Compliance and ultimate strength of composite arterial prostheses

The Biomimetic Evolution of Composite Materials: From Straw Bricks to Engineering Structures and Nanocomposites

Advanced polymer-based composite materials have revolutionized the structural material arena since their appearance some 60 years ago. Yet, despite their relatively short existence, they seem to be taken for granted as if they have always been there. One of the reasons for this state of affairs is that composite materials of various types have accompanied human history for thousands years, and their emergence in the modern era could be considered a natural evolutionary process. Nevertheless, the continuous line that leads from early days of composites in human history to current structural materials has exhibited a number of notable steps, each generating an abrupt advance toward the contemporary new science of composite materials. In this paper, I review and discuss the history of composites with emphasis on the main steps of their development.

Feasibility of a Dielectric Elastomer Augmented Aorta

Although heart transplantation is a gold standard for severe heart failure, there is a need for alternative effective therapies. A dielectric-elastomer aorta is used to augment the physiological role of the aorta in the human circulatory system. To this end, the authors developed a tubular dielectric elastomer actuator (DEA) able to assist the heart by easing the deformation of the aorta in the systole and by increasing its recoil force in the diastole. In vitro experiments using a pulsatile flow-loop, replicating human physiological flow and pressure conditions, show a reduction of 5.5% (47 mJ per cycle) of the heart energy with this device. Here, the controlled stiffness of the DEA graft, which is usually difficult to exploit for actuators, is perfectly matching the assistance principle. At the same time, the physiological aortic pressure is exploited to offer a prestretch to the DEA which otherwise would require an additional bulky pre-stretching system to reach high performances.

Mimicking "J-Shaped" and Anisotropic Stress-Strain Behavior of Human and Porcine Aorta by Fabric-Reinforced Elastomer Composites

An ex vivo heart perfusion device preserves the donor heart in a warm beating state during transfer between extraction and implantation surgeries. One of the current challenges includes the use of rigid and noncompliant plastic tubes, which causes injuries to the heart at the junction between the tissue and the tube. The compliant and rapidly strain-stiffening mechanical property that generates a "J-shaped" stress-strain behavior is necessary for producing the Windkessel effect, which ensures continuous flow of blood through the aorta. In this study, we mimic the J-shaped and anisotropic stress-strain behavior of human aorta in synthetic elastomers to replace the problematic noncompliant plastic tube. First, we assess the mechanical properties of human (n = 1) and porcine aorta (n = 14) to quantify the nonlinear and anisotropic behavior under uniaxial tensile stress from five different regions of the aorta. Second, fabric-reinforced elastomer composites were prepared by reinforcing silicone elastomers with embedded fabrics in a trilayer geometry. The knitted structures of the fabric provide strain-stiffening as well as anisotropic mechanical properties of the resulting composite in a deterministic manner. By optimizing the combination between different elastomers and fabrics, the resulting composites matched the J-shaped and anisotropic stress-strain behavior of natural human and porcine aorta. Finally, improved analytical constitutive models based on Gent's and Mooney-Rivlin's constitutive model (to describe the elastomer matrix) combined with Holzapfel-Gasser-Ogden's model (to represent the stiffer fabrics) were developed to describe the J-shaped behavior of the natural aortas and the fabric-reinforced composites. We anticipate that the suggested fabric-reinforced silicone elastomer composite design concept can be used to develop complex soft biomaterials, as well as in emerging engineering fields such as soft robotics and microfluidics, where the Windkessel effect can be useful in regulating the flow of fluids.

