The biocompatibility and adhesive properties of polymer–clay nanocomposite hydrogels

Clay–polymer nanocomposite hydrogels represent an increasingly utilised material for use in biomedical applications. Because of their enhanced mechanical properties compared to traditionally cross-linked materials, they are lending themselves to a wide variety of potential applications. One such possibility is the use in neural controlled next-generation prostheses.

As part of their development, it was necessary to investigate the ability of these hydrogels to produce coherent layers to determine their ability to be used in additive manufacturing systems. Additionally, cell studies can be undertaken to investigate the in vitro biocompatibility of these materials on neuronal cells, thereby assessing their ability to be used in a device demonstrating neural control.

In order to conduct peel and pull apart adhesion testing, a novel method of attaching the hydrogel to a stiffer substrate required developing. This involved the use of atoms regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) to grow polymer brushes onto an aluminium substrate. The adhesion of the hydrogel was then facilitated by polymerising the precursor liquid in contact with the polymer brush.

This research demonstrates the adhesive properties and biocompatibility of laponite–polymer nanocomposite hydrogels. It also investigates the use of poly(D-lysine) coatings to improve the cellular attachment and differentiation of human lymphoblastoma SH-SY5Y cells when seeded onto these materials. Using a combination of peel and pull apart tests, the adhesive properties of these hydrogels have begun to be elucidated in order to assess the ability of layer-by-layer formation. The data showed that thermal initiated materials produce hydrogels with inferior adhesive ability compared to UV-initiated materials.

The adhesion tests on the UV-initiated materials showed that the hydrogel attached more firmly to the polymer-brush coated substrate than to layers of itself. This demonstrated that it is possible to utilise polymer brushes as an attachment mechanism to attach hydrogels to other materials, as the attachment to the polymer brush was greater than the internal strength of the hydrogel.

In testing the biocompatibility of these materials, it was shown that the thermal- initiated samples provided a greater reduction in the cell attachment compared to the UV-initiated materials. Irgacure 184 is also widely known to be a less biocompatible initiator than Irgacure 2959.

Finally, poly(D-lysine) coating of nanocomposite hydrogels were shown to increase the biocompatibility of the Irgacure 184 initiated samples as shown by the increase in cell attachment and the increase in the average length of neurites. These coatings did not affect the differentiation of SH-SY5Y cells into a neuronal phenotype, as demonstrated by no change in the average number of neurites or the average length of the neurites on all other samples.

UV-initiated hydrogels have begun to demonstrate the properties required in an additively manufactured prosthetic device exhibiting neural control. Future work is necessary to further develop the potential to provide guided neuronal growth.