Jukka Tuominen

Chain extending of lactic acid oligomers. 2. Increase of molecular weight with 1,6-hexamethylene diisocyanate and 2,2′-bis(2-oxazoline)

Abstract Lactic acid polymers were synthesised from oligomers by the addition of highly reactive 1,6-hexamethylene diisocyanate (HMDI) and 2,2′-bis(2-oxazoline) (BOX) as chain extenders. The effects were studied of adding the extenders simultaneously and sequentially and in different amounts. The reactions were followed and the polymers obtained were structurally characterised by size exclusion chromatography and spectroscopy (NMR and FTIR). High molecular weight poly(lactic acids) were obtained by both simultaneous and sequential addition. The mode of addition and the amounts of extenders had a considerable effect on the branching. The addition of HMDI before BOX in the sequential linking produced more highly branched polymers than did the simultaneous addition. Branching and crosslinking reactions were identified as side reactions of the chain extending reactions. The gel formation was attributed to the reactions of BOX and HMDI with urethane and amide but not with oxamide.

Chain extending of lactic acid oligomers. Effect of 2,2′-bis(2-oxazoline) on 1,6-hexamethylene diisocyanate linking reaction

Abstract New l -lactic acid polymers were synthesised with the use of highly effective carboxyl and hydroxyl reactive chain extenders. The chain extending behaviour of 2,2′-bis(2-oxazoline) (BOX) and 1,6-hexamethylene diisocyanate (HMDI) with two different oligomers was followed by means of acid value (AV) and molecular weight increments measured by size exclusion chromatography (SEC). Both chain coupling agents were found to react selectively with end-groups, forming oxamide and urethane groups as characterised by spectroscopy (FTIR and 1H NMR). Reaction between BOX and carboxyl groups of lactic acid oligomer led to hydroxyl terminated prepolymer with low AV, which provided significant increase of molecular weight in the HMDI linking reaction. In addition, an improvement in the thermal stability of the resulting polymers was observed with excess amount of BOX. The material properties of the polymers were characterised by dynamic mechanical thermal analysis (DMTA), dynamic rheometry and tensile testing. Introduction of oxamide groups into the polymer structure increased the chain stiffness, which was detected in mechanical properties and increment in glass transition temperature.

Resorbable composites with bioresorbable glass fibers for load-bearing applications. In vitro degradation and degradation mechanism.

An in vitro degradation study of three bioresorbable glass fiber-reinforced poly(l-lactide-co-dl-lactide) (PLDLA) composites was carried out in simulated body fluid (SBF), to simulate body conditions, and deionized water, to evaluate the nature of the degradation products. The changes in mechanical and chemical properties were systematically characterized over 52 weeks dissolution time to determine the degradation mechanism and investigate strength retention by the bioresorbable glass fiber-reinforced PLDLA composite. The degradation mechanism was found to be a combination of surface and bulk erosion and does not follow the typical core-accelerated degradation mechanism of poly(α-hydroxyacids). Strength retention by bioresorbable glass fiber-reinforced PLDLA composites can be tailored by changing the oxide composition of the glass fibers, but the structure-property relationship of the glass fibers has to be understood and controlled so that the phenomenon of ion leaching can be utilized to control the degradation rate. Therefore, these high performance composites are likely to open up several new possibilities for utilizing resorbable materials in clinical applications which could not be realized in the past.

Hydrolysis of lactic acid based poly(ester-urethane)s

The hydrolysis behaviour of lactic acid based poly(ester-urethane)s has been studied in a buffer solution of pH 7·00 at 37 and 55°C. Samples were prepared using a straight two step lactic acid polymerization process. The lactic acid was first polymerized by condensation with a low molecular weight by hydroxyl terminated telechelic prepolymer and the molecular weight then was increased with a chain extender such as a diisocyanate. In the hydrolysis study, the effect on the hydrolysis rate of different stereostructures (different amount of D-units in the polymer chain) and the length of the ester units were studied. The rate of hydrolysis was examined by various techniques including weighing (water absorption and weight loss), GPC (molecular weight and polydispersity), and DSC (thermal properties). GPC measurements showed that at 37°C the weight average molecular weight of the poly(ester-urethane)s started to decrease slowly during the first week of hydrolysis, but that at 55°C the weight average molecular weight decreased dramatically during the first week of hydrolysis. Significant mass loss occurred later at both temperatures.

In vitro behaviour of three biocompatible glasses in composite implants

Poly(l,dl-lactide) composites containing filler particles of bioactive glasses 45S5 and S53P4 were compared with a composite containing a slowly dissolving glass S68. The in vitro reactivity of the composites was studied in simulated body fluid, Tris-buffered solution, and phosphate buffered saline. The high processing temperature induced thermal degradation giving cavities in the composites containing 45S5 and S53P4, while good adhesion of S68 to the polymer was observed. The cavities partly affected the in vitro reactivity of the composites. The degradation of the composites containing the bioactive glasses was faster in phosphate buffered saline than in the two other solutions. Hydroxyapatite precipitation suggesting bone tissue bonding capability was observed on these two composites in all three solutions. The slower dissolution of S68 glass particles and the limited hydroxyapatite precipitation suggested that this glass has potential as a reinforcing composition with the capability to guide bone tissue growth in biodegradable polymer composites.

Self-reinforcement and hydrolytic degradation of amorphous lactic acid based poly(ester-amide), and of its composite with sol-gel derived fibers.

The self-reinforcing and hydrolytic degradation of an amorphous poly(ester-amide) (PEA) based on lactic acid have been studied and compared with those of poly-L-lactide (PLLA). The studied PEA-rods were self-reinforced (SR) by solid-state die drawing resulting double shear strength. The hydrolytic degradation of PEA was studied during exposure to phosphate buffered saline at pH 7.4 and at 37 °C for 18 weeks. The degradation and mechanical properties of PEA were also followed in a self-reinforced composite structure consisting of PEA and sol-gel derived SiO2-fibers (SGF, 8 wt %). The hydrolytic degradation of the SR-PEA-rods with and without SG-fibers was significantly faster than that of SR-PLLA-rods. The weight average molecular weight (Mw) of PEA decreased by 90% from the initial Mw during the first 6 weeks in hydrolysis, when the Mw of the PLLA decreased by 10%.

Dissolution behavior of high strength bioresorbable glass fibers manufactured by continuous fiber drawing

This article describes the dissolution behavior of three silica-based resorbable glasses manufactured by an industrial-type continuous fiber drawing process yielding fibers with tensile strength of 1800-2300MPa. The results of a long-term in vitro degradation testing of the manufactured high strength bioresorbable glass fibers are presented. The degradation was performed by exposing the glass fibers to SBF and TRIS for 26 weeks at physiological conditions at 37°C. All fibers showed continuous resorption throughout the study and two of the fibers revealed bioactivity by forming a calcium phosphate (CaP) layer in SBF.