Hydrogels That Allow and Facilitate Bone Repair, Remodeling, and Regeneration

Abstract

Bone defects can originate from a variety of causes, including trauma, cancer, congenital deformity, and surgical reconstruction. Success of the current "gold standard" treatment (i.e., autologous bone grafts) is greatly influenced by insufficient or inappropriate bone stock. There is thus a critical need for the development of new, engineered materials for bone repair. This review describes the use of natural and synthetic hydrogels as scaffolds for bone tissue engineering. We discuss many of the advantages that hydrogels offer as bone repair materials, including their potential for osteoconductivity, biodegradability, controlled growth factor release, and cell encapsulation. We also discuss the use of hydrogels in composite devices with metals, ceramics, or polymers. These composites are useful because of the low mechanical moduli of hydrogels. Finally, the potential for thermosetting and photo-cross-linked hydrogels as three-dimensionally (3D) printed, patient-specific devices is highlighted. Three-dimensional printing enables controlled spatial distribution of scaffold materials, cells, and growth factors. Hydrogels, especially natural hydrogels present in bone matrix, have great potential to augment existing bone tissue engineering devices for the treatment of critical size bone defects.

Figure 1

The Bone Healing Process. Upon the fracturing of a bone (A), blood vessels are severed allowing for the creation of a blood clot, or hematoma, within the bone cavity (B). As the blood clot breaks down (C), granulation tissue forms, allowing angiogenesis to occur within the injured area. Meanwhile, the periosteum of the healthy bone replicates and transforms into chondroblasts, creating a cartilaginous scaffold within the cavity (D). The next phase begins as the process known as endochondral ossification transforms the cartilaginous tissue into trabecular bone matrix (E). Once the cartilage callus has been transformed into laminar bone (F), the bone remodelling process begins to transform the outer laminar bone into compact bone. Image modified, with permissions, from Lissenberg-Thunnissen, S. N. et al., Int Orthop, 2011, 1271.

Figure 2

Chemical schemes for hydrogel synthesis. (A) Radical polymerization utilizes a radical initiating agent and ultraviolet light to initiate polymerization. In this example, poly(ethylene glycol)-dimethacrylate (PEG-DMA) utilizes a radical producing curing agent to polymerize the hydrogel via methacrylate groups. (B) Polymerization through “click” mechanisms allows for dipolar cycloaddition, or Huisgen reaction, to occur between azido and alkyne groups in aqueous solutions. In this example, hyaluronic acid (HA) was modified using (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride) (EDC) to attach either an azide or alkyne functional group. Addition of the modified HA to a CuCl solution allows for polymerization to occur. (C) Michael addition reactions occur in the presence of a carbanion and an unsaturated carbonyl to create C-C bonds. In this example, poly(ethylene glycol) vinyl sulfone was reacted with free thiol groups located on purified peptides to create functionalized hydrogels. Images modified, with permissions, from Backstrom, S. et al., Materials Sciences and Applications, 2012, 425; Crescenzi, V. et al., Biomacromolecules, 2007, 1844; Lutolf, M. P. et al., Biomacromolecules, 2003, 713.

Figure 3

Osteoblasts demonstrate little attachment to unmodified alginate hydrogels (A), but preferentially attach to alginate hydrogels modified with the RGD peptide (B). Image modified, with permissions, from Alsberg, E. et al., Journal of Dental Research, 2001, 2025.

Figure 4

Scanning electron microscopy shows that bioactive glass nanoparticles incorporated into chitosan hydrogels support the formation of apatite over time: (a) 0 days (control), (b) 3 days, (c) 7 days and (d) 14 days. Image modified, with permissions, from Couto, D. S. et al., Acta Biomaterialia, 2009, 115.

Figure 5

Human osteoblasts show increased adhesion and spreading on pHEMA surfaces modified with fumed silica, as seen under scanning electron microscopy 24 h after osteoblast plating: (A) pHEMA, (B) pHEMA + 10% fumed silica, (C) pHEMA + 23% fumed silica (Mag: 12,000X). Image modified with permissions from Schiraldi, C. et al., Biomaterials, 2004, 3645.

Figure 6

After 12 weeks, 6 mm rabbit, cranial defects were analysed via histology after treatment with: (A) PBS, (B) gelatin hydrogel, (C,D) 100 μg of free β-FGF, (E,F) β-FGF-incorporating gelatin hydrogel. Incorporation of a β-FGF gelatin hydrogel shows an increase of infiltrative osteoblasts, as well as new bone formation. B=bone, DM=duramater, C=connective tissue, NB=new bone, OB=osteoblast. (A, B, C and D; ×40, E and F; ×400) Image modified, with permissions, from Tabata, Y. et al., Biomaterials, 1998, p.807.

Figure 7

Live/Dead staining of MC3T3-E1 cells cultured in the presence of calcium phosphate cement (CPC) show no living cells, in green (A) and extensive cell death, in red (B). Whereas, cells encapsulated within alginate hydrogel beads (C) show a significant increase in cell survival, green. Image modified, with permissions, from Weir, M.D. et al., Journal of Biomedical Materials Research Part A, 2006, 487.

Figure 8

Three-dimensional printing offers great potential for the creation of cell favourable, multiplexed constructs for tissue engineering. (A) Represents a modified stereolithographic printer adapted for multiple materials, while still achieving layer-by-layer deposition through the bottom-up approach. Insets depict a representation of the geometric shapes being created. Fluorescence staining of the encapsulated cells within a PEG-dimethacrylate hydrogel confirms that the multiplexed geometries are maintained during printing: Top-down view (B) and the cross-sectional view (C). Image modified, with permissions, from Chan, V. et al., Lab Chip, 2010, 2062.