Table 1.

The major natural and synthetic biomaterials that have been studied for their potential use for treating intervertebral disc degeneration (DD). In the context of intervertebral disc (IVD) applications, potential strengths (+) and drawbacks (−) for each material are provided as well as the IVD-specific studies accompanying them.

Hydrogels for IVD TE
Natural Hydrogels
Type	Material Biomechanical Properties	IVD Studies
Alginate	+Injectable, biocompatible, tailorable properties, anionic properties attract cationic PG [358]. −Diminishing structural integrity over time in calcium cross-linked hydrogels [370].	In vitro/ex vivo: In situ gelation of calcium carbonate cross-linked alginate hydrogel showing ability to maintain disc height over cyclic loading regime [371]. In vitro: Porcine AF, NP, and transition zone cells were cultured in sodium chloride cross-linked alginate beads demonstrating IVD relevant ECM expression but diminished mechanical properties [370]. In vitro: Bovine NP cells encapsulated in a photo-cross-linkable alginate hydrogel showed decrease cell viability over 14-d culture period [372].
Fibrin	+Biocompatible, gelation time control, biodegradable, promotes matrix synthesis stem cell-derived chondrocytes, non-immunogenic [366]. −Soft in nature (however, can be modified to overcome this) [373].	In vitro/ex vivo: Fibrin-genipin hydrogel with silk scaffold for AF and NP repair demonstrated cytotoxicity to cells in vitro and no recovery of disc height but matrix comparable to healthy disc in bovine organ culture [374]. Ex vivo/in vivo: Fibrin-genipin adhesive hydrogel tested in bovine organ culture and in a mouse model for AF defect repair demonstrating biocompatibility and biomechanics’ restoration [375]. Clinical: A phase II, randomized, double-blind, placebo-controlled study. Assessment of safety and preliminary efficacy of juvenile chondrocytes delivered using a fibrin carrier (NuQu®) for treating disc pain [376].
Collagen	+Good cell adhesion, biocompatibility, and proliferation. Non-immunogenic. Major component of IVD. −Poor mechanical properties with high degradation rate [361].	In vitro: Dense collagen I hydrogel demonstrating comparable functional characteristics to NP [377]. Ex vivo: Condensed collagen gel for NP replacement showed disc height restoration but extrusions of implant during stress testing [378]. Composites: In vitro: Human NP cells encapsulated type II collagen-hyaluronic acid hydrogel crosslinked 1-ethyl-3 (3-dimethyl aminopropyl) carbodiimide demonstrating cell proliferation but no increase in matrix gene expression compared with control gel [379]. In vivo: Transplantation of HA/collagen hydrogel into porcine nucleotomy model causing localized annular damage and inflammation [380].
Atelo-collagen	+Low immunoreactivity, injectable due to liquid–solid transition when warmed to body temperature, support high-density cell cultures [360].	Ex vivo: Autologous MSC encapsulated in atelocollagen II gel and transplanted into IVD of rabbit disc degeneration model resulting in disc height recovery and PG accumulation [381]. In vivo: Autologous MSC encapsulated in atelocollagen II gel and injected into degeneration-induced NP of rabbits resulting in comparable PG accumulation to healthy control [382].
Chitosan	+Supports IVD cell encapsulation, cationic properties retain PG, thermoresponsive [383]. −Cell adhesion and mechanical properties not ideal for IVD [358].	In vitro: Bovine IVD cells encapsulated in chitosan hydrogel showed retention of NP-produced PG within gel. Gel cytotoxic towards AF cells [383]. In vitro: Human MSC differentiation into NP-like cells in a chitosan-glycerophosphate hydrogel [384].
Gellan gum	+Thermo-reversible gel properties, acid and heat resistant, non-cytotoxic, gelation without the need of harsh reagents, supports chondrocyte ECM deposition [360,385]. −Mechanically weak, requires high gelling temperatures, and lacks anchorage sites for adherent dependent cells [386].	In vitro: Ionic and photo-cross-linked methacrylated gellan gum showed lower water uptake ability but improved mechanical properties than gellan gum alone in the context of NP repair [385]. In vitro/In vivo: Encapsulated MSC in gellan gum hydrogel show cell viability in vitro and signs of chondrogenesis in mouse subcutaneous implant [387]. In vitro: Gellan gum hydrogels reinforced with nanocellulose demonstrated AF biomechanical properties and bovine AF cell support [388].
Hyaluronan (hyaluronic acid)	+Retains water, non-immunogenic, anti-inflammatory, and low cost. Bioactive by binding with cell surface receptors and ECM proteins, which promotes cell infiltration [358]. −Osteogenic properties and cytotoxic at high concentrations [361].	In vivo: MSC injected into rat IVD using 15% hyaluronic acid hydrogel. Initial significant cell loss followed by proliferation. An increase in disc height was shown [389]. In vivo: Injection of hyaluronic acid hydrogel in rat tail disc degeneration model demonstrating signs of pain marker reduction and attenuation of inflammation [390]. Clinical: Prospective, multicenter, double-blinded, controlled phase 2 study. Safety and efficacy assessment of allogenic MSC injected with hyaluronic acid in disc degeneration patients (no results posted) [391]. Composites: Ex vivo: Bovine NP cells cultured in a fibrinogen-hyaluronic acid-based hydrogel showed maintenance of some NP markers and disc height recovery in ex vivo organ culture [392].
