On the fractional transversely isotropic functionally graded nature of soft biological tissues: Application to the meniscal tissue.

This paper focuses on the origin of the poroelastic anisotropic behaviour of the meniscal tissue and its spatially varying properties. We present confined compression creep test results on samples extracted from three parts of the tissue (Central body, Anterior horn and Posterior horn) in three orientations (Circumferential, Radial and Vertical). We show that a poroelastic model in which the fluid flow evolution is ruled by non-integer order operators (fractional Darcy's law) provides accurate agreement with the experimental creep data. The model is validated against two additional sets of experimental data: stress relaxation and fluid loss during the consolidation process measured as weight reduction. Results show that the meniscus can be considered as a transversely isotropic poroelastic material. This behaviour is due to the fluid flow rate being about three times higher in the circumferential direction than in the radial and vertical directions in the body region of the meniscus. The 3D fractional poroelastic model is implemented in the finite element software to estimate the weight loss during the confined compression tests.

Energy storage and stress-strain characteristics of a prosthetic foot: a priori design and analysis with experiments.

This work proposes an experimentally validated numerical approach for a systematic a priori evaluation of the energy storage and stress-strain characteristics of a prosthetic foot during the stance phase of walking. Boundary conditions replicating the rocker based inverted pendulum model were incorporated. The mechanically complex Ottobock Solid Ankle Cushioned Heel (SACH) foot was opted as the test device. A non-linear finite element (FE) model of the device was developed inclusive of its sources of non-linearity. A custom rig was fabricated to investigate the prosthetic foot's mechanics in a gait analysis facility and experimentally validate the finite element approach. The numerical and experimental outcomes showed fair agreement, with the centre of pressure at the foot-floor interface deviating by 1.56 mm at the instance of highest strain. The magnitude and the distribution of the energy stored and a series of stress and strain parameters were analyzed for the test device using the proposed approach. The novel methodology proposed may act as an effective tool for the design, analysis, and prescription of energy storage and return prosthetic feet.

Roll-over shape of a prosthetic foot: a finite element evaluation and experimental validation.

Prosthetic feet have generally been designed experimentally by adopting a trial-and-error technique. The objective of this research is to introduce a novel numerical approach for the a priori evaluation of the roll-over shape (ROS) of a prosthetic foot for application in its systematic design and development. The ROS was achieved numerically by employing a non-linear finite element model incorporating the augmented Lagrangian and multi-point constraint contact formulations, a hyperelastic material model and a higher-order strain definition. The Ottobock Solid Ankle Cushion Heel (SACH) foot was chosen to experimentally validate the numerical model. The geometry of the foot was evaluated from optical scans, and the material properties were obtained from uniaxial tensile, shear and volumetric compression tests. A new setup was designed for an improved experimental determination of the ROS, with the inclusion of an extended moment arm and variable loading. Error analysis of the radius of curvature of the ROS between the numerical and experimental results showed the percentage error to be 7.52%, thereby establishing the validity of the model. A numerical design model of this kind can be utilised to vary the input design parameters to arrive at a prosthetic foot with specified performance characteristics effectively and economically. Graphical abstract.