The Effects of Lipids on the Mechanical Properties of Polyglycolic Acid Sutures

Muscle Tissue as a Surrogate for In Vitro Drug Release Testing of Parenteral Depot Microspheres

Despite the importance of drug release testing of parenteral depot formulations, the current in vitro methods still require ameliorations in biorelevance. We have investigated here the use of muscle tissue components to better mimic the intramuscular administration. For convenient handling, muscle tissue was used in form of a freeze-dried powder, and a reproducible process of incorporation of tested microspheres to an assembly of muscle tissue of standardized dimensions was successfully developed. Microspheres were prepared from various grades of poly(lactic-co-glycolic acid) (PLGA) or ethyl cellulose, entrapping flurbiprofen, lidocaine, or risperidone. The deposition of microspheres in the muscle tissue or addition of only isolated lipids into the medium accelerated the release rate of all model drugs from microspheres prepared from ester-terminated PLGA grades and ethyl cellulose, however, not from the acid-terminated PLGA grades. The addition of lipids into the release medium increased the solubility of all model drugs; nonetheless, also interactions of the lipids with the polymer matrix (ad- and absorption) might be responsible for the faster drug release. As the in vivo drug release from implants is also often faster than in simple buffers in vitro, these findings suggest that interactions with the tissue lipids may play an important role in these still unexplained observations.

Bioresorbable polymers: heading for a new generation of spinal cages

The use of polymer-based bioresorbable materials is now expanding to the realm of spinal interbody fusion. Bioresorbable polymers have important advantages over metals, because they are temporary, much less stiff, and radiolucent. Most promising is a group of alpha-polyesters, in particular polylactide acids (PLAs). Their biocompatibility is excellent, and they have sufficient stiffness and strength to provide initial and intermediate-term stability required for bone healing. However, polylactides have characteristics that make them vulnerable to complications if not properly controlled. Degradation rate strongly depends on polymer type, impurities, manufacturing process, sterilization, device size, and the local environment. The fact that larger implants degrade faster is contra-intuitive, and should be considered in the design process. Also optimal surgical techniques, such as careful bone bed preparation, are required for a successful application of these materials. The purpose of this paper is to highlight the specific properties of these bioresorbable polymers and to discuss their potential and limitations. This is illustrated with early preclinical and clinical data.Bioresorbable cage technology is just emerging: their time-engineered degradation characteristics allow controlled dynamization in interbody applications, facilitating spinal fusion. Their radiolucency improves image assessment of fusion healing. Acceptance and use of bioresorbable implants may increase as further research and clinical studies report on their safety, efficacy, and proper usage.

The role of superoxide ions in the degradation of synthetic absorbable sutures

The objective of this study was to examine the effect of superoxide ion-induced degradation on synthetic absorbable biomaterials. Synthetic absorbable sutures were used as the model compounds. Inflammatory cells, particularly leukocytes and macrophages, are able to produce highly reactive oxygen species, such as superoxide (. O(2)(-)), during inflammatory reactions to foreign materials. Superoxide ions may act as oxygen nucleophile agents to attack biomaterials. In this study, the changes in tensile breaking force, thermal properties, and the surface morphology of five commercial (2/0 in size) synthetic absorbable sutures (Dexon, Vicryl, PDS II, Maxon, and Monocryl) as a function of superoxide ion concentration at 25 degrees C for 24 h were studied. Among the five absorbable sutures and over the concentration range of this study, the monofilament Monocryl suture was the most sensitive toward superoxide ion-induced degradation, followed by Maxon, Vicryl, Dexon, and PDS II sutures. The amount of tensile breaking force loss over a 24 h period ranged from as low as 3% to as high as 80%, depending on the type of absorbable sutures, the reaction time, and the superoxide ion concentration. All five absorbable sutures showed significant reductions in both the T(m) and T(g). Unlike the surface morphological changes of absorbable sutures in conventional buffer solutions, the effect of superoxide ion-induced degradation on the surface morphologies of these five absorbable sutures was unique, particularly the moon-crater-shaped impressions of various sizes and depths found in Monocryl and Maxon sutures, which defied the anisotropic characteristics of fibers.

Chemical stress relaxation of polyglycolic acid suture

Chemical stress relaxation methods are employed to study chemical and mechanical factors influencing the degradation of uncoated polyglycolic (PG) sutures. Specially constructed instrumentation is used to study the kinetics of the load bearing capability of PG (Dexon¿) 3-0 sutures in hydrolytic solution. The effects of pH, temperature, strain rate, and initial load on the rate of chemical stress relaxation are presented. Data show how mechanical factors such as the rate of loading (related to the speed of knot tying), as well as the final tension, are related to the rate of structural degradation. Maximum stability is observed at approximately 40 degrees C, with slower degradation both above and below this point. Results show that the slower and tighter the suture is pulled, the greater its ability to sustain tensile loads during hydrolysis.

