Novel Application of Biomedical Hydrogels for Treating Degenerative Conditions of the Spine

Hydrogels are water imbibing polymeric networks that have an enormous capacity to attract and hold water. Composed of homopolymer and copolymer chains, hydrogels are not water soluble yet have the capacity to soak up large quantities of the solvent; sequestering it in polymer domains that have high affinity for polar molecules. Commonly, synthetic hydrogels are produced through cross-linkage and polymerisation of low-molecular-weight hydrophilic polymers or through chemical cross-linkage of high molecular-weight hydrophilic polymer chains. In both instances, a 3D network of polymer chains is formed with large regions of highly polar and oftentimes ionically charged chemical moieties that both attract and retain water in their pore structure. In addition to chemical synthesis, hydrogels abound in nature and constitute much of the structure of joint spaces providing for viscoelasticity and shock absorption. ¹ Naturally derived hydrogels, such as hyaluronic acid, can be extracted from tissues or obtained from bacterial fermentation processes.² Hyaluronic acid-based biopolymers can hold up to 99% of their weight in water and have molecular weights in the hundreds of thousands in the MDalton range. They are very complex chemical structures that are often incompletely characterised. By virtue of the fact that much of their weight is in the form of water, some hydrogels offer little in the way of mechanical strength. However, an interesting class of hydrogels exhibiting mechanical strength in compression, tension and tear resistance has recently been developed for medical applications, in particular, medical applications that have traditionally required mechanical strength and durability. ³ The chemistry of the polymer system, hydrolysed polyacrylonitrile (HPAN) is based on acid- or base-promoted hydrolysis of the nitrile functional group of polyacrylonitrile (PAN). A segmented, block copolymer is formed consisting of hydrophylic acrylic acid and amide groups (the polar, globular region) and a nitrile crystalline block that provides mechanical stiffness and strength. PAN polymer is initially formed from assembly of subunits of acrylonitrile. The resulting PAN polymer is then hydrolysed to produce HPAN.

The chemical structure of PAN, the high-molecular-weight precursor polymer used in the preparation of HPAN, is depicted in Figure 1. The molecular weight of the starting material is approximately 220,000 Daltons. The starting polymer PAN consists of both crystalline and amorphous domains.

The nitrile or CN side chain groups attached to the aliphatic (C–C) polymer backbone are the sites at which the polymer undergoes hydrolysis during the production of HPAN. Hydrolysis of the CN side chains does not affect the molecular weight of the polymer backbone, which is important to its mechanical strength. The resultant HPAN polymer can be shaped into a wide array of forms and then dried into nearly any desired configuration. Once rehydrated, the HPAN implant resumes the original hydrated shape and retains ‘memory’.

Functional Application of Hydrolysed Polyacrylonitrile Hydrogel for Use in Degenerative Conditions of the Spine
There are a number of medical applications of hydrogels where mechanical strength and durability are necessary requirements for function. Orthopaedic and spinal implants that seek to replace a joint- or weight-bearing function must bear considerable load. Not surprisingly, implants dedicated to these activities are largely composed of metal.

Spinal Stenosis and Interspinous Spacer Devices
Interspinous spacer devices (ISDs) are a class of lumbar spinal devices gaining acceptance as an alternative to fusion for the treatment of lumbar spinal stenosis (LSS), which is a common degenerative disease that causes a narrowing of the spinal canal and neural foramen. Stenosis leads to the painful condition, neurogenic intermittent claudication (NIC). The degree of stenosis is exacerbated in extension of the spine. In extension, painful symptoms worsen due to increased neurologic compression. Flexion in the spine relieves these symptoms.” An interspinous device distracts (flexes) the two adjacent spinous processes at the afflicted level and prevents the pathological extension. Degenerative spinal changes can be observed in 95% of people by 50 years of age,⁴ and stenosis is the most common cause of serious back pain in adults ≥60 years of age.⁵