D. Bakker

Surface modification of hydroxyapatite to introduce interfacial bonding with polyactiveTM 70/30 in a biodegradable composite

A method was developed to improve the interfacial bonding between hydroxyapatite and a biodegradable copolymer PolyactiveTM 70/30. Hydroxyapatite was first surface modified by the polyelectrolytes polyacrylic acid or poly(ethylene-co-maleic acid) in aqueous solutions. Subsequently the surface-modified hydroxyapatite was used as filler in composites with PolyactiveTM 70/30. The strength, elongation at break and elastic modulus of the composite in aqueous environment were significantly improved by this method. Based on these experimental results, it is believed that the interface improvement is due to hydrogen bonding and/or dipole interactions formed between polyelectrolyte molecules and polyethylene glycol segments in the polymer matrix. Due to the introduction of interfacial bonding by using such method, a new biodegradable bone-bonding composite can be made.

Cell-seeding and in vitro biocompatibility evaluation of polymeric matrices of PEO/PBT copolymers and PLLA

A bilayered matrix has been evaluated in vitro as a carrier for autografts of cultured epidermal keratinocytes and dermal fibroblasts, to be used as a skin substitute in deep dermal skin defects. A poly-L-lactide (PLLA) and an elastomeric and biodegradable poly(ethyleneoxide)-poly(butyleneterephthalate)(PEO-PBT++ +)copolymer, called Polyactive, were chosen as the constituents of the matrix. The substrate properties of the bilayers for human and rat epidermal keratinocytes and dermal fibroblasts were assessed. Keratinocytes attached and expanded into confluent sheets on both the routine cell culture plastic (TCPS) and the experimental substrates. Morphology of the cells cultured on the biomaterials was found to be comparable with the morphology of those grown on TCPS. In contrast to dense films, porous PEO:PBT copolymer and PLLA appeared poor substrates for fibroblasts. Long-term (in vivo) degradation of the biomaterials was mimicked in vitro to screen the biomaterials for any release of toxic substances. Culturing keratinocytes and fibroblasts in media based on the artificially aged biomaterials did not result in any negative effects on proliferative activity or morphological appearance of the cells.

Effect of implantation site on phagocyte/polymer interaction and fibrous capsule formation

Implants of Silastic, Estane, polypropylene oxide and an HPOE/PBT segmented polyether polyester copolymer were qualitatively and quantitatively evaluated, with respect to interaction with mononuclear and multinucleated phagocytes as well as fibrous capsule formation, after implantation at three sites in the rat middle ear. The volume of the phagocyte exudate surrounding the implants, the degree of implant degradation and fragmentation and the thickness of the fibrous capsules were found to be correlated with the implantation site. From these findings, it can be concluded that it is important to assess the biological performance of a biomaterial at carefully chosen implantation sites.

Polyacids as bonding agents in hydroxyapatite polyester-ether (Polyactive 30/70) composites.

A previously developed method to improve the interface between hydroxyapatite (HA) and a polyester-ether (PolyactiveTM 70/30) by using polyacrylic acid or poly(ethylene-co-maleic acid) has been applied to HA/PolyactiveTM 30/70 composites noting that polyactiveTM 30/70 contains less polyethylene glycol (PEG) segments and a higher concentration of rigid poly(butylene terephthalate) (PBT) segments. The mobility of the PEG segments is significantly affected by the existence of a high concentration of rigid PBT segments. Our experimental results show that this method is indeed suitable for making HA/PolyactiveTM 30/70 composites. The hydrogen bond/dipole interaction forming ability of the PEG segment is not affected by the existence of relatively large amounts of PBT segments. By using these coupling agents, the mechanical properties of the composite can be significantly improved both in dry and wet states. A fractographical study of the fracture surfaces revealed that the surface modified HA particles maintain better contact at fracture. It also showed that larger HA particles may initiate cracks and that such particles may be responsible for a decrease in the tensile strength of the composites.

The bone-bonding polymer Polyactive 80/20 induces hydroxycarbonate apatite formation in vitro.