Synthetic Hydrogels
Type	Material Biomechanical Properties	IVD Studies
Poly N-isopropyl-acrylamide (pNIPAM)-based hydrogels	Laponite crosslinked pNIPAM-co-DMAc:
	+Thermo-responsive hydrogel injectable above body temperature and solidifies upon cooling to 37 degrees C. [393], supports differentiation of human MSC into NP cells [394], excellent biocompatibility [395]	In vitro: Assessment of hMSC to NP cell differentiation in pNIPAM hydrogel in normoxia and hypoxia [394]. Ex vivo: Human MSC and bovine NPs encapsulated in pNIPAM hydrogel and injected into papain-induced bovine disc degeneration model [393]. In vitro: Laponite cross-linked pNIPAM-co-DMAc encapsulation of hMSC showed NP differentiation was not affected by catabolic culture conditions [356]. In vitro/ex vivo: HA-pNIPAM hydrogel seeded with autologous human NP cells and implanted in intact human IVD explant, demonstrating matrix synthesis [337].
	HA-pNIPAM hydrogel:
	+Solidifies beyond 32 °C and is injectable at room temperature [130]	In vitro/ex vivo: Improved NP-like differentiation of hMSC in vitro in HA-pNIPAM hydrogel with GF compared to alginate hydrogel. Direct implantation of hMSC/HA-pNIPAM into bovine organ culture better than pre-differentiating hMSC [396]. In vitro/ex vivo NP cell support and ECM deposition in HA-pNIPAM hydrogel compared to alginate beads, in vitro. Implanted cell-hydrogel construct in bovine disc organ culture showed cell viability [397].
Polyethylene glycol (PEG)-based hydrogels	+Non-cytotoxic, easily synthesized, PEG-based hydrogels have high hydration properties. Photo-polymerizable composites [358]. −Low biorecognition of cells, which affects cell adhesion properties, non-biodegradable [358].	Composites: In vitro: PEG-hyaluronic acid hydrogel screen for porcine AF and NP cell proliferation and sGAG production identified lower molecular weight hyaluronic acid gels were most suitable for the IVD [398]. In vitro: Porcine NP cell 2D and 3D culture in photo-crosslinked PEG-laminin 111 hydrogel showed support of cell viability and matrix deposition [399]. In vitro/in vivo: PEG-albumin hydrogel tested with human disc cells by comparing 2D, 3D, and mouse subcutaneous implant culture. Significantly higher SOX9 and HAS expression but not of aggrecan or collagen types I/II [400]. Ex vivo: Injection of the photo-polymerizable PEG dimethacrylate nano-fibrillated cellulose composite hydrogel into a bovine organ model of IVD resulting in disc height restoration [401]. In vitro: Bovine NPs cultured in high-molecular-weight hyaluronic acid cross-linked with PEG-amine showed reduced pro-inflammatory markers [402].
Polyvinyl alcohol (PVA)-based hydrogels	+Mechanical properties easily tailorable to IVD via PVA concentration adjustments [403,404].− Non-degradable and expensive, limited biological testing for IVD applications [361].	In vitro: PVA cryogel biomechanical testing found 3% PVA concentration with 3 freeze-thaw cycles was optimum for mimicking compression properties of the NP [404]. In vitro: Elastic modulus similar to native articular cartilage was attained from the fabrication of PVA and bacterial cellulose nanofiber nanocomposite however, not directly compared to IVD biomechanical properties [405]. Composites: In vitro: PVA containing laponite and bacterial cellulose nanocomposites were mechanically tested showing tailorable stiffness. Wear and fatigue properties were enhanced with nanofiller adjustments and two-component PVA hydrogel could be tailored to mimic IVD compression properties [406]. Ex vivo: PVA-polyvinyl pyrrolidone composite hydrogel showed good fatigue properties and restored compressive stiffness in human cadaver models [403]. In vitro: PVA-silk composite cyrogel showing silk improved cell adhesion and survival of rabbit adipose stem cells over culture period. No proliferation was observed or capacity to encourage NP differentiation [407].
Self-assembling peptide hydrogels (SAPH)	+Provide the advantages of natural and synthetic biomaterials while overcoming their individual disadvantages, easily tunable biomechanics via peptide sequence modifications, biodegradable and biocompatible, chemically defined, self-healing [408,409]. +Functionalization: Functionalize the peptide with motifs that replicate useful biological molecules, e.g., BMP [410]. Graphene incorporation can act as delivery vehicles for biological factors [411]. Graphene oxide is biocompatible and promotes cell adhesion [412]. −Stability and variable immunogenic concerns remain a challenge [408].	In vitro: Good cell viability of 3D cultured, de-differentiated human NP cells in FEFEFKFK SAPH. NP phenotype (except aggrecan) and GAG synthesis was significantly higher at days 7 and 14 compared to day 1 of 3D culture [413]. In vitro: 3D culture of rabbit NP cells in KLD-12 SAPDH demonstrating increased cell viability and GAG release into media compared to hydrogel only control [414]. In vitro: Rabbit NP cells showed greater cell viability, inward migration, and ECM synthesis in RLN functionalized RADA16 compared with pure RADA16 [415]. In vitro: Human degenerated NP cells 3D cultured in RKP (BMP7 motif) functionalized RAD16-I SAPH showed increased migration, proliferation, and expression of NP marker genes compared to RADA16-I alone [416]. In vitro: Graphene oxide flakes incorporated into FEFEFKFK self-assembling peptide hydrogel mechanically similar to NP and supports bovine NP cells [417].
