Polymers for biodegradable medical devices. 1. The potential of polyesters as controlled macromolecular release systems

Resorbable polymers for internal fixation

Bone fractures are usually fixed with metallic internal and/or external fixation devices. Such devices have to be removed from the implantation site, once the bone fracture is healed. Metallic devices may also corrode in vivo releasing ions into the tissues. An ideal internal fixation device should allow adequate fracture fixation, immediate recovery of mobility of the limb, and be produced from a material which can be safely left at the implantation site when the fracture healing is completed. Resorbable polymers are candidates for construction of such devices.

Resorbable polyesters: composition, properties, applications

Synthetic polymers, in particular polyesters based on lactic acid, glycolic acid and other raw materials are described for medical and technical applications. Their main advantages are compatibility and safety. As they are degradable in the human body they are particularly suitable for implants needed only temporarily for the healing process. Resorbable polyesters are produced by direct polycondensation or ring opening polymerization resulting in a broad range of polymers with different features. The main properties such as degradation (hydrolytic and thermal), crystallinity and mechanical strengths are discussed. These polyesters, which can be processed like other thermo-plastic polymers by extrusion and injection moulding, are used mainly for wound closure products (sutures, staples, clips), fracture fixation devices (nails, screws, plates) and parenteral depot systems for the sustained release of drugs. Possibilities for polymer and process improvement to develop better implants and additional applications are presented.

Salt formation of lactic acid oligomers as matrix for sustained release of drugs

The calcium and sodium salts of L- and D,L-lactic acid oligomers were obtained by immersion of the oligomer powders (weight average mol. wt 3000-10,000) in aqueous solutions of CaCl2 and NaCl, respectively, at room temperature (22 degrees C). The salt formation was analysed by atomic absorption measurement. The thermal properties, including glass transition temperature, melting temperature, and softening temperature, were altered by conversion of the terminal free acid of the oligomer to its salt. An in-vitro release test of an anti-cancer drug from the oligomer beads showed that the drug was released more rapidly from the beads prepared from the non-converted oligomers than those prepared from the salts when the release test was carried out in media containing no calcium and no sodium ions.

Poly(hydroxybutyrate-hydroxyvalerate) microspheres containing progesterone: preparation, morphology and release properties

The biodegradable polyesters, poly(hydroxybutyrate) (PHB) and poly(hydroxybutyrate-hydroxyvalerate) (PHBV) were investigated for use as sustained delivery carriers of a model drug, progesterone. Spherical microspheres containing the drug were prepared by an emulsion solvent-evaporation method with gelatin as an emulsifier. Methylene chloride as the polymer solvent yielded smoother microspheres than chloroform. The surface texture was also dependent upon the temperature of the preparation and polymer used. Surface crystals were observed when the drug loading was increased beyond 5 per cent w/w. Thermograms of the microspheres did not show an endotherm corresponding to the melting of the drug because the drug dissolved in the melted polymer while heating. The amount of residual solvent in the microspheres (gas chromatographic assay) ranged from 3.4 to 58.4 ppm and was dependent on the processing temperature, concentration of the polymer in the solvent and the polymer composition. In vitro release of the drug was slowest from microspheres made from copolymer containing 9 per cent hydroxyvalerate. A less porous microsphere matrix was formed by this copolymer.

X-ray photoelectron spectroscopy analysis of the surface chemical structure of some biodegradable poly(orthoesters)

The surface analysis of some biodegradable poly(orthoesters) has been undertaken using X-ray photoelectron spectroscopy (XPS). There is very good agreement between the experimental and theoretical surface compositions. All the carbon environments of the poly(orthoesters) are detected within the high-resolution C1s peak envelopes and again, good agreement is observed between the theoretical and experimental proportions of each carbon environment. The data confirm the high surface purity of these polymers and demonstrate the potential for the use of XPS in future studies of the solid state in-situ degradation of poly(orthoesters).

