**4.3 Plasma syn-irradiation of biodegradable polyesters**

As described in section 3.2.3, a polymer can also be grafted on the surface of a biodegradable polyester by pre-adsorption of the monomer followed by a plasma treatment. However for the specific case of biodegradable polymers, we were able to track only one research paper using this plasma approach (Ding et al., 2004). In this paper, Ding et al. tried to modify the surface of PLLA films with a chitosan layer. However, results indicated poor cell adhesion, but acceptable cell proliferation.

### **4.4 Plasma polymerization on biodegradable polyesters**

Plasma polymerization differs from plasma grafting in that respect that it coats the substrate rather than covalently binds species to a plasma-modified polymer surface (Barry et al., 2005). Allylamine is one of the most frequently used monomers to plasma polymerize on biodegradable polymers such as PLLA, PCL and PHBV (Barry et al., 2005, Guerrouani et al., 2007, Carlisle et al., 2000). Plasma polymerized allylamine films on biodegradable polymers resulted in highly hydrophilic surfaces with contact angles of 20° or lower due to the amine groups on the surface.

As there is a great interest in the surface modification of 3D implants, plasma polymerization has also been performed on the surface of 3D PLA scaffolds. Plasma grafting has been compared with plasma polymerization using allylamine (Barry et al., 2005). In the case of plasma grafting, the scaffolds were first pre-treated with an oxygen plasma and afterwards exposed to allylamine vapour, while plasma polymerization was carried out by exposing the scaffolds to an allylamine vapour plasma after an oxygen plasma pretreatment. XPS measurements of the scaffolds at different points across the scaffold diameter demonstrated that the grafting process resulted in a more homogeneous nitrogen concentration through the scaffold while the concentration of nitrogen on the internal surface of the scaffold on which the plasma deposit was formed decreased from the edge to the core of the scaffold, as can be seen in Figure 6. However, at the lowest nitrogen concentration, the nitrogen concentration on the internal surface of the plasma-polymerized scaffold was still greater than that of the grafted surface. The plasma-coated scaffolds also showed a higher metabolic activity than the plasma-grafted samples. Moreover, fibroblasts were detected in the centre of the plasma-coated samples, which was not the case for the grafted scaffolds (Barry et al., 2005).

### **5. Conclusion**

The growing research fields of tissue engineering and regenerative medicine are a leverage for surface engineering of biodegradable polymers. Next to chemical surface modification techniques which encounter problems with the use of hazardous organic solvents in relation to cell viability, non-thermal plasma technology knows a steep growth as solvent-free technique. Plasma treatments are already commonly performed on biodegradable polymers such as PLA and PLGA, while treatment of more advanced biodegradable polymers (such as PCL, PHBV, PBS and composites) and other plasma-based techniques (such as plasma grafting and plasma polymerization) are only at the verge of breaking through. Nonthermal plasma technology can greatly enhance cell-material interactions, however, a better understanding of these interactions is of crucial importance. This knowledge can provide us information on which plasma-based strategies should exactly be pursued.

Fig. 6. Nitrogen concentration as determined by XPS at set points across the internal diameter of grafted and plasma-polymerized allylamine (ppAAm) scaffolds (Reprinted from (Barry et al., 2005) with permission from Wiley-VCH Verlag GmbH & Co. KGaA).

### **6. References**

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were detected in the centre of the plasma-coated samples, which was not the case for the

The growing research fields of tissue engineering and regenerative medicine are a leverage for surface engineering of biodegradable polymers. Next to chemical surface modification techniques which encounter problems with the use of hazardous organic solvents in relation to cell viability, non-thermal plasma technology knows a steep growth as solvent-free technique. Plasma treatments are already commonly performed on biodegradable polymers such as PLA and PLGA, while treatment of more advanced biodegradable polymers (such as PCL, PHBV, PBS and composites) and other plasma-based techniques (such as plasma grafting and plasma polymerization) are only at the verge of breaking through. Nonthermal plasma technology can greatly enhance cell-material interactions, however, a better understanding of these interactions is of crucial importance. This knowledge can provide us

information on which plasma-based strategies should exactly be pursued.

Fig. 6. Nitrogen concentration as determined by XPS at set points across the internal

(Barry et al., 2005) with permission from Wiley-VCH Verlag GmbH & Co. KGaA).

diameter of grafted and plasma-polymerized allylamine (ppAAm) scaffolds (Reprinted from

Agrawal, C. M., Best, J., Heckman, J. D. & Boyan, B. D. 1995. Protein Release Kinetics of A Biodegradable Implant for Fracture Non-Unions. *Biomaterials,* 16**,** 1255-1260.

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**5. Conclusion** 

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