Jack Zhou

Morphological development in absorbable poly(glycolide), poly(glycolide-co-lactide) and poly(glycolide-co-caprolactone) copolymers during isothermal crystallization

Abstract Morphological development of the homopolymer, poly(glycolide), PGA; its random copolymers, poly(glycolide-co-lactide), PGA-co-PLA (5:95 and 90:10) and segmented block copolymer, poly(glycolide-co-caprolactone), PGA-co-PCL (75:25) during isothermal crystallization was studied using simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques with synchrotron radiation. It was found that the lamellar morphology could best describe the superstructure of these polymers. During crystallization, both average long period (L) and lamellar thickness (lc) exhibited notable decreases with time, which were attributed to the mechanism of secondary crystallization in the form of lamellar-stacks insertion. Both values of L and lc were found to increase with temperature. In the chosen crystallization temperature range (100–200°C), homopolymer PGA exhibited the fastest crystallization rate and the lowest values of L and lc, probably due to the largest degree of supercooling. As a result, in copolymers with higher content of PGA, the crystallization rate increased and the values of L and lc decreased. The value of amorphous layer thickness (la) was the highest in PGA-co-PLA (5:95), but those in PGA, PGA-co-PLA (90:10) and PGA-co-PCL (75:25) were about the same. In addition, the values of L, lc and the crystallinity were the highest in PGA-co-PLA (5:95). Corresponding degrees of crystallinity in PGA homopolymer and PGA-co-PCL (75:25) and PGA-co-PLA (90:10) copolymers were relatively low.

Crystallization study on absorbable poly(p-dioxanone) polymers by differential scanning calorimetry

An investigation was carried out on the crystallization behavior of p-dioxanone polymers using differential scanning calorimetry (DSC). Kinetic analyses were performed on data collected primarily during isothermal crystallization. Isothermal data were treated within the framework of the classical Avrami equation. Using this approach, both the Avrami exponent, n, and the crystallization half-time, t1/2, were evaluated and their implications are discussed for each system studied. It is shown that a small change in the polymers composition greatly affects the crystallization kinetics, as well as the crystallizability of the materials. Additionally, nonisothermal crystallization under controlled heating and cooling rates was explored. In the case of cooling from the melt, the Ozawa theory and the recently proposed Calculus method were employed to describe the nonisothermal crystallization kinetics. In view of our results, the validity of these two estimation techniques for determining important kinetic and morphological parameters is also discussed.

A study on in vitro degradation behavior of a poly(glycolide-co-L-lactide) monofilament.

The in vitro degradation behaviors of a poly(glycolide-co-l-lactide) 90/10 monofilament were investigated in phosphate buffer solution at pH 7.4 over a temperature range of 27.5-47.5 degrees C. The property changes of the monofilament with time at different temperatures were evaluated by tensile mechanical test, gel permeation chromatography analysis, and image techniques (optical microscopy, scanning electron microscopy and atomic force microscopy). The interrelationships among material properties, in vitro time and experimental conditions were explored. The results showed that the polymer monofilament gradually lost its tensile strength and molecular weight with increasing in vitro time. The hydrolytic degradation of the monofilaments followed a first order behavior. Higher temperatures accelerated the degradation process significantly. It was found that for a given tensile breaking strength retention (BSR), the dependence of degradation time on temperature could be illustrated by an Arrhenius-type equation, from which the activation energy was derived. Further analysis indicated that there are well-defined relationships between molecular weight and tensile strength, which could be illustrated mathematically. Finally, the microscopic evaluation of the monofilament samples revealed visible changes in morphology on the surface and cross-section area during degradation process. The results from atomic force microscopy showed that the surface roughness of the monofilament tended to increase with the in vitro time.

Time-resolved isothermal crystallization of absorbable PGA-co-PLA copolymer by synchrotron small-angle X-ray scattering and wide-angle X-ray diffraction

Abstract The isothermal crystallization behavior of absorbable dyed and undyed PGA- co -PLA copolymers was investigated by time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) methods with synchrotron radiation. The morphological parameters extracted from time-resolved SAXS profiles show that long period and lamellar thickness decrease slightly after primary crystallization. The unit cell parameters a and b and the apparent lateral crystal sizes L 110 and L 020 were extracted from the corresponding WAXD profiles. A significant decrease in the unit cell parameters and a substantial increase in the apparent crystal sizes are seen during the initial crystallization stage. Both scattering invariant ( Q from SAXS) and crystallinity ( X c from WAXD) results indicate that the crystallization rate is the fastest at 130°C. These copolymers show a bell-shape crystallization rate curve with temperature, where the dyed copolymer has a faster crystallization rate than the undyed one even though the inclusion of the low molecular weight organic dye is very small (ca. 0.2% by weight). We conclude that the dye molecule, which enhances the visibility during surgery, acts as a nucleating agent that increases the overall crystallization rate. The crystallization rate at 90°C is significantly slower than that at 130°C, however, the long period and lamellar thickness formed at 90°C are much lower than those formed at higher temperatures. The dyed and undyed PGA- co -PLA copolymers have almost the same morphological parameters at the same temperature. This indicates that morphological parameters of the lamellar structures in the polymers depend primarily on the crystallization temperature rather than on the crystallization rate. It is evident that the thermodynamic factor driven by temperature principally determines the lamellar morphology. The final unit cell parameters a and b and the final apparent crystal sizes all increase with temperature, indicating that crystal perfection prevails at high temperatures.

Density Determination of Medical Sutures by Pycnometry

In this study, we evaluated the density of medical sutures using a Micromeritics pycnometer. The types of sutures, their size, and experimental conditions were investigated to determine their effects on density measurement. Based on the results of two types of suture materials and six suture sizes, the pycnometer is a relatively fast method for density determinations for biomaterials and medical devices with small sizes, such as for sutures. The instrument yields repeatable and precise results; however, the measurement error increases when the sample weight becomes very small.

A Bioabsorbable Microcatheter for the Continuous and Direct Treatment of Surgical Sites

A bioabsorbable microcatheter comprising of a polyglycolideco-lactide microtube and an absorbable braid is developed for the continuous delivery of therapeutic fluids directly to a surgical site. Studies conducted with tissue staining dye and radio-opaque solution using porcine models indicate that infusion of fluids in the wound site is rapid and complete. The tissue reaction to the microtube and braid is generally characterized by minimal to mild chronic inflammation or foreign body reaction that decreases in severity as the material is absorbed. The microtube is considered to be absorbed at 56 days postimplantation while the bulk of the braid is considered to be absorbed by 70 days postimplantation.