In vitro degradation of a PHB/PHV copolymer and a new technique for monitoring early surface changes

The use of polyhydroxbutyrate and its copolymers as degradable materials for surgical use is showing increasing promise due to their long degradation profile. Of considerable interest were the interfacial changes and their role in affecting the overall or bulk degradation pattern. The role of pH and its effect on the degradation pattern were examined as polymer degradation significantly altered, depending on whether it was subjected to an alkaline or an acidic medium. In vitro surface changes were monitored using two techniques (scanning electron microscopy and goniophotometry); bulk changes were monitored by gravimetric, mechanical and molecular weight techniques. It was concluded that the bimodal molecular weight distribution plays a significant part in the degradation pattern and that this pattern is also considerably altered by pH.

Biocompatibility of poly (DL-lactic acid/glycine) copolymers

In this review the authors discuss the polymer chemical, physical and cell biological aspects of poly (DL-lactic acid/glycine) copolymers, both in vitro and in vivo. The mechanism and rate of degradation and the degree of foreign body reaction were evaluated as a function of the molecular composition of the (co)polymer, its initial molecular weight and changes in crystallinity. Data from the literature concerning poly(lactic acid), poly(glycolic acid) and poly(amino acids) are included in this review. The choice to apply the polymers mentioned was determined by their nature: all are present in the human body as natural residues. Upon degradation, biocompatibility will thus not be impaired. The authors conclude that the degradation mechanism of poly(lactic acid), poly(glycolic acid) and poly(amino acids) are similar, i.e. bulk hydrolysis of ester bonds. The initial molecular weight and the chemical composition, determine the rate of degradation and the degree of foreign body reaction.

In vitro model for the degradation of alkylcyanoacrylate nanoparticles

A photometric assay was developed to study the surface erosion of polymeric nanoparticles. The hydrolytic degradation of polyalkylcyanoacrylate particles was studied in different environments (NaOH, buffer, cell culture medium and serum). The influence of particle modification on the degradation rate was assessed. Particularly, the effect of polymer coating for particle targeting and fluorescence labelling was investigated. From the absorption data, a t50% and t100% can be calculated for fast degrading particles and obtained by an extrapolation in case of a slow degradation process. The degradation rate was found to decrease with increasing alkyl chain length from methyl-, ethyl-, isobutyl- to isohexylcyanoacrylate particles. Polymer coating and fluorescent labelling had little effect on the rate of degradation.

In vitro degradation of glycine/DL-lactic acid copolymers

The in vitro degradation of glycine-DL-lactic acid copolymers was studied as a function of the composition. These polydepsipeptides were prepared by ring-opening copolymerization of 6-methyl-2,5-morpholinedione and DL-lactide. The degradation of discs of the copolymers was performed in a phosphate buffer at pH 7.4 and 37 degrees C. The decrease in molecular weight and weight was determined until complete weight loss had occurred. Poly(DL-lactide) was used as a reference material. All (co)-polymers show an immediate decrease in molecular weight, whereas the weight remains almost unchanged during a longer period of time. Decrease in weight started earlier as the glycine content of the co-polymer increased. The lactic acid content of the residual material increased during the weight loss showing a higher solubility of polymer fragments with a relatively high content of glycine residues. From the hydrolysis constants it was concluded that the degradation was best described by hydrolysis of ester bonds via a bulk erosion process, autocatalyzed by the generated carboxylic acid end groups. The rate constants varied from 4-7 X 10(-2) (day-1) for all (co)polymers. All (co)polymers show an increase in the molecular weight distribution upon weight loss.

XPS and SSIMS analysis of the surface chemical structure of poly(caprolactone) and poly(beta-hydroxybutyrate-beta-hydroxyvalerate) copolymers

The surface chemical structures of poly(beta-hydroxybutyrate), poly(caprolactone) and poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate) have been analysed using static secondary ion mass spectrometry and X-ray photoelectron spectroscopy. The X-ray photoelectron spectroscopy data confirm the purity of the polyester surfaces and there is close agreement between the stoichiometric and experimentally determined ratios of the peaks and different carbon environments within the C1s envelopes. The static secondary ion mass spectrometry analysis reveals general fragmentation pathways which permit the ready distinction between the different polyesters examined. The differentiation of the different monomer repeat units in the copolymer together with the detection of some ions representative of the random copolymer sequence are also possible in the static secondary ion mass spectrometry analysis.

