Shape Memory Hydrogels – A Novel Material for Treating Age-related Degenerative Conditions of the Spine

Hydrogels are able to absorb large quantities of water relative to their initial weight because of their intrinsic hydrophilicity. As a result of this propensity to imbibe large quantities of water, the material can be implanted into the body in a collapsed, low-volume, dehydrated state and then expanded in vivo through absorption of body fluids to assume a different shape comprising a much greater volume. Given that the composition by mass of the expanded polymer is principally water and/or body-derived fluids, the swollen object is highly biocompatible, producing minimal inflammation following implantation.

Although many hydrogels have been developed based on various chemistries, hydrolysed polyacrylonitrile (HPAN) has been extensively studied and used in the formulation of contact lenses, drug delivery, gynaecological and orthopaedic implants.^1,2 This group of thermoplastic hydrogels is based on acrylic multiblock copolymers. A heterogeneous reaction of the hydrophobic base polymer polyacrylonitrile with sodium hydroxide produces a water-soluble block copolymer that upon phase separation yields crystalline clusters of hydrophobic nitrile functional groups and amorphous hydrophilic water-binding domains. The hydrogel is produced through a simple chemical reaction using no monomers, cross-linkers, catalysts or other toxic residuals.³

Advantageous properties of the HPAN block copolymer include biocompatibility and biodurability. HPAN also exhibits similar elasticity and tensile strength compared with tissues such as vitreous body, cartilage and the nucleus pulposus of the intervertebral disc. The elasticity of the hydrogel can also be controlled by adjusting the chemistry and the water content, allowing the material to be used in various applications such as replacement of the aforementioned tissues.

The shape memory property of the hydrogel is a unique attribute that provides treatment options in situations where insertion dimensions are crucial. These properties are particularly well suited to indications in minimally invasive spine surgery wherein the collapsed, minimised, insertion form of the hydrogel configured to a particular shape, transforms into a larger and different functional shape upon implantation. In this instance, the minimised shape facilitates insertion with little tissue damage and the fully hydrated state allows the implant to function as either a nucleus augmentation implant or an interspinous spacer (see Figure 1). In vivo expanding hydrogel implants in a nucleus augmentation (left) and interspinous spacer (right) application.

Prior to clinical introduction, hydrogel implants were subjected to an extensive battery of in vitro and in vivo animal tests to evaluate the safety and functional properties of the material. The in vitro and animal tests included but were not limited to acute and chronic toxicity, genotoxicity, systemic toxicity, irritation, and intramuscular implantation testing. All testing was conducted using Good Laboratory Practices (GLP) at an accredited independent laboratory.