Surface chemical modification of PCL films for peripheral nerve repair

Nerve injury is a very common trauma affecting 300,000 people in Europe every year.Although autografts are currently the gold standard in surgery, they can cause loss ofsensation and scar tissue formation. Artificial nerve conduits are a valid alternative forperipheral nerve repair. They can provide a confined environment during theregeneration process, enabling axons sprouting from the proximal to the distal nervesegments as well as reducing scar tissue formation. Poly-e-caprolactone (PCL) is abiocompatible and biodegradable polymer suitable for the fabrication of nerveguidances. In particular, previous works demonstrated that neural cells are able toadhere and proliferate on micropitted PCL films obtained through solvent casting. Also,short term studies showed that axons were able to bridge 1cm injury gap. In this work a18 weeks long term in vivo experiment using a rat model was performed to investigatethe reinnervation of end organ skin and muscle. PCL conduits were compared toautografts, with no significant differences in terms of regeneration and reinnervation.However, Schwann cells (SCs), the most important glial cells in the peripheral nervoussystem, showed poor attachment in vitro on PCL scaffolds; hence, surface modificationwas carried out in order to improve the material biocompatibility. The effect of bothhydrophilicity and functional groups on SCs was first investigated. PCL films were thenhydrolysed and aminolysed to modify the surface with carboxylic and amino groupsrespectively. Hydrolysed films increased remarkably the surface hydrophilicity,although topography and mechanical properties were not affected. Conversely, thetensile modulus and strength were significantly reduced by aminolysis, but still suitablefor the desired application. The two treatments influenced also the morphology of SCs.It was demonstrated that cell elongation was induced by hydrophilic surfaces, whilstcells preferred cell-cell interaction when cultured on aminolysed films. However, cellproliferation was remarkably increased on the latter surfaces, confirming previousresults obtained on substrates characterised by amino groups. These results confirmedthat a good balance between hydophilicity and surface chemistry is necessary toguarantee the best cell response. In order to enhance both proliferation and morphologyof SCs, arg-gly-asp (RGD) sequences were immobilised on the PCL film surface usingtwo different reaction mechanisms. Carbodiimide chemistry was compared to a newmechanism developed in the present study based on the Thiol chemistry. Biologicaltests performed on these modified films demonstrated the improvement of SC responseafter the peptide immobilisation using the novel approach. Cell attachment andproliferation were three times higher compared to untreated PCL films. It was alsoobserved that the presence of peptides on the film surface induced the formation of focaladhesion plaques by SCs, important for the perception of cellular signals when incontact with a particular substrate. Hence, a good balance between focal adhesion andadhesion forces was achieved after peptide immobilisation. Overall the results of thisstudy showed that material functionalisation is very important for SC response and itwill be fundamental for the production of artificial nerve conduits.