Pablo C. Caracciolo

Effect of the hard segment chemistry and structure on the thermal and mechanical properties of novel biomedical segmented poly(esterurethanes)

Two series of biomedical segmented polyurethanes (SPU) based on poly(ε-caprolactone) diol (PCL diol), 1,6-hexamethylene diisocyanate (HDI) or l-lysine methyl ester diisocyanate (LDI) and three novel chain extenders, were synthesized and characterized. Chain extenders containing urea groups or an aromatic amino-acid derivative were incorporated in the SPU formulation to strengthen the hard segment interactions through either bidentate hydrogen bonding or π-stacking interactions, respectively. By varying the composition of the hard segment (diisocyanate and chain extender), its structure was varied to investigate the structure-property relationships. The different chemical composition and symmetry of hard segment modulated the phase separation of soft and hard domains, as demonstrated by the thermal behavior. Hard segment association was more enhanced by using a combination of symmetric diisocyanate and urea-diol chain extenders. The hard segment cohesion had an important effect on the observed mechanical behavior. Polyurethanes synthesized using HDI (Series H) were stronger than those obtained using LDI (Series L). The latter SPU exhibited no tendency to undergo cold-drawing and the lowest ultimate properties. Incorporation of the aromatic chain extender produced opposite effects, resulting in polyurethanes with the highest elongation and tearing energy (Series H) and the lowest strain at break (Series L). Since the synthesized biodegradable SPU possess a range of thermal and mechanical properties, these materials may hold potential for use in soft tissue engineering scaffold applications.

Segmented poly(esterurethane urea)s from novel urea–diol chain extenders: Synthesis, characterization and in vitro biological properties

This work describes the preparation, physicochemical characterization, mechanical properties and in vitro biological properties of two bioresorbable aliphatic segmented poly(esterurethane urea)s (SPEUU) based on poly(epsilon-caprolactone) diol (PCL diol), 1,6-hexamethylene diisocyanate and two novel urea-diol chain extenders. To strengthen the interactions through hydrogen bonding in the hard segments of SPEUU, novel chain extenders containing urea groups were synthesized and used in the SPEUU formulation. The different chemical structures of the chain extenders modulated the phase separation of soft and hard segments, as demonstrated by the thermal behavior. The hard segment association was enhanced using a diurea-diol chain extender. The biological interactions between the obtained materials and blood were studied by in vitro methods. Research on the protein adsorption, platelet adhesion and thrombus formation is presented. Studies of protein adsorption onto polymeric surfaces showed that SPEUU adsorbed more albumin than fibrinogen. Studies on platelet adhesion and thrombus formation of SPEUU-coated coverslips indicated the antithrombogenic behavior of these surfaces. The synthesized SPEUU revealed no signs of cytotoxicity to Chinese hamster ovary cells, showing satisfactory cytocompatibility.

Optimization of poly(L-lactic acid)/segmented polyurethane electrospinning process for the production of bilayered small-diameter nanofibrous tubular structures.

The present study is focused on the electrospinning process as a versatile technique to obtain nanofibrous tubular structures for potential applications in vascular tissue engineering. A bilayered scaffolding structure composed of poly(L-lactic acid) (PLLA)/bioresorbable segmented polyurethane (SPEU) blends for small-diameter (5mm) vascular bypass grafts was obtained by multilayering electrospinning. Polymer blend ratios were chosen to mimic the media and adventitia layers. The influence of the different electrospinning parameters into the fiber formation, fiber morphology and fiber mean diameter for PLLA, SPEU and two PLLA/SPEU blends were studied. Flat and two-parallel plate collectors were used to analyze the effect of the electrostatic field on the PLLA nanofiber alignment in the rotating mandrel. Membrane topography resulted in random or aligned nanofibrous structures depending on the auxiliary collector setup used. Finally, composition, surface hydrophilicity, thermal properties and morphology of nanofibrous scaffolds were characterized and discussed. Since the development of tissue engineered microvascular prostheses is still a challenge, the prepared scaffolding tubular structures are promising candidates for vascular tissue engineering.

Mechanical behavior of bilayered small-diameter nanofibrous structures as biomimetic vascular grafts

To these days, the production of a small diameter vascular graft (<6mm) with an appropriate and permanent response is still challenging. The mismatch in the grafts mechanical properties is one of the principal causes of failure, therefore their complete mechanical characterization is fundamental. In this work the mechanical response of electrospun bilayered small-diameter vascular grafts made of two different bioresorbable synthetic polymers, segmented poly(ester urethane) and poly(L-lactic acid), that mimic the biomechanical characteristics of elastin and collagen is investigated. A J-shaped response when subjected to internal pressure was observed as a cause of the nanofibrous layered structure, and the materials used. Compliance values were in the order of natural coronary arteries and very close to the bypass gold standard-saphenous vein. The suture retention strength and burst pressure values were also in the range of natural vessels. Therefore, the bilayered vascular grafts presented here are very promising for future application as small-diameter vessel replacements.

