Daniel Cohn

Phase separation in poly(ethylene glycol)/poly(lactic acid) blends

Abstract Studies were conducted on the morphology of poly(ethylene glycol)/poly(lactic acid) (PEG/PLA) blends. Mixtures comprising PLA and PEG chains of different molecular weights (1500, 3400, 6000 and 35,000) were prepared; their structures were investigated by DSC studies and i.r. spectroscopy. The results suggest that, whenever one of the components is present at more than 20% by wt, it is able to crystallize; such blends consist of two semi-miscible crystalline phases dispersed in an amorphous matrix. For more extreme compositions, only the major component is able to crystallize; the noncrystalline matrix which develops, consists of the minor constituent and the amorphous phase of the major component of the blend. The molecular weight of PEG affects the morphology generated due to the enhanced crystallizability of the longer PEG chains. Crystallization phenomena are viewed as a fundamental driving force for microphase segregation.

3D Printing of Shape Memory Polymers for Flexible Electronic Devices

The formation of 3D objects composed of shape memory polymers for flexible electronics is described. Layer-by-layer photopolymerization of methacrylated semicrystalline molten macromonomers by a 3D digital light processing printer enables rapid fabrication of complex objects and imparts shape memory functionality for electrical circuits.

Improved reverse thermo-responsive polymeric systems

Novel reverse thermo-responsive (RTG) polymeric systems displaying superior rheological properties were generated by polymerization of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) segments. Two basic synthetic pathways were followed: (1) The bulk polymerization of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock (Pluronic(RTM) F127) (MW=12,600, 70wt% PEO) with hexamethylene diisocyanate (HDI) and (2) The covalent binding of poly(ethylene glycol) and poly(propylene glycol) chains, using phosgene as the connecting molecule. While in the former, the basic amphiphilic F127 repeating unit is known for its own RTG behavior, the latter polymers consist of segments unable of exhibiting reverse thermal gelation of their own. These new materials achieved viscosities at least 15 times higher than F127, at 37 degrees C. Dynamic light scattering measurements revealed that the microstructures formed by these novel polymers were markedly larger than those generated by PEO-PPO-PEO triblocks. While the size of Pluronic F127 micelles ranged from 15 to 20nm, the higher molecular weight amphiphiles generated much larger nanostructures (20-400nm). Finally, the ability of reverse thermo-sensitive gels to perform as drug delivery systems was exemplified by releasing an anti-restenosis model drug (RG-13577). A 30% P[F127](4) gel delivered the drug over 40 days, whereas a F127 gel having the same concentration released the drug over a 7 days period.

A study on the in vitro degradation of poly(lactic acid).

The in vitro degradation of samples of L- and D,L-lactic acid polymers, P(L)LA and P(DL)LA respectively, having different molecular weights, morphology and/or geometry, has been studied through the determination of viscometric molecular weight, mass and mechanical properties as function of the immersion time in Ringer solution at 37 degrees C. In particular have been compared the degradation kinetics of P(L)LA, amorphous and crystalline, and of P(L)LA and P(DL)LA having different molecular weight and sample geometry. From the molecular weight versus the degradation time data, a degradation rate has been defined, as the derivative of the function best fitting the data, normalized to the molecular weight of the polymer at each time. The behavior of the degradation rate curves, plotted against the degradation time, has been interpreted and compared with relation to the initial physical and geometrical characteristics of the PLA samples.

Amorphous and crystalline morphologies in glycolic acid and lactic acid polymers

Abstract This paper describes an investigation of a number of parameters which affect the physical structures of glycolic acid (GA) and lactic acid (LA). It has been found that the cooling rate of a quenching process determines the amorphous-crystalline morphology balance, and that the effect of a similar quenching process will vary with the molecular weight of the polymer. At very high molecular weights, even very rapid quenching does not produce higher degrees of amorphous phase. Copolymerization of PGA or of PLA with poly(ethylene oxide) results in either phase blending or phase separation, depending on the copolymer composition and the segmental chain length. The degree of crystallinity of the PGA or PLA component in the copolymer is mostly affected by copolymerization in a state of phase blending.

Crosslinkable PEO-PPO-PEO-based reverse thermo-responsive gels as potentially injectable materials

This paper describes the functionalization and crosslinking of PluronicRTM derivatives in aqueous solution at 37° C. Pluronic dimethacrylate was obtained by reacting native PEO-PPO-PEO triblocks with methacryloyl chloride and then crosslinking them by free radical polymerization at 37° C, using a redox system. The resulting gel and its rheological behavior were characterized by different techniques. The swelling study of the crosslinked polymer was indicative of its reverse thermo-responsive behavior, as illustrated by the almost 800% water uptake of the polymer at 37° C, as opposed to the 1600% attained by the polymer at 25° C. As expected, while the Pluronic dimethacrylate gel displayed an E c value of 142.5 ± 29.7 kPa at 37° C, the crosslinked system attained a Youngs modulus three times higher: 415.2 ± 45.7 kPa. Finally, the environmental SEM analysis revealed the porous microstructure of the crosslinked gels.

