## 4 Translational Preclinical Models Used to Investigate the Intervertebral Disc Degeneration
\begin{tabular}{p{108.4 pt} p{142.3 pt}} \hline
**High fibrin** \\ \hline \multirow{9}{*}{**Collagen and HA Incorporated**} & & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\ \hline \end{tabular}  & \begin{tabular}{p{108.4 pt} p{142.3 pt}} \hline
**High fibrin** \\ \hline \multirow{9}{*}{**Collagen and HA Incorporated**} & & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\ \hline \end{tabular}  & \begin{tabular}{p{108.4 pt} p{142.3 pt}} \hline
**High fibrin-concentration enhanced cell viability and proliferation.** \\ \hline \multirow{9}{*}{**Collagen and HA Incorporated**} & & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\ \hline \end{tabular}  & \begin{tabular}{p{108.4 pt} p{142.3 pt}} \hline
**High fibrin-concentration enhanced cell viability and proliferation.** \\ \hline \multirow{9}{*}{**Collagen and HA Incorporated**} & & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\ \hline \end{tabular}  & \begin{tabular}{p{108.4 pt} p{142.3 pt}} \hline
**High fibrin-concentration increased cell viability and proliferation.** \\ \hline \multirow{9}{*}{**Collagen and HA Activative**} & & & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\ \hline \end{tabular}  &
\begin{tabular}{p{108.4 pt} p{142.3 pt}} \hline
**High fibrin-concentration increased cell viability and proliferation. ** \\ \hline \multirow{9}{*}{**Collagen and HA Activative**} & & & & & \\There is an increasing requirement for three-dimensional (3D) scaffolds to regenerate IVDs. HA has a diverse range of applications when integrated into such 3D structures. Different materials were incorporated with HA to form composites with biocompatible properties including biological performance, stiffness, and degradation both in vitro and in vivo. Such composites, including collagen, Fibrin and chondro sulphate [142; 143; 146; 156], have been investigated in order to tackle extracellular matrix degradation by enhancing the synthesis of key matrix modulating proteins, GAGs and increasing cellular viability. Pre-clinical models of IVD degeneration to test the efficacy of HA-based materials have been developed in mice [155], rats [60], rabbit [143], sheep [153], goats [154], pigs [156], cows [142], and primates [144]. HA formulations have been cross-linked using many methods including UV cross-linking [142], divinyl sulfone [144], polyethyleneglycol (PEG)-amine with EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide), NHS (N-hydroxysuccinimide) cross-linking [59], fibrin [150], batroxobin [152], butanediol diglycidyl ether [157] and enzymatic cross-linking using transglutaminase in a fibrin/HA material [154]. HA hydrogels utilized in treating the different parts of the IVD, including AF and NP, not only acted as composite materials but also as a reservoir for growth factors and therapeutic drugs [142; 150; 154], as well as a cell delivery vehicle [147; 155; 155]. In summary, these studies report on the hydrophilic properties of HA to restore disc height [151] and MRI signal intensity [144], the ability of HA to promote ECM synthesis [147] in resident or injected cells, and of course the anti-inflammatory [145] and anti-nociceptive [148] properties of HA in the setting of IVD degeneration. Several clinical trials investigating the efficacy of HA-based materials are ongoing, with few reporting results at this early stage. However, initial results have been promising, where MSCs delivered in a HA carrier system demonstrated an excellent safety profile with several patients reporting improved VAS and ODI reflecting better MRI signal intensity [157]. Furthermore, HA is being trialled in extra-discal injections in cases of facet joint degeneration [158]. HA has demonstrated equivalent efficacy to glucocorticoid administration in this patient cohort.