Synthesis of HMDI-based segmented polyurethanes and their use in the manufacture of elastomeric composites for cardiovascular applications

For short-term cardiovascular application, segmented polyurethanes (SPUs) based on 4,4-methylenebis(cyclohexyl isocyanate) (HMDI), polytetramethylenglycol (PTMG) and 1,4-butanediol (BD) were synthesized and characterized by spectroscopy (FT-IR, (1)H-NMR) and thermal (TGA, DMA, DSC) and mechanical techniques. The segmented nature of the SPUs was not easily established by spectroscopic means; however, TGA allowed the quantification of the rigid segments content by the significant mass loss between 348 and 356 degrees C. The alpha transition was detected by DMA and related to the T(g) of the soft segments at -50 degrees C, while DSC showed the presence of an endothermic transition above 80 degrees C attributed to the melting of rigid segments. Two types of composites were prepared using the synthesized SPUs and Lycra (either T162B or T162C). The first one consisted of a two layers casting laminated while the second one was a classic unidirectional fibre-reinforced material. Laminate composites prepared with SPU containing 23.9% and 33.9% of rigid segments and Lycra T162C exhibited a higher tensile modulus but lower tensile strength than composites prepared with Tecoflex SG-80A (39.7% of rigid segments). The energy of adhesion between layers on these composites ranged from 475 to 2150 J. Fibre-reinforced SPUs exhibited higher moduli than the two layer laminated composites with increasing amounts of rigid segments in the matrix and by increasing Lycra T162C content (up to 10%). This behaviour was explained by SEM, which showed a good fibre-matrix bonding.

Self-healing biomaterials

The goal of this review is to introduce the biomaterials community to the emerging field of self-healing materials, and also to suggest how one could utilize and modify self-healing approaches to develop new classes of biomaterials. A brief discussion of the in vivo mechanical loading and resultant failures experienced by biomedical implants is followed by presentation of the self-healing methods for combating mechanical failure. If conventional composite materials that retard failure may be considered zeroth generation self-healing materials, then taxonomically speaking, first generation self-healing materials describe approaches that "halt" and "fill" damage, whereas second generation self-healing materials strive to "fully restore" the prefailed material structure. In spite of limited commercial use to date, primarily because the technical details have not been suitably optimized, it is likely from a practical standpoint that first generation approaches will be the first to be employed commercially, whereas second generation approaches may take longer to implement. For self-healing biomaterials the optimization of technical considerations is further compounded by the additional constraints of toxicity and biocompatibility, necessitating inclusion of separate discussions of design criteria for self-healing biomaterials.

Designing biodegradable multiblock PCL/PLA thermoplastic elastomers

A series of poly(epsilon-caprolactone)/poly(L-lactic acid) (PCL/PLA) biodegradable poly(ester-urethane)s, was synthesized and characterized. The first step of the synthesis consisted of the ring opening polymerization of L-lactide, initiated by the hydroxyl terminal groups of the PCL chain, followed by the chain extension of these PLA-PCL-PLA triblocks, using hexamethylene diisocyanate (HDI). The trimers comprised PCL2000 flexible segments, while the length of each PLA block covered the 550-6000 molecular weight range. The morphology of the copolymers gradually changed, as the length of the PLA blocks increased. The multiblock copolymers produced displayed enhanced mechanical properties, with ultimate tensile strength values around 32 MPa, Young's modulus as low as 30 MPa and elongation at break values well above 600%. The longer the PLA block, the slower the in vitro degradation of the material, with all copolymers degrading faster than the respective homopolymers.

Novel synthetic selectively degradable vascular prostheses: a preliminary implantation study

BACKGROUND

Vascular grafts perform less well than autologous arterial or vein grafts. The purpose of this study was to evaluate the short-term performance of selectively biodegradable filament-wound vascular prostheses, comprising elastomeric poly(ether urethane) (Lycra) scaffolds and flexible, hydrophilic biodegradable coatings.

MATERIALS AND METHODS

Two types of selectively biodegradable vascular grafts were manufactured, comprising a filament-wound Lycra scaffold, subsequently coated with a biodegradable poly(ethylene glycol)/poly(lactic acid) (PELA) block copolymer. The two types of grafts differed in both the overall porosity of the scaffold and the hydrophilicity of the biodegradable constituent. A 60-mm-long and 6-mm-diameter filament-wound and polytetrafluoroethylene (ePTFE) grafts were implanted as interposition prostheses, randomly, at the right- and left-side carotid arteries.

