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.

Modelling the anisotropic behaviour of filament wound vascular grafts

The model according to the Law of Laplace, describing the mechanical behaviour of blood vessels and vascular grafts, was applied to filament wound arterial prostheses, which have been manufactured with different winding angles. By varying the winding angle, the anisotropic behaviour of the grafts could be changed and fitted to the anisotropic properties of natural blood vessels. Thus, the Laplace model had to be modified, and answers now to the requirement of responding to the anisotropic behaviour in hoop versus axial direction of the grafts. The experimental data of hoop and axial compliances obtained by biaxial inflation tests could be then correlated to the material properties of the vascular grafts measured by uniaxial tensile loading. It is shown that with the modified Laplace model the changes in the anisotropic behaviour due to different winding angles can be described and predicted. The calculated compliance values derived from the uniaxial tensile tests fitted the experimental data obtained by the biaxial inflation tests, although the calculated hoop compliance values tended to be higher than the experimental data.

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.

Introducing a selectively biodegradable filament wound arterial prosthesis: a short-term implantation study

This article introduces a new compliant and selectively biodegradable filament wound vascular graft and reports the findings of a short-term implantation study. A basic feature of filament winding is its ability to tailor and better control the mechanical properties of the prosthesis, so that a closer match with the anisotropic properties of native arteries is achieved. The elastomeric vascular grafts comprise poly(ether urethane urea) fibers (Lycra) embedded in a two-component matrix consisting of poly(ether urethane) (Pellethane) and a highly flexible poly(ethylene glycol)/poly(lactic acid) biodegradable segmented copolymer (PELA). Typical tensile modulus values fall in the few megapascals (MPa) range, this being comparable to that of natural arteries. The wound graft exhibits excellent handling and suturability characteristics as well as enhanced burst strength. Furthermore, due to its biodegradable constituent, the prosthesis combines minimal intraoperative blood loss and high healing porosity. The graft displays initially negligible in vitro water permeation, which increases gradually with time. In this short-term study, the prostheses were implanted in the canine carotid, and their biological performance was compared to that of expanded Gore-Tex. The luminal surface of the wound grafts was coated with a thin layer of pseudointima, strongly adhered to the prosthesis surface. Contrasting with the very stiff Gore-Tex grafts, the filament wound prostheses retained their high compliance, being highly pulsatile upon explanation. Histological studies fully corroborated these findings, underscoring the healing properties of these new filament wound vascular prostheses.

Compliance and ultimate strength of composite arterial prostheses

A further stage is reported in a comprehensive project aimed at developing new composites for soft tissue implants. The composite prostheses are made by a filament-winding technique comprising Lycra elastomeric fibres embedded in an elastomeric matrix of mostly Pellethane. New experimental results of compliance of filament-wound tubes and of the ultimate strength are presented and compared with theoretical predictions. It is shown that the functions of these properties in the winding angle are given by common composite material models. The compliance is predicted accurately by the expression based on a transformation of the principal elastic constants, whereby a maximum is expected at a winding angle of around 45 degrees. The ultimate strength is compared with two failure criteria, of which the maximum work criterion gives an excellent fit.

The failure analysis of composite material flight helmets as an aid in aircraft accident investigation

Understanding why a flying helmet fails to maintain its integrity during an accident can contribute to an understanding of the mechanism of injury and even of the accident itself. We performed a post-accident evaluation of failure modes in glass and aramid fibre-reinforced composite helmets. Optical and microscopic (SEM) techniques were employed to identify specific fracture mechanisms. They were correlated with the failure mode. Stress and energy levels were estimated from the damage extent. Damage could be resolved into distinct impact, flexure and compression components. Delamination was identified as a specific mode, dependent upon the matrix material and bonding between the layers. From the energy dissipated in specific fracture mechanisms we calculated the minimum total energy imparted to the helmet-head combination and the major injury vector (MIV) direction and magnitude. The level of protection provided by the helmet can also be estimated.

Utilization of composite laminate theory in the design of synthetic soft tissues for biomedical prostheses

There are several advantages of using composite design considerations for the preparation of biomedical soft tissues. Using a composite laminate design, a wide range for compliance results, proving that the prosthesis compliance can be altered without a concomitant variation of other properties. The trend of compliance as a function of the reinforcement angle is discussed for an angle-ply composite of low compliance constituents, as well as the implications for stress-strain behaviour. Experimental examples pertinent to prosthetic arterial design are presented.

New arterial prostheses by filament winding

A new manufacturing method for vascular grafts, based on a filament winding technique, is unveiled. The concept that pilots this method is presented and analysed in detail alongside the experimental results. A basic feature of filament winding is its ability to produce a two-phase structure built of a continuous fibre-reinforced polymeric matrix, shaped according to the shape of a mandrel. This structure offers a number of advantages over common vascular graft designs, e.g. better control of the mechanical properties and closer match with anisotropic properties of native arteries, and more degrees of design freedom with respect to pore size, biodegradability and biocompatibility. The experimental section offers a range of potential constituent materials, and presents an example of a Lycra fibre/Pellethane matrix prototypical prosthesis.