Controlled delivery of therapeutics from microporous membranes. I. Fabrication and characterization of microporous polyurethane membranes containing polymeric microspheres

Effects of Growth Factor Presence on Mineralization of Porous Poly(Lactide-Co-Glycolide) Scaffolds In Vitro

Basic design requirements of scaffolds for bone tissue engineering applications include biocompatibility, temporally controlled degradability, osteoconductivity, mechanical integrity, and mass transport capabilities. A recent study has attempted to meet design criteria via the growth of a carbonated apatite (bone-like) mineral on the inner pore surfaces of a highly porous 85:15 poly(lactide-co-glycolide) scaffold using a biomimetic process. It has also been recently demonstrated that the mineralization strategy can be combined with sustained growth factor delivery to induce vascular tissue ingrowth. The present study examines the effect of protein presence on the mineralization process by measuring the amount of protein incorporated into the scaffold during the process of mineral formation. Surprisingly, vascular endothelial growth factor incorporates more readily into control scaffolds than into scaffolds subjected to mineralization treatment. This finding suggests that there is little incorporation of protein into the growing mineral film, and offers insight into the mechanism for sustained drug delivery from the mineralized scaffolds.

Development of a 'mechano-active' scaffold for tissue engineering

In this study. we investigate the potential for manipulating bone cell mechanotransducers in tissue engineering. Membrane ion channels such as voltage operated calcium channels (VOCC) have been shown to be a critical component of the bone cell transduction pathway with agonists and inhibitors of this pathway having profound effects on the load signal. By encapsulating a calcium channel agonist with slow release within a poly(L-lactide) (PLLA) scaffold, we can generate a 'mechano-active' scaffold for use in skeletal tissue engineering. PLLA scaffolds with and without a calcium channel agonist, BAY K8644, were seeded with primary human bone cells or the human MG63 bone cell line and cultured for 13 weeks followed by mechanical stimulation with a four-point bending model. Our results show that addition of the agonist for slow release is sufficient to enhance the load-related responses in bone cells within the scaffolds. Specifically, collagen type I expression and the ratio of alkaline phosphatase to protein are elevated in response to cyclical mechanical stimulation of approximately 1000 microstr which is then further enhanced in the mechano-active' scaffolds. As the agonists only act when the calcium channels are open by attenuating the calcium flux, the stimulation is specifically targeted to scaffolds subjected to load either in vitro or ultimately in vivo. Our results suggest that manipulating the VOCC and attenuating the opening of the calcium channels may be an effective technique to amplify matrix production via mechanical stimulation which may be applied to bone tissue engineering and potentially engineering of other load-bearing connective tissues.

Sustained release of vascular endothelial growth factor from mineralized poly(lactide-co-glycolide) scaffolds for tissue engineering

Strategies to engineer bone tissue have focused on either: (1) the use of scaffolds for osteogenic cell transplantation or as conductive substrates for guided bone regeneration; or (2) release of inductive bioactive factors from these scaffold materials. This study describes an approach to add an inductive component to an osteoconductive scaffold for bone tissue engineering. We report the release of bioactive vascular endothelial growth factor (VEGF) from a mineralized, porous, degradable polymer scaffold. Three dimensional, porous scaffolds of the copolymer 85 : 15 poly(lactide-co-glycolide) were fabricated by including the growth factor into a gas foaming/particulate leaching process. The scaffold was then mineralized via incubation in a simulated body fluid. Growth of a bone-like mineral film on the inner pore surfaces of the porous scaffold is confirmed by mass increase measurements and quantification of phosphate content within scaffolds. Release of 125I-labeled VEGF was tracked over a 15 day period to determine release kinetics from the mineralized scaffolds. Sustained release from the mineralized scaffolds was achieved, and growth of the mineral film had only a minor effect on the release kinetics from the scaffolds. The VEGF released from the mineralized and non-mineralized scaffolds was over 70% active for up to 12 days following mineralization treatment, and the growth of mineral had little effect on total scaffold porosity.

Controlled delivery of therapeutics from microporous membranes. II. In vitro degradation and release of heparin-loaded poly(D,L-lactide-co-glycolide)

In vitro degradation and release of five types of heparin/surfactant-loaded poly(D,L-lactide-co-glycolide 50:50) (PLG) microspheres alone and also incorporated within microporous polyurethane tubes were studied over a 3-month period. Degradation was studied with scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). Heparin release was characterized using a modified Azure A assay. SEM suggests that microspheres may be entrapped within polyurethane fibrils of the polyurethane tubes, thereby reducing contact with their hydrated environment. FTIR transmittance spectra confirm microsphere incorporation within the polyurethane tubes and PLG ester hydrolysis occurring over the 3-month period. A correlation was observed between decreasing molecular weights and glass transition temperatures (Tg). The microspheres alone exhibited a change in Tg but not when incorporated within the microporous tubes. Release profiles revealed a burst effect occurring during the first 4h and total release of the heparin from the microspheres by 12 weeks.