Preliminary effects of in vitro lipid exposure on absorbable poly(ortho ester) films

Bioabsorbable films show promise in preventing postoperative interfacial tissue adhesion. Absorbable polymers in film form are generally more sensitive to chemical environments, due to their large surface area to volume ratio. The in vivo environment contains lipids such as cholesterol, triglycerides, and phospholipids, which are known to affect the degradation of permanent and absorbable polymeric biomaterials. Preliminary investigations of in vitro lipid exposure of bioabsorbable poly(ortho ester) (POE) films for implant use are described. POE has been studied previously for use in controlled drug delivery and fracture fixation. Six-week in vitro exposure of 65:35 POE films to a cholesterol emulsion (1 g/L) showed no apparent difference in hydrolytic degradation rates of mechanical properties or mass loss compared to deionized water exposed films. Decreases of 28 and 6% in inherent viscosity were observed after 5 weeks for cholesterol and deionized water exposed films, respectively, suggesting cholesterol may have some effect. Further examination of the data, due to sample variation, revealed that clear, uniform films showed only minor changes in mass loss and mechanical properties after 6 weeks in either of the in vitro media. But slightly cloudy films possessing microscopic bubbles showed accelerated degradation in both media, indicating the effect of cholesterol was inconclusive due to sample variation. Control of the microbubble formation process could have utility in controlling hydrolytic degradation of POE films.

Molecular biointeractions of biomedical polymers with extracellular exudate and inflammatory cells and their effects on the biocompatibility, in vivo

The stability of biomedical polymers in physiological environments is crucial for the normal operation of devices, as well as determining their effect on the tissue response. Degradation is an important factor in polymer biocompatibility, since the environment of the human body can be aggressive to polymers. Most implanted polymers suffer degradation to some extent, and the kinetics and mechanisms of the processes can be affected significantly by various biologically active species, especially enzymes, lipids, peroxides, free radicals and phagocytic cells. The degradation of poly(caprolactone) and poly(DL-lactic acid) under controlled in vivo conditions was studied using a poly(methyl methacrylate) chamber designed to control the exposure of polymers to physiological environments. In particular they may be designed to allow access of extracellular exudate only or access to cells as well as the fluid. The chambers, sealed with filters of pore size either 0.45 micron (impervious to cells) or 3.0 microns (allowing cells to enter the chamber), were implanted subcutaneously into experimental animals for 10, 20 and 30 wk periods. Degradation and molecular interactions of the polymers were characterized by gel permeation chromatography and scanning electron microscopy. The extracellular exudate formed within the implanted chamber is active in promoting the degradation of some biomedical polymers. Inflammatory cells are involved in the biodegradation of implanted polymers by releasing biologically active species such as free radicals into the area surrounding the implant. The data have demonstrated that the hydroxyl radical is likely to be one of the main causes of polymer degradation.

The effect of gamma irradiation on the enzymatic degradation of polyglycolic acid absorbable sutures

The purpose of the study was to examine the effect of gamma irradiation on the enzymatic as well as the in vivo degradation of polyglycolic acid sutures. The sutures of size 2-0 were irradiated at dosage levels of 0-20 mrad. The three enzymes chosen for this study were esterase, alpha-chymotrypsin, and trypsin. The irradiated sutures were both immersed in the enzyme solutions; their corresponding buffer controls, and implanted in inbred black-and-white hooded hister rats (Liverpool strain). The degradation of PGA sutures was determined mechanically. Among the three enzymes studied, esterase showed the highest enzymatic effect on the degradation of the unirradiated and irradiated PGA sutures. Trypsin's effect on PGA sutures was not observed until 20 mrad. The findings of trypsin demonstrated the hypothesis that synthetic high molecular weight polymers, which are initially resistant to enzymatic degradation, could become prone to enzymatic attack after altering their physical and chemical structures. Implanted PGA sutures maintained a similar or slightly higher mean tensile breaking strength in in vivo degradation compared to in vitro degradation (0.1M tris buffer of pH = 7.5); these degradation profiles suggest that PGA does not display similar behavior in in vivo and in vitro degradations. The magnitude of dissimilarity depends on the radiation dosage and on the duration of degradation, and is speculated to be attributable to the specific action of enzymes with respect to the configuration and chemical structure of the PGA sutures.