Structural characterization of electrospun micro/nanofibrous scaffolds by liquid extrusion porosimetry: A comparison with other techniques

Poly(ε-caprolactone) micro/nanofibrous scaffolds obtained by electrospinning technique from polymer solutions were characterized in terms of fiber diameter (as measured by scanning electron microscopy-SEM), pore size and its distribution (as measured by liquid extrusion porosimetry), and porosity (as determined by gravimetric measurement, liquid intrusion method, SEM image analysis and liquid extrusion porosimetry - LEP). Nonwoven micro/nanofibrous scaffolds were formed by uniform bead-free fibers with mean diameters in the range of 0.4 to 7 μm. The results indicate that pore size and pore size distribution are strongly associated to fiber diameter. Porosity results were analyzed taking into account the accuracy and limitations of each method. LEP resulted as the most suitable technique for measuring through-pore diameter and porosity. In order to compare empirical data of pore size from LEP, a theoretical multiplanar model for stochastic fiber networks was applied. The results predicted by the model were in good agreement with the experimental data provided by LEP for mean diameters higher than 1 μm. The present study shows the potential of LEP as a valuable instrumental technique for characterizing the porous structure of electrospun fibrous scaffolds.

Surface-modified bioresorbable electrospun scaffolds for improving hemocompatibility of vascular grafts

The replacement of small-diameter vessels is one of the main challenges in tissue engineering. Moreover, the surface modification of small-diameter vascular grafts (SDVG) is a key factor in the success of the therapy due to their increased thrombogenicity and infection susceptibility caused by the lack of a functional endothelium. In this work, electrospun scaffolds were prepared from blends of poly(L-lactic acid) (PLLA) and segmented polyurethane (PHD) with a composition designed to perform as SDVG inner layer. The scaffolds were then successfully surface-modified with heparin following two different strategies that rely on grafting of heparin to either PLLA or PHD functional groups. Both strategies afforded high heparin density, being higher for urethane methodology. The functionalized scaffolds did not cause hemolysis and inhibited platelet adhesion to a large extent. However, lysozyme/heparin-functionalized scaffolds obtained through urethane methodology achieved the highest platelet attachment inhibition. The increase in hydrophilicity and water absorption of the surface-functionalized nanostructures favored adhesion and proliferation of human adipose-derived stem cells. Heparinized surfaces conjugated with lysozyme presented microbial hydrolysis activity dependent on heparin content. Overall, a better performance obtained for urethane-modified scaffold, added to the fact that no chain scission is involved in urethane methodology, makes the latter the best choice for surface modification of PLLA/PHD 50/50 electrospun scaffolds. Scaffolds functionalized by this route may perform as advanced components of SDVG suitable for vascular tissue engineering, exhibiting biomimetic behavior, avoiding thrombi formation and providing antimicrobial features.

High pressure assessment of bilayered electrospun vascular grafts by means of an Electroforce Biodynamic System

Introduction: Tissue engineering offers the possibility of developing a biological substitute material in vitro with the inherent properties required in vivo. However, the inadequate performance in vascular replacement of small diameter vascular grafts (VG) reduces considerably the current alternatives in this field. In this study, a bilayered tubular VG was produced, where its mechanical response was tested at high pressure ranges and compared to a native femoral artery. Materials and Method: The VG was obtained using sequential electrospinning technique, by means of two blends of Poly(L-lactic acid) and segmented poly(ester urethane). Mechanical testing was performed in a biodynamic system and the pressure-strain relationship was used to determine the elastic modulus. Results: Elastic modulus assessed value of femoral artery at a high pressure range (33.02×106 dyn/cm2) was founded to be 36% the magnitude of VG modulus (91.47×106 dyn/cm2) at the same interval. Conclusion: A new circulating mock in combination with scan laser micrometry have been employed for the mechanical evaluation of bioresorbable bilayered VGs. At same pressure levels, graft elasticity showed a purely “collagenic” behavior with respect to a femoral artery response.

Evaluation of in vitro cytotoxic activity of mono-PEGylated StAP3 (Solanum tuberosum aspartic protease 3) forms

Highlights • Selective StAP3 cytotoxic activity is not affected by PEGylation.• mono-PEGylated StAP3 forms exhibit higher antifungal activity than the native protein.• PEGylation improves StAP3 cytotoxicity against Gram-positive bacteria.

Micro/nanofiber-based scaffolds for soft tissue engineering applications: Potential and current challenges

Bioresorbable micro/nanofiber-based structures are being studied as promising candidates for tissue engineering applications. Among the existing techniques for producing these matrices, electrospinning has attracted interest in many technological fields as a versatile and powerful processing technique. Electrospun micro/nanofibers possess high surface-area-to-volume ratio, high porosity and pore interconnectivity, and tunable fiber morphology and orientation. Moreover, submicron fibers are found in the extracellular matrix of natural organs and tissues. To date, many synthetic and natural polymers, biodegradable or non-biodegradable polymers, ceramics and composite materials, have been successfully electrospun using a plethora of techniques. Although in the beginning electrospinning was focused in producing two-dimensional structures, nowadays three-dimensional structures are also being developed. Besides the progress in the electrospinning process achieved in recent years, there still remain a number of challenges, such as mechanical, physical, and chemical biomimeticity, pore size enlargement, surface functionalization, therapeutic agent/cell loading, vascularization, and cell infiltration. This chapter reviews the research advances made in electrospun scaffolds for soft tissue engineering applications focusing on wound dressing, cartilage, muscle, cardiovascular, nerve, and skin tissues.