Controlling the switching temperature of biodegradable, amorphous, shape-memory poly(rac-lactide)urethane networks by incorporation of different comonomers.

Biodegradable shape-memory polymers have attracted tremendous interest as potential implant materials for minimally invasive surgery. Here, the precise control of the material's functions, for example, the switching temperature T(sw), is a particular challenge. T(sw) should be either between room and body temperature for automatically inducing the shape change upon implantation or slightly above body temperature for on demand activation. We explored whether T(sw) of amorphous polymer networks from star-shaped rac-dilactide-based macrotetrols and a diisocyanate can be controlled systematically by incorporation of p-dioxanone, diglycolide, or epsilon-caprolactone as comonomer. Thermomechanical experiments resulted that T(sw) could be adjusted between 14 and 56 degrees C by selection of comonomer type and ratio without affecting the advantageous elastic properties of the polymer networks. Furthermore, the hydrolytic degradation rate could be varied in a wide range by the content of easily hydrolyzable ester bonds, the material's hydrophilicity, and its molecular mobility.

The influence of a multifunctional, polymeric biomaterial on the concentration of acute phase proteins in an animal model.

The concentrations of the acute phase proteins alpha1-Acid Glycoprotein (AAG) and haptoglobin were determined in Sprague-Dawley-rats after implantation of a novel biodegradable multifunctional polymeric biomaterial for the reconstruction of a gastric wall defect (polymer group; n=42). For comparison, the concentrations of AAG and haptoglobin were measured as well after primary wound closure of the gastric wall defect without biomaterial implantation (control group; n=21) and in rats without any surgical procedure (baseline group; n=21). The implantation periods were 1 week, 4 weeks and 6 months. The concentrations of AAG and haptoglobin were measured by an ELISA assay. Gastrointestinal complications like fistula, perforation or peritonitis did not occur in any of the animals. No statistically significant differences in the concentrations of AAG and haptoglobin were detected between the polymer and the control group. An adequate mechanical stability of the polymeric biomaterial was detectable under the extreme pathophysiological conditions of the stomach milieu. In further examinations the correlation between the intraperitoneal cytokine levels of the animals and the following systemic inflammatory markers should be analysed. Further investigations are needed to analyse the mechanisms of the tissue integration of a biomaterial as well as the process of the tissue remodeling and the influence of the immune system on these mechanisms. The knowledge of these processes is necessary to adapt the multifunctional biomaterial and prepare it thus for the use and implantation in different body locations and to develop novel therapeutical options in medicine.

[First results of the investigation of the stability and tissue integration of a degradable, elastomeric copolymer in an animal model]. / Erste Ergebnisse zur Untersuchung der Stabilität und Gewebeintegration eines abbaubaren, elastischen Copolymers im Tiermodell.

The stability and tight integration into adjacent tissue of a novel, degradable, elastic copolymer were examined in an animal model. The biomaterial was used for the reconstruction of a gastric wall defect in Sprague-Dawley rats (n=42) to test the polymeric material under the extreme chemical, enzymatical and mechanical conditions of the stomach. In the control group (n=21) the same defect of the gastric wall was primarily closed without biomaterial implantation. In the baseline group (n=21) the animals were kept under standard conditions without any surgical procedure. The implantation periods were 1 week, 4 weeks and 6 months. The animals' weight was determined preoperatively and before explantation. After explantation, air was pumped into the stomach and the pressure was measured by using a pressure-gauge in order to test whether the surgically produced union of the stomach wall and the polymer patch was gas-tight. After 1 week of implantation time a statistically significant increase of the body weight of the animals was found only in the baseline group. Four weeks and 6 months after the abdominal surgical procedure, a statistically significant increase of the animals' weight was found in the implantation group, the control and the baseline group. Gastrointestinal complications like fistula, perforation or peritonitis did not occur in any of the animals. The measurement of the stomach pressure after maximal gas insufflation did not show significant differences between the implantation group, the control and the baseline group in any of the time periods investigated. Despite very high strains of the gastric wall, no gas leakage was detected. There was a tight connection between the polymer and the adjacent stomach wall in all animals investigated. An adequate mechanical stability of the biomaterial was detectable under the extreme pathophysiological conditions of the stomach milieu. A fast and unfavourable degradation of the degradable polymer was not found in any of the animals. Further investigations are needed to analyse the mechanisms of the tissue integration of the biomaterial as well as the degradation kinetic of the polymer and the process of the tissue remodeling. The knowledge of these processes is necessary to adapt the novel biomaterial and thus prepare it for the use and implantation in different body locations and to develop novel therapeutical options in medicine.

Shape-memory polymers.

Material scientists predict a prominent role in the future for self-repairing and intelligent materials. Throughout the last few years, this concept has found growing interest as a result of the rise of a new class of polymers. These so-called shape-memory polymers by far surpass well-known metallic shape-memory alloys in their shape-memory properties. As a consequence of the relatively easy manufacture and programming of shape-memory polymers, these materials represent a cheap and efficient alternative to well-established shape-memory alloys. In shape-memory polymers, the consequences of an intended or accidental deformation caused by an external force can be ironed out by heating the material above a defined transition temperature. This effect can be achieved because of the given flexibility of the polymer chains. When the importance of polymeric materials in our daily life is taken into consideration, we find a very broad, additional spectrum of possible applications for intelligent polymers that covers an area from minimally invasive surgery, through high-performance textiles, up to self-repairing plastic components in every kind of transportation vehicles.

Biodegradable multiblock copolymers based on oligodepsipeptides with shape-memory properties.

Thermoplastic phase-segregated multiblock copolymers with polydepsipeptides and PCL segments were prepared via coupling of diol and PCL-diol using an aliphatic diisocyanate. The obtained multiblock copolymers showed good elastic properties and a shape memory. Almost complete fixation of the mechanical deformation, resulting in quantitative recovery of the permanent shape with a switching temperature around body temperature, was observed. In hydrolytic degradation experiments, a quick decrease of the molecular weight without induction period was observed, and the material changed from elastic to brittle in 21 d. These materials promise a high potential for biomedical applications such as smart implants or medical devices.

Shape-memory polymer networks from oligo[(epsilon-hydroxycaproate)-co-glycolate]dimethacrylates and butyl acrylate with adjustable hydrolytic degradation rate.

Degradable shape-memory polymer networks intended for biomedical applications were synthesized from oligo[(epsilon-hydroxycaproate)-co-glycolate]dimethacrylates with glycolate contents between 0 and 30 mol % using a photopolymerization process. In addition AB copolymer networks were prepared by adding 60 wt % n-butyl acrylate as comonomer. All synthesized polymer networks are semicrystalline at room temperature. A melting transition T(m) between 18 and 53 degrees C which can be used as switching transition for the shape-memory effect can be attributed to the crystalline poly(epsilon-hydroxycaproate) phase. At temperatures below T(m) the elastic properties are dominated by these physical cross-links. At temperatures higher than T(m) the E modulus of the amorphous polymer networks is lowered by up to 2 orders of magnitude, depending on the chemical cross-link density. Copolymer networks based on macrodimethacrylates with a M(n) of up to 13,500 g x mol(-1) and a maximum glycolate content of 21 mol % show quantitative strain recovery rates in stress-controlled cyclic thermomechanical experiments. Hydrolytic degradation experiments of polymer networks performed in phosphate buffer solution at 37 degrees C show that the degradation rate can be accelerated by increasing the glycolate content and decelerated by the incorporation of n-butyl acrylate.