Tissue Response

The tissue response to polymers is investigated in a variety of studies [200, 211-214].

There are different attributes, which complicate the use of polymer vascular implants. Especially for small-diameter vascular grafts the blocking is an important problem. For example, calcification decreases the vascular diameter. So calcification of subdermally implanted polyurethane is enhanced by calciphylaxis as shown by SEM [194]. Another aim is to decrease the thrombogenicity [195] of polymeric biomaterials. These "heparin-like" polymers have a specific affinity for antithrombin III and thus are able to catalyze the inhibition of thrombin. In this study sulfur, sodium, fluorine, and carbon are determined by SEM combined with energy dispersive X-ray analysis. The objective of another project [196] was to define a series of assays for the evaluation of hemocompatibility of cardiovascular devices. In [193] a polyamino-acid urethane copolymer coated vascular prosthesis (1.5 mm diameter) was developed to enhance endothelialization. SEM showed the homogeneous coating of clinically available synthetic PTFE grafts. Prosthetic heart valves made of isotropic pyrolytic carbon (LTIC) are a successful biomaterial application which is investigated together with other difficult-to-image biomaterials in [17]. Low-voltage, high-resolution scanning electron microscopy shows that LTIC valve leaflets are significantly rougher than previously described, and that LTIC induces extensive platelet spreading in vitro, even in the presence of considerable albumin.

Calcification is also crucial in the use of intraocular lenses [201]. To assess this, intraocular lens optic materials were implanted intramuscularly and/or subcutaneously in rabbits for up to 90 days. SEM and energy dispersive X-ray spectroscopy (EDX) were used to detect discrete nodules containing both calcium and phosphate. Calcification only was noted on intramuscularly, subcutaneously, and intraocularly implanted experimental acrylic and intramuscularly implanted hydrogel material. In contrast, the intramuscularly or subcutaneously implanted silicon, PMMA, and acrylic optic materials showed no calcification.

Glassy polymeric carbon was treated by bombardment with energetic ions in different curing states. The roughness that leads to a better thromboresistance was monitored with SFM [18].

An approach to manage chronic osteomyelitis (inflammation of the periosteum) utilizes the implementation of antibiotic-impregnated PMMA beads for local delivery of antibiotics. The study of the getamicin sulphate release from a commercial acrylic bone cement was presented in [197].

A very interesting feature is the biodegradability of some polymers either natural or synthetic. The surface structure of blends with different composition of poly lactic acid (PLA) and poly sebaic acid (PSA) and their degradation properties were investigated. Single component PLA exhibits a smooth surface whereas PSA films possess spherulites at the surface. Blends of 70%, 50%, and 30% PLA exhibited granular respectively pitted surface structures but no fibrous structures visible in SFM. The monitoring of the erosion within 10 minutes showed that PLA is present in granules [16]. Phase imaging with the SFM yields phase shifts at different compositions ranging between 45° and 52°. The presence of PSA microdomains in blend of 50% PSA could be confirmed [215].

Screws, made of poly-L-lactic acid, were inserted axi-ally into the right distal femur in 18 rabbits. The degradation and phagocytosis were assessed histologically and by TEM [198]. In the TEM specimens, polymeric particles of an average area of 2 /m2 were seen to be located intercellularly with phagocytic cells. In 4.5-year specimens, the size of the polymeric particles, measured as area and perimeter, was significantly smaller than that of the 3-year specimens. The findings indicate that the ultimate degradation process of PLA is much longer than it was previously thought. Porous structured polymers to mimic natural extracellular matrix were investigated in [216] and in [217]. A biodegradable blend of starch with ethylene-vinyl alcohol copolymer (SEVA-C) with hydroxyapatite as filler was investigated, monitoring the degradation and the buildup of calcium phosphate crystals over a time span of up to 30 days in simulated body fluid. Only with the filler the buildup of the calcium phosphate layer could be observed. Within 8 hours the roughness increased, after 24 hours calcium phosphate nuclei covered the surface, and after 126 hours a dense and uniform layer was present [218].

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