Scaffolds for IVD TE
Natural Polymers
Type	Material Biomechanical Properties	IVD Studies
Silk fibroin	+Compressive and tensile strength, slow degradation, cytocompatible, modifiable with covalent attachment of additional peptides [360].	In vitro: Bovine AF cells seeded onto porous silk RGD-modified scaffolds demonstrating cell support and ECM deposition and higher collagen II and aggrecan expression than nonmodified scaffold [418]. In vitro: Porcine AF and chondrocyte cells seeded onto biphasic silk scaffold for AF and fibrin/hyaluronic acid for the NP. Increase in GAG and collagen over four-week culture [419]. In vitro/in vivo: Porcine AF cells and hMSC show cell viability and appropriate differentiation toward AF phenotype on multi-layered silk scaffold compared to native AF cells. Subcutaneous mouse implant showed negligible immune response [420].
Alginate	+Biocompatible, biodegradable, anti-microbial, cheap, high porosity, support cell adhesion and growth [360]. −Poor native mechanical strength needs to be overcome with cross-linking strategies [421].	Composites: In vitro: Alginate-chitosan scaffold showed fiber alignment similar to AF and supports canine AF cell growth and ECM (collagen, aggrecan) deposition [422]. Human NP cells cultured on alginate scaffold demonstrated a fall in cell number over the 21-day culture period [423].
Atelo- collagen	+Low-immunogenic derivative of collagen, safe and biocompatible, supports stem cell and disc cell regeneration of the IVD [424].	In vitro: Rabbit NP cells seeded on atelocollagen types I and II scaffolds supplemented with BMP demonstrating anabolic gene and protein expression in type II but not type I scaffolds, compared with control [424]. In vivo: Rabbit AF cells cultured on atelocollagen honeycomb-shaped scaffold and transplanted into rabbits showed cell proliferation and production of hyaline-like cartilage [425].
Synthetic Polymers
Type	Material biological and mechanical properties	IVD Studies
Poly-urethane (PU)	+Biocompatible, biodegradable, decomposes to water and carbon dioxide and high biomechanical properties [358].	In vivo: PU mass transfer device transplanted into punctured porcine AF showed similar biomechanical properties to control group as well as enhanced energy production [426]. Ex vivo: Implantation of biphasic PU scaffold into nucleotomy bovine whole organ culture model demonstrated restoration of disc height, cytocompatibility with native cells, and downregulation of catabolic and upregulation of anabolic genes [427].
Polylactic acid polyglycolic acid (PGA) Copolymer: polylactic-co-glycolic acid (PLGA)	+Biocompatible and biodegradable, approved by FDA for in-human use [358]. − Potentially encourages nerve in-growth in the disc [428].	In vivo: Sheep AF cells seeded onto PLGA scaffold and implanted in mice demonstrating collagen I expression [429]. Composites: In vitro: Human MSC cultured on a biphasic polyL-lactic acid nanofibrous outer scaffold and inner hyaluronic acid hydrogel to mimic the architecture of AF and NP, respectively. Increased IVD ECM protein accumulation in both regions over 28d culture period [430]. In vivo: PLGA-fibrin gel plugs implanted into empty disc defects resulting in increased nerve ingrowth than empty disc controls [428].
Poly D,L-lactide (PDLLA)	+Supports adhesion, infiltration, and proliferation of MSC [431]. −Has been shown to encourage osteogenic differentiation of MSC [431].	In vitro: Human AF cells cultured within PDLLA/Bioglass® foam demonstrated proliferation over 4-week culture period and increased PG and collagen deposition than no Bioglass® foam control [432]. Composites: In vitro: b-TCP and calcium carbonate particles loaded into acrylic-terminated oligo[D,L-lactide-co-(ε-caprolactone)]. Biomechanical tests were performed, demonstrated that the addition of fillers aided achieving properties similar to the IVD [433].
Poly-ε-caprolactone (PCL)	+Biodegradable, FDA approved for in-human use, controlled decomposition time through alternative polymer combinations, high elasticity [358].	In vitro: Electrospun PCL scaffold (AF) combined with cell-seeded hydrogel for rat disc replacement showed effective cell infiltration [434]. In vitro: Bovine MSC seeded onto nanofibrous anistropic PCL scaffold demonstrating collagen deposition and alignment comparable to native bovine AF [435]. In vitro: PCL fibrous scaffold fabricated by wet-spinning showed rabbit AF cell adherence, proliferation, and increased collagen and aggrecan expression over 3-week culture [436].