Silk Fibroin Immobilization On PP/PET And Hap Mineralization On Surface-modified Substrates

Polypropylene (PP) and Poly(ethylene terephthalate)(PET) are good materials for industrial and daily applications because of their desirable bulk properties, such as excellent mechanical strength, good stability against body fluids, and high radiation resistance for sterilization, etc. However, PP and PET also have surface problems such as bio-inert, which make it necessary to modify the substrate surfaces for more biological and medical applications.Silk fibroin (SF), a protein produced by silkworms and derived from silk fiber, is a material with good biocompatibility, good oxygen and water vapor permeability, biodegradability as well as lower inflammatory response, and it is commercial availability at relatively low cost. Comprehensive studies on its applications in biological and biomedical fields have emerged such as enzyme immobilization, drug carrier and delivery, scaffolds for tissue engineering including infarcted cardiac tissues, wound-dressing, ligament, and bone tissues, etc. In these studies, SF matrixes can be used to culture various cells like fibroblasts, chondrocytes and osteoblasts. In ligament and bone tissues engineering, silk fibroin shows to be non-immunogenic, biocompatible and capable of supporting the attachment, spreading, growth and differentiation of mesenchymal stem cells (MSCs) as well as eliciting a negligible response.Hydroxyapatite (HAP), a naturally derived material which can be found in body parts such as bones and teeth, is innovative and promising materials for bioapplications due to its biocompatibility, bioactiveness, osteoconductivity and osteophilicity. One method to prepare HAP coatings is mineralization, and HAP-coated silk scaffolds prepared by alternate soaking process, deposition in simulated body fluid and homogeneous precipitation have been widely reported.Base on these, PP and PET surfaces can be modified by SF immobilization and HAP minralization to improve their biocompatibility. Two different methods were employed for SF immobilization based on this thesis:1) plasma-induced acrylic acid graft polymerization and subsequent covalent immobilization of SF (PP-PAA-SF or PET-PAA-SF), and2) Plasma pretreatment followed by SF dip coating (PP-SF or PP-PAA-SF). HAP deposition was also carried on surface modified PET in our work.For PP film surface modification, graft polymerization of acrylic acid was firstly carried out, the influence of synthesis conditions, such as plasma treatment time, plasma power, monomer concentration and temperature on the degree of grafting was investigated, and recomended synthesis conditions for higher grafting degree were given as followed:plasma treatment time-120s, plasma power-100W, monomer concentration-40%(v/v) and polymerization reation tempreture-80℃. After SF immobilization, the biocompatibility of both PP-PAA-SF and PP-SF were evaluated by MSCs culture, which indicated that PP-SF supported MSCs growth in vitro, while the cells growth on PP-PAA-SF was inhibited.The differences of two SF immobilization methods were further studied with PET surface modification. ATR-FTIR, XPS, AFM and WCA measurements were performed to find out the essence of the modification process. ATR-FTIR indicated the introduction of SF on PET films according to the adsorption peaks of SF amide groups. XPS showed the higher amount of SF on PET-SF surfaces than that on PET-PAA-SF films, which may be due to higher surface roughness on plasma-treated PET. PET-SF film surfaces had lower WCA than PET-PAA-SF, its surface roughness calculated from AFM images was slightly lower than PET-PAA-SF, yet much higher than that of the virgin PET. In the biological study, similar results to PP-PAA-SF and PP-SF were found on PET films, PET-SF again enhanced the proliferation and adhesion of MSCs, while PET-PAA-SF inhibited the cells growth due to the toxic effect of PAA.Our work indicated that SF dip coating with plasma pretreatment was much simple and efficient for biocompatibility improvement compared to covalent immobilization of SF onto PAA grafting layer of polymer substrates. It provided the potential of polymer surface modification in batch production for biomaterials in tissue engineering, such as materials for artificial ligament.In ligament tissue engineering, except for the biocompatibility of the materials, the osteoconductivity and osteophilicity of the two ends of ligament connected to bone cannel should be ensured, and this can be solved by HAP mineralization in our work. In this part, we started with the alternate soaking of surface-modified PET nonwoven fabric in CaCl2and Na2HPO4solutions, SEM and TGA analysis showed that with PAA grafting and SF immobilization, the amount of HAP depositing on PET nonwoven fabric surface was enhanced accounted for the chelation interactions between Ca2+and carboxyl groups of PAA and SF. This effect was diminished with the increase of mineralization cycles when the SF and PAA were covered with HAP and can’t interact with more Ca2+.For the above PET-PAA-SF and PET-SF films,10cycles of HAP mineralization in CaCl2and Na2HPO4solutions were carried out, the following MSCs culture indicated that with HAP depositing, the biocompatibility of PET-SF was further improved, while that of PET-PAA-SF films was enhanced yet not enough since the contribution of depositing HAP on biocompatibility was diluted by the toxic effect of grafted PAA.In conclusion, our work provides a good recommendation for artificial ligament materials surface modification:for biocompatibility improvement, SF can be immobilized by dip coating with plasma pretreatment; and for the osteoconductivity and osteophilicity of the two ends, HAP can be introduced by simple alternate soaking of SF-immobilized materials in CaCl2and Na2HPO4solutions. Batch production is possible using our methods as it’s simple and can be well controlled.