RESULTS

Implantation studies proved the grafts to be patent and pulsatile for periods of up to 3 months. Increasing the scaffold porosity and enhancing the hydrophilicity of the biodegradable component improved both the transmural tissue ingrowth process and the vascularization of the prosthesis wall. Also, a well-adhered peripheral tissue and a thin, uniform intima and endothelial lining were obtained. All ePTFE graft controls, although patent, were rather stiff and nonpulsatile. A thick pseudointima, poorly attached to the prosthesis inner surface, was observed. The compliance of the wet grafts was significantly higher than in the dry state, stemming mainly from the water-plasticizing effect on the biodegradable component. The grafts explanted after a period of 6 weeks exhibited compliance only slightly lower than that of the wet grafts. After 12 weeks, however, the hoop compliance was 20% lower than that prior to implantation. At 100 mm Hg, for example, the original compliance of the wet graft was 2.5%/100 mm Hg decreasing to 2.0%/100 mm Hg after a 3-month implantation. The compliance reduction with implantation is attributed to the ingrowth of the perigraft tissue as revealed by the histological study. A compliance of 2.0%/100 mm Hg is slightly better than that of a standard PTFE graft with an original compliance of 1.6%/100 mm Hg. Yet it is still an order of magnitude smaller than that of a canine carotid artery.

CONCLUSIONS

The improved mechanical properties and enhanced healing of the highly porous filament-wound Lycra scaffold graft coated with hydrophilic biodegradable PELA has the potential of being a highly effective small caliber prosthetic graft.

Elastomers for biomedical applications

Current topics in elastomers for biomedical applications are reviewed. Elastomeric biomaterials, such as silicones, thermoplastic elastomers, polyolefin and polydiene elastomers, poly(vinyl chloride), natural rubber, heparinized polymers, hydrogels, polypeptides elastomers and others are described. In addition biomedical applications, such as cardiovascular devices, prosthetic devices, general medical care products, transdermal therapeutic systems, orthodontics, and ophthalmology are reviewed as well. Elastomers will find increasing use in medical products, offering biocompatibility, durability, design flexibility, and favorable performance/cost ratios. Elastomers will play a key role in medical technology of the future.

Elastic response of filament wound arterial prostheses under internal pressure

Filament wound synthetic prostheses have anisotropic material properties and are therefore able to match closely the elastic properties of the replaced host vessels. Highly porous prosthesis walls are required to allow ingrowth of capillar cells from the outer surface of the graft in order to increase endothelium coverage of the luminal surface. The coating of highly porous grafts with biodegradable polymers has been shown to result in a sealed structure at the time of implantation followed by controlled porosity during the healing process. Accordingly, a new manufacturing process for a coated filament wound vascular graft is proposed in this work, which combines high potential porosity with high mechanical compliance. In vitro testing of its mechanical properties shows that the compliance can be controlled by changing the reinforcement angle and the coating material. Whereas the initial compliance of the coated structure expresses a composite material response, the post-degradation compliance reflects the highly compliant response of the filament wound non-woven scaffold. Hence, higher compliance values can be achieved by the proposed technique, compared with those of the commonly used synthetic grafts.

Stiffness variability and stress-dependent elastic response of synthetic fibre-reinforced composites for biomedical applications

A major design requirement of biomaterial prostheses is to match their elastic properties with those of the natural host tissue. Composite materials address this requirement because their elastic properties can be altered accurately through composition and directionality parameters, and they can be designed to match closely the elastic properties of the biological tissues, in isocompliance, modulus gradient and anisotropy. This adds to a range of advantages of synthetic composite materials with respect to potential biomedical applications, which draw on their heterogeneity and anisotropy. This paper focuses on the elastic properties of synthetic fibre-reinforced composite materials that pertain to biomedical applications, and demonstrates the range of stiffnesses obtainable through selection of constituents and by choice of angle of reinforcement.