Gerardo A. Montero

Bioresorbable elastomeric vascular tissue engineering scaffolds via melt spinning and electrospinning

Current surgical therapy for diseased vessels less than 6mm in diameter involves bypass grafting with autologous arteries or veins. Although this surgical practice is common, it has significant limitations and complications, such as occlusion, intimal hyperplasia and compliance mismatch. As a result, cardiovascular biomaterials research has been motivated to develop tissue-engineered blood vessel substitutes. In this study, vascular tissue engineering scaffolds were fabricated using two different approaches, namely melt spinning and electrospinning. Small diameter tubes were fabricated from an elastomeric bioresorbable 50:50 poly(l-lactide-co-epsilon-caprolactone) copolymer having dimensions of 5mm in diameter and porosity of over 75%. Scaffolds electrospun from two different solvents, acetone and 1,1,1,3,3,3-hexafluoro-2-propanol were compared in terms of their morphology, mechanical properties and cell viability. Overall, the mechanical properties of the prototype tubes exceeded the transverse tensile values of natural arteries of similar caliber. In addition to spinning the polymer separately into melt-spun and electrospun constructs, the approach in this study has successfully demonstrated that these two techniques can be combined to produce double-layered tubular scaffolds containing both melt-spun macrofibers (<200microm in diameter) and electrospun submicron fibers (>400nm in diameter). Since the vascular wall has a complex multilayered architecture and unique mechanical properties, there remain several significant challenges before a successful tissue-engineered artery is achieved.

Nanofibrous scaffolds electrospun from elastomeric biodegradable poly(L-lactide-co-ε-caprolactone) copolymer

Electrospinning has recently received much attention in biomedical applications, and has shown great potential as a novel scaffold fabrication method for tissue engineering. The nano scale diameter of the fibers produced and the structure of the web resemble certain supramolecular features of extracellular matrix which is favorable for cell attachment, growth and proliferation. There are various parameters that can alter the electrospinning process, and varying one or more of these conditions will result in producing different nanofibrous webs. So the aim of this study was to investigate the effect of material variables and process variables on the morphology of electrospun 50:50 poly(L-lactide-co-epsilon-caprolactone) (PLCL) nanofibrous structures. The morphology of the nanofibers produced was strongly influenced by parameters such as the flow rate of the polymer solution, the electrospinning voltage and the solution concentration. The diameter was found to increase with solution concentration in a direct linear relationship. Finally, it has been successfully demonstrated that by increasing the rotation speed of the collector mandrel, the alignment of the fibers can be controlled in a preferred direction. These findings contribute to determining the functional conditions to electrospin this biodegradable elastomeric copolymer which has potential as a scaffold material for vascular tissue engineering.

Reducing problems of cyclic trimer deposits in supercritical carbon dioxide polyester dyeing machinery

Abstract The present paper describes an alternative procedure for the reduction or elimination of oligomeric polyester residues, in particular the cyclic trimer (CTR), in supercritical fluids (SCFs). Polyethylene terephthalate is the largest, (by volume) man-made synthetic fiber produced in the world owing to its favorable properties, such as durability, strength, stability during heat setting, abrasion resistance, and resistance to sunlight, acids, alkalis, and bleaches. In addition, polyester fibers have very good crease recovery and are durable to washing. Because of these characteristics, polyester has many important uses including home furnishings, apparel fabrics, automotive interior fabrics, and carpeting (Ind. Eng. Chem. Res. 39 (2000) 4806). Due to the large volume of polyester dyed, fundamental research has given attention to alternatives for conventional aqueous processes. The application of SCFs, especially supercritical carbon dioxide (SC-CO2), in the textile industry has recently become an alternative technology for developing a more environmentally friendly dyeing process. SCF dyeing technology has the potential to overcome several environmental and technical issues in many commercial textile applications such as yarn preparation, dyeing and finishing. SCFs represent a potentially unique media for either transporting chemical into or out of a polymeric substrate, because of their thermo-physical and transport properties. SCFs exhibit gas-like viscosities and diffusivities and liquid-like densities. Carbon dioxide is appealing for use as a SCF because it is inexpensive, non-toxic, non-flammable, environmentally friendly, and chemically inert under many conditions (J. Org. Chem. 49 (1984) 5097). In order to improve efficiency and address some of the environmental concerns with SCF technology, researchers at North Carolina State University (NCSU), College of Textiles, have constructed a single-package-pilot-plant system for dyeing polyester using SC-CO2. Based in part on data gathered from this investigation, the technical and economic feasibility of this process has been demonstrated and SCF dyeing appears to be on the leading edge of emerging technologies. However, it has been shown that the removal of precipitated oligomers mainly from the inside walls of all parts of the dyeing machine (i.e. vessels, spindle tube, sight glasses, valves, tubing, and fittings) is highly desirable. Experimental pressure measurements across a few sections of the SC-CO2 dyeing machine show that significant pressure losses can occur where oligomer, predominately CTR, builds up. Consequently, the maximum CO2 volume flow rate in the dyeing machine can decrease 30–35% (Conf. Eng. Note (1998); Conf. Eng. Note (1999)). A preliminary investigation shows that highly insoluble CTR has a reduced affinity to adhere to these stainless steel surfaces at higher SC-CO2 pressure. Although the knowledge and expertise base in this new textile research area has been increased considerably, an economic removal procedure for CTR in SC-CO2 machinery has not been found (Proc. 6th Conf. Supercrit. Fluids Appl. (2001) 571).

Modeling of supercritical fluid flow through a yarn package

Abstract Steady-state supercritical fluid flow through both isotropic and anisotropic cylindrical yarn packages is modeled as 2-D, axisymmetric flow through porous media. A numerical flow model using a finite-difference method predicts pressure and velocity profiles based on user-defined package geometry, permeability profile, and fluid properties. The use of variable permeability in the model allows simulation of typical package heterogeneities that result from radial variations and relatively denser edges usually associated with package winding. The numerical model is compared with results obtained from analytical expressions for radial flow, axial flow, and 2-D flow in an annulus of isotropic, porous material. The model is then verified using experimental pressure drop measurements for a range of supercritical CO 2 flows through polyester yarn packages. Model predictions show very good agreement with experimental data.

A Heterogeneous Kinetic Model for the Cutinase‐Catalyzed Hydrolysis of Cyclo‐tris‐ethylene Terephthalate

The kinetics of enzyme‐catalyzed hydrolysis of the polyester oligomer cyclo‐tris‐ethylene terephthalate, commonly known as cyclic trimer, using a developmental cutinase is reported. The effect of substrate surface area and enzyme concentration, in a largely aqueous medium, on the rate of hydrolysis was measured via spectrophotometric measurement using high performance liquid chromatography (λ 254 nm) at 60 °C in a glycine buffer (pH 8). The rate was strongly dependent on the substrateapos;s surface characteristics. When the substrate surface area was relatively small and the substrate was relatively low in crystallinity, the reaction followed zero order kinetics, whereas a first order rate constant was obtained when the substrate surface area was increased considerably and the crystallinity was relatively high.