AAc: Acrylic acid; AAm: Acrylamide; EGDA: Ethylene glycol diacrylate; HEMA: 2-hydroxyethyl methacrylate; MBA: N,N′-methylenebis(acrylamide); mPEGMA: Methoxyl poly(ethylene glycol) monoacrylate; NIPAm: N-Isopropylacrylamide; PEGDA: Poly(ethylene glycol) diacrylate.

Figure 3. Structures of macromers, PLA–PEG–PLA and PEG–PLA–PEG diacrylates, and PEG(SS)DA for preparing degradable synthetic hydrogels.<

br>PEG: Poly(ethylene glycol); PEG(SS)DA: Disulfide-containing PEG diacrylate; PLA: Poly(lactic acid).

Figure 4. Model of bioactive synthetic hydrogels

Cell-adhesive and enzyme-sensitive peptides can be incorporated into hydrogels to make hydrogels as cell-adhesive and biodegradable scaffolds. Growth factors can also be covalently attached on, or reversely bind with, the hydrogel network to mediate cellular response and regulate tissue formation.

Figure 5. Model of complex 3D structure of the natural extracellular matrix and the interactions between cells and the extracellular matrix components

Extracellular matrix proteins such as collagen, laminin and fibronectin are embedded in highly negatively charged polysaccharide-rich glycans, including glycosaminoglycans and proteoglycans. The extracellular matrix components provide cell-adhesive domains for binding cell-surface receptors, such as intergrins, selectins, CD44 and syndecan.

Figure 6. Preparation of cell-adhesive hydrogels

(A) Model of cell-adhesive hydrogels. Cell-adhesive peptides can be incorporated into the hydrogel network by various methods, such as free radical copolymerization, Michael addition and Click chemistry. (B) Structures of RGD-modified poly(ethylene glycol) macromers: RGD-MA, RGD-PEGMA and RGD-PEGDA. CAP: Cell-adhesive peptide; MA: Monoacrylate; PEGDA: Poly(ethylene glycol) diacrylate; PEGMA: Poly(ethylene glycol) monoacrylate; RGD: Arg–Gly–Asp.

Figure 7. Phase contrast images of 2D seeding and culturing of human umbilical vein endothelial cells on hydrogels

(A & B) 2 and 24 h after seeding human umbilical vein endothelial cells (HUVECs) on 10% (w/v) poly(ethylene glycol) diacrylate (PEGDA) hydrogels, respectively; (C & D) 2 and 24 h after seeding HUVECs on Arg–Gly–Asp (RGD)-PEGDA hydrogels made by copolymerization of RGD-poly(ethylene glycol) monoacrylate (PEGMA; 1%, w/v) and PEGDA (9%, w/v), respectively. The images show that HUVECs seeded on RGD-PEGDA hydrogels exhibited higher initial cell attachment, greater cell spreading, and higher cell density than on PEGDA hydrogels. (Scale bar: 100 μm).

Figure 8. Schematic of the methods for the preparation of enzyme-sensitive hydrogels

(A) Free radical polymerization of ESP-containing PEGDA (ESP-EGDA). ESP-PEGDA can be synthesized by the conjugation of ESP-2NH₂ with acrylate-PEG-NHS. (B) Michael addition of ESP-2SH and multiarm PEG sulfone, such as PEG-4VS. ESP: Enzyme-sensitive peptide; ESP-2NH₂: ESP diamine; ESP-2SH: ESP-dithiol; PEG-4VS: 4-arm poly(ethylene glycol) vinyl sulfone; PEGDA: Poly(ethylene glycol) diacrylate.

Figure 9. Schematic of growth factor-bearing hydrogels

(A) Direct loading: GFs are encapsulated into hydrogels directly during hydrogel preparation. (B) Carrier systems: carrier systems like micro- or nano-particles are used to encapsulate GFs first, which are subsequently embedded in hydrogels during hydrogel preparation. (C) Covalent bonding: GFs are covalently attached on the hydrogel network through chemical conjugation or copolymerization. (D) Reverse binding: GF-binding polymers or short peptides are incorporated into hydrogels by various reactions, such as free radical copolymerization, Michael addition and chemical conjugation. The resulting hydrogels can control the delivery of GFs through the reverse binding between GFs and the incorporated GF-binding polymers or peptides. CAP: Cell-adhesive peptide; GF: Growth factor.