The bone-bonding polymer known as Polyactive is a block copolymer composed of a polyethylene oxide (PEO) soft segment and a polybutylene terephthalate (PBT) hard segment. This study focuses on the in vitro induction of hydroxyapatite by Polyactive. Our results show that Polyactive is capable of inducing hydroxycarbonate apatite (HCAp) formation from a metastable calcium phosphate solution analogous to a physiological solution. In a 4-day incubation, the HCAp formation extended approximately 100 microm deep from the surface. A great number of globules about 1 microm large were found in the calcified Polyactive. These globules were composed of HCAp crystals embedded in the polymer matrix. There were so many globules in the surface that they connected with each other and formed a calcified layer. Next to the calcified layer was a zone where the globules were scattered. The calcified surface may have acted to promote HCAp growth from the solution, bringing about the formation of a HCAp layer on top of the calcified layer. The transition of solid Polyactive into a Polyactive hydrogel in calcium phosphate solution permitted HCAp formation within the polymer. It is proposed that the COOH groups produced during hydrolysis of Polyactive play an important role in nucleating hydroxyapatite. A remarkable affinity of the PEO segment of the polymer for calcium ions may facilitate moving calcium and phosphate from the solution into the polymer for the growth of HCAp.

Reactions of cells at implant surfaces

The surface of implants is an important parameter in host-implant integration. Several strategies can be used to obtain integration, such as the application of grooves or pores at the implant surface. Most of these surface alterations, however, will lead to an increase of total implant surface area which might influence the inflammatory response to an implant. As far as integration with bone is concerned several biomaterials have been successful in mimicking this material, by having similar crystals at their surface (calcium phosphate ceramics) or by containing a certain amount of calcium and phosphorus. Polyactive, a poly(ethylene oxide)-poly(butylene terephthalate) segmented copolymer, also possesses favourable integration properties with bone, but initially lacks calcium and phosphorus. It is proposed that the application of hydrogels as biomaterial may add a new dimension to integration capacity.

A new biodegradable matrix as part of a cell seeded skin substitute for the treatment of deep skin defects: a physico-chemical characterisation

A bilayered matrix, as an integral part of a cell seeded skin substitute for deep dermal skin injury, is described. The skin substitute has been given a dense top layer, to function as a substrate for keratinocyte culture, and a porous under layer for wound adhesion and to serve as a template for dermal regeneration. This porous under layer may be seeded with dermal fibroblasts for improvement of neodermis formation. The elastomeric hydrogel Polyactive¿, a biodegradable polyether/polyester block copolymer, has been selected as the top layer component and Polyactive or Poly-L-lactide (PLLA) as the constituents of the under layer; this to ensure that the matrix is degradable and fulfils certain mechanical requirements. Polyactive top layers were found to be highly permeable to water vapour (+/-30 g/m -2 h -1 kPa) -1 and to molecules up to 150 kD (human IgG). Tensile properties of the bilayered matrices were to a considerable extent dependent on top layer and under layer composition and thickness. Elasticity moduli were in the range of those previously reported for human skin. Varying the weight ratios of the soft/hard segments of the Polyactives and/or substituting PLLA for Polyactive as the under layer component, allows the modulation of the matrix to specific application site dependent demands.

Tympanic membrane structure during a Staphylococcus aureus-induced middle ear infection. A study in the rat middle ear.

In response to a Staphylococcus aureus-induced middle ear infection the tympanic membrane showed infiltration of polymorphonuclear granulocytes, lymphocytes, and macrophages and increased areas covered by ciliary and secretory epithelium. These reactions, which were comparable to the cellular and mucociliary responses seen in the middle ear mucosa during infection, were restricted to the pars flaccida and to predominantly the annular and manubrial regions of the pars tensa. This showed that the greater part of the tympanic membrane, where the lamina propria is composed of collagenous bundles and only very thin layers of loose connective tissue, is hardly affected by or barely responds to the inflammatory stimulus.

Polyactive: A Bone-Bonding Polymer

Polyactive a 55/45 poly(ethylene oxide)/poly(butylene terephthalate) copolyether ester, was investigated as regards its general in vitro and in vivo biocompatibility. Special attention was directed to the interactions of the polymer with bone.

Biodegradation and Phagocyte/Polymer Interaction

The degree of implant degradation was found to be correlated with both the implantation site and the properties of the implant materials under study. Implant degradation was found to be associated with interactions between the implants and macrophages or foreign-body giant cells: the numbers of these phagocytes varied with the implantation site in patterns corresponding with those accompanying implant degradation, and the morphology of the phagocytes also varied with the degree of implant degradation.