The effect of thermal history on the crystallinity of different molecular weight PLLA biodegradable polymers

Crystallinity and viscosimetric molecular weights (Mwv) of four l-lactic acid polymers (PLLA) having different initial molecular weight, i.e. 18 000, 31000, 156000 and 425000, have been determined as functions of the polymer thermal history. Treatments performed in a differential scanning calorimeter (DSC) at various rates showed the different capability of materials to crystallize, due to their different crystallization kinetics and depending on their molecular weight. In the case of cooling from melt, for instance, even at a cooling rate of 0·5 °C/min, only lower molecular weight polymers were able to develop crystallinity. In all materials, moreover, and depending on the initial molecular weight, a parallel degradation effect was observed, which is attributed to thermal cleavage occurring during the treatment.

Evaluation of polyethylene glycol/polylactic acid films in the prevention of adhesions in the rabbit adhesion formation and reformation sidewall models

OBJECTIVE To assess the efficacy of bioresorbable films consisting of various polyethylene glycol 6000 and polylactic acid block copolymers on the formation and reformation of adhesions in rabbit models of adhesion development between the sidewall to the adjacent cecum and bowel. The composition of the different polymers was expressed by the number of monomeric units in the block, namely, ethylene oxide (EO) and lactic acid (LA), respectively. DESIGN Studies of the efficacy of EO/LA films were conducted in rabbit sidewall adhesion formation studies in the presence and absence of blood and in rabbit adhesion reformation studies. REPEL (Life Medical Sciences, Edison, NJ), a film of EO/LA ratio 3.0 manufactured under commercial conditions, was also tested in these animal models. SETTING University-based laboratory. ANIMALS New Zealand white rabbits. INTERVENTION(S) Placement of films of various EO/LA ratios at the site of injury to the parietal peritoneum. MAIN OUTCOME MEASURE(S) Adhesion formation and reformation. RESULT(S) Films of various EO/LA ratios, Seprafilm (Genzyme, Cambridge, MA) and Interceed (Johnson and Johnson Medical, Arlington, TX) placed over an area of excised sidewall at the time of initial injury were highly efficacious in the prevention of adhesion formation. A film of EO/LA ratio 3.7, in contrast with Interceed, was also shown to maintain maximal efficacy in the reduction of adhesion formation in the presence of blood. Further, a film of EO/LA ratio 3.0 produced under commercial conditions, REPEL, was highly efficacious in reducing adhesion development in the rabbit models of adhesion and reformation. CONCLUSION(S) These studies suggest that bioresorbable EO/LA films reduced adhesion development in rabbit models of adhesion formation and reformation.

Repair of Bone Defect Using Bone Marrow Cells and Demineralized Bone Matrix Supplemented with Polymeric Materials

We present a novel, reverse thermo-responsive (RTR) polymeric osteogenic composite comprising demineralized bone matrix (DBM) and unmanipulated bone marrow cells (BMC) for repair of bone defects. The polymers investigated were low viscosity aqueous solutions at ambient temperature, which gel once they heat up and reach body temperature. Our goal to supplement DBM-BMC composite with RTR polymers displaying superior rheological properties, was to improve graft integrity and stability, during tissue regeneration. The osteogenic composite when implanted under kidney capsule of mice, proved to be biocompatible and biodegradable, with no residual polymer detected in the newly formed osteohematopoietic site. Implantation of the osteogenic composite into a large area of missing area of parietal bone of the skull of rats, resulted in an extensive remodeling of DBM particles, fully reconstituted hematopoietic microenvironment and well integrated normal flat bone within thirty days. The quality and shape of the newly created bone were comparable to the original bone and neither local or systemic inflammatory reactions nor fibrosis at the junction of the new and old calvarium could be documented. Furthermore, combined laser capture microdissection (LCM) technique and PCR analysis of male BMC in female rats confirmed the presence of male derived cells captured from the repaired/ regenerated flat bone defect. The use of active self sufficient osteogenic DBM-BMC composite supported by a viscous polymeric scaffold for purposive local hard tissue formation, may have a significant potential in enhancement of bone regeneration and repair following trauma, degenerative or inflamatory lesion, iatrogenic interventions and cosmetic indications.

Use of high-performance polyethylene fibres as a reinforcing phase in poly(methylmethacrylate) bone cement

First results are presented concerning the elastic and ultimate mechanical behaviour of p(MMA) bone cement reinforced with as-received and surface-modified Spectra 900 polyethylene fibres. Even though the surface chemistry and reactivity of the fibres was modified, the surface oxidation and surface grafting treatments of the polyethylene fibres apparently did not significantly affect the mechanical properties of the polyethylene-reinforced p(MMA) bone cement or improve the interfacial bonding. This may be attributed to the rather unfavourable area-to-volume ratio of PE fibres for such treatments, as well as to the necessarily low content of PE fibres in the bone cement which does not allow a clear differentiation between the various samples.