In vivo and in vitro degradation of glycine/DL-lactic acid copolymers

A series of copolymers of glycine and DL-lactic acid with various compositions was synthesized and their in vivo and in vitro degradation behavior was studied. For the in vivo examination, discs of the copolymer films were subcutaneously implanted in rats. The in vitro studies were carried out in phosphate buffer at pH = 7.4 and 37 degrees C. The decrease in molecular weight, the loss of weight, and the tissue reactions of the different copolymers were determined after 2, 5, and 10 weeks. Poly(DL-lactic acid) was used as reference material. The in vivo and in vitro degradation behavior of the polymers was comparable. The decrease of molecular weight of the copolymers and poly(DL-lactic acid) in time was similar. The weight loss for copolymers with a higher mole fraction of glycine units started earlier. The copolymer with the highest content of glycine units disappeared completely within 10 weeks both in vivo and in vitro. The poly(DL-lactic acid) implant lost only 25% weight over the same period. Tissue reactions against all materials started with an acute inflammatory reaction caused by the trauma of implantation, followed by wound-healing processes, ending in a very mild foreign body reaction for the poly(DL-lactic acid) and a more excessive macrophage mediated foreign body reaction for the glycine/DL-lactic acid copolymers. The tissue reaction was more severe for polymers having a higher rate of degradation.

Polymers for biodegradable medical devices. VI. Hydroxybutyrate-hydroxyvalerate copolymers: accelerated degradation of blends with polysaccharides

The hydrolytic degradation of poly(hydroxybutyrate)-poly(hydroxyvalerate) (PHB-PHV) copolymers in the form of blends with the polysaccharides amylose, dextran, dextrin and sodium alginate, has been studied under a range of conditions (pH 2.3, 7.4 and 10.6 and at 37 degrees C and 70 degrees C). The hydrolytic degradation of the PHB-PHV copolymers was found to be dramatically affected by the presence of polysaccharides. Its progress was characterized by an initial increase in the wet weight, with concurrent decrease in the dry weight as the polysaccharides eroded from the matrix. Surface energy measurements and goniophotometry proved to be particularly useful in monitoring this stage of the degradation process. The concurrent increase in internal porosity leads to the eventual collapse of the matrix, a process which occurs, but less rapidly, in the degradation of the unblended PHB-PHV copolymers. Information obtained from molecular weight and crystallinity studies enabled a comprehensive profile of the overall degradation process to be built up.

Polymers for biodegradable medical devices. III. Polymerization and copolymerization of cyclic derivatives of tartronic acid

The syntheses of anhydrosulphite and anhydrocarboxylate derivatives of tartronic acid are described. These compounds, more correctly named 5-carboxy-1,3,2-dioxathiolan-4-one-2-oxide and 5-carboxy-1,3-dioxolan-2,4-dione have been shown to undergo polymerization and copolymerization to poly-alpha-esters containing the tartonic acid residue, characterized by the presence of a pendant carboxyl group. Thermal polymerization of the anhydrosulphite appears to proceed in a substantially identical manner to other members of the series in which the monomer decomposes by a first order process to yield an alpha-lactone which polymerizes by a rapid chain growth reaction. Because the rate of thermal polymerization of tartronic acid anhydrosulphite is much more rapid than simple alkyl substituted anhydrosulphites, copolymerization favours the former residues. More successful copolymerization was achieved by the use of tertiary base initiators and the anhydrocarboxylate derivative of tartronic acid. The polymers and copolymers described are of potential value in the synthesis of drug-carrying biodegradable matrices or more hydrophilic analogues of poly(glycolic acid) which combine the known bioerodibility of the moiety with that presence of a pendant carboxyl group.

Polymers for biodegradable medical devices. II. Hydroxybutyrate-hydroxyvalerate copolymers: hydrolytic degradation studies

The hydrolytic degradation of poly(hydroxybutyrate) together with a series of hydroxybutyrate-hydroxyvalerate copolymers has been studied. The effects of copolymer composition and molecular weight are presented together with the results of varying pH and temperature on the degradation rate. Degradation has been monitored by weight loss and water uptake measurements together with goniophotometric, surface energy and scanning electron microscopic studies. Some comparisons with the more widely used so-called 'biodegradable' polymers, poly(glycolic acid), poly(dioxanone) and the glycolic-lactic acid (90:10) copolymers are presented together with the effect of blood plasma on the degradation process.