Hydrogels consist of a network of hydrophilic polymer chains held together by cross-links. These cross-links prevent the individual motion of polymer chains leading to the formation of a ‘gel’. Hydrogels closely resemble biological tissues due to their high water content and environmental sensitivities. This makes them attractive for biomedical applications, including tissue engineering and drug delivery.
The cross-links between polymer chains can be either physical or chemical. Physically cross-linked gels tend to be weak and the physical cross-links between the polymer chains are reversible. The weak mechanical properties limit the use of physically cross-linked gels in wider applications.
Chemically cross-linked gels are formed following a chemical reaction, where a chemical cross-linking agent is added to a solution of polymer chains. By varying the quantity of cross-linking agent, the degree of cross-linking can be altered to optimise the gel properties for different applications. Although chemically cross-linked gels have attractive properties, the toxicity of cross-linking agents limits their use in biomedical applications.
Polymer centre academic Dr. Biqiong Chen has attempted to overcome these boundaries by synthesising novel, physically cross-linked, graphene oxide (GO)-gelatin nanocomposite hydrogels by self-assembly.
Both gelatin hydrogels and graphene-based materials have previously been studied for applications in drug delivery and tissue engineering. Gelatin is a denatured biopolymer, derived from collagen (found in skin and muscle). It has advantageous properties such as being biocompatible, biodegradable and low cost. GO, the oxidised form of graphene, has low toxicity. Gelatin-functionalised GO nanosheets are non-toxic and can be removed from the body by metabolism. Although these gels are physically cross-linked, multiple hydrogen-bonding and electrostatic interactions between the gelatin chains and GO sheets increase the strength of the 3D network significantly. These gels also exhibit self-healing properties.
The GO-gelatin hydrogels are also pH responsive and can be used for pH-sensitive drug release. At acidic pH (pH 1.7) the GO sheets form tightly packed aggregates preventing drug release. At neutral pH (pH 7.4) the pore size increases, promoting the diffusion of an encapsulated drug from the hydrogel to its surrounding liquid environment. This pH-sensitive behaviour would allow selective drug release into the intestine (pH 6.6-7.5) with minimal release in the stomach (pH 1.0-2.5). Conventional drug delivery methods can carry drugs to a specific location but are unable to protect the drug against the acidic and enzymatic environment of the stomach which can lead to the drug being released early or being altered. In contrast, these GO-gelatin hydrogels can protect the drug from enzymatic attack in the stomach. This allows the drug to be maintained and then released in a more controlled manner.
This research was published by Dr Biqiong Chen’s research group in the Journal of Polymer Science Part B and was supported by the University of Sheffield. Original Article: Y. Piao, B. Chen, J. Polym. Sci. Part B: Polym. Phys, 2014, 53, 356 -367.
Article by Amy Cockram; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.