Andrew J. Taberner

Mechanical characterisation of in vivo human skin using a 3D force-sensitive micro-robot and finite element analysis

The complex mechanical properties of skin have been the subject of much study in recent years. Several experimental methods developed to measure the mechanical properties of skin in vivo, such as suction or torsion, are unable to measure skin’s anisotropic characteristics. An experiment characterising the mechanical properties of in vivo human skin using a novel force-sensitive micro-robot is presented. The micro-robot applied in-plane deformations to the anterior forearm and the posterior upper arm. The behaviour of the skin in each area is highly nonlinear, anisotropic, and viscoelastic. The response of the upper arm skin is very dependent on the orientation of the arm. A finite element model consisting of an Ogden strain energy function and quasi-linear viscoelasticity was developed to simulate the experiments. An orthogonal initial stress field, representing the in vivo skin tension, was used as an additional model parameter. The model simulated the experiments accurately with an error-of-fit of 17.5% for the anterior lower forearm area, 6.5% for the anterior upper forearm and 9.3% for the posterior upper arm. The maximum in vivo tension in each area determined by the model was 6.2 Nm−1 in the anterior lower forearm, 11.4 Nm−1 in anterior upper forearm and 5.6 Nm−1 in the posterior upper arm. The results also show that a finite element model with a neo-Hookean strain energy function cannot simulate the experiments with the same accuracy.

Experimental Study of a TET System for Implantable Biomedical Devices

Time-varying magnetic fields can be used to transfer power across the skin to drive implantable biomedical devices without the use of percutaneous wires. However, the main challenges of a transcutanoues energy transfer (TET) system are the temperature rise caused by power loss in the implanted circuitry and the changes in positioning between the external and internal coils due to fitting and changes in posture. This study presents a TET system with a closed-loop frequency-based power regulation method to deliver the right amount of power to the load under variable coil coupling conditions. After implanting a TET system into adult sheep, the temperature rise in the internal and external coils of a TET system was measured for power delivery in the range of 5 W to 15 W. The sheep was housed in a temperature controlled (16 plusmn1degC, humidity 50plusmn10%) room, in accordance with the standard protocols implemented at the University of Auckland for sheep studies. A power-loss analysis for the overall system was performed. The system was capable of regulating power for axially aligned separations of up to 16 mm. The maximum power efficiency of the overall system was 82.1% and a maximum temperature rise of 2.7degC was observed on the implanted secondary coil.

Measurement of the force–displacement response of in vivo human skin under a rich set of deformations

The non-linear, anisotropic, and viscoelastic properties of human skin vary according to location on the body, age, and individual. The measurement of skins mechanical properties is important in several fields including medicine, cosmetics, and forensics. In this study, a novel force-sensitive micro-robot applied a rich set of three-dimensional deformations to the skin surface of different areas of the arms of 20 volunteers. The force-displacement response of each area in different directions was measured. All tested areas exhibited a non-linear, viscoelastic, and anisotropic force-displacement response. There was a wide quantitative variation in the stiffness of the response. For the right anterior forearm, the ratio of the maximum probe reaction force to maximum probe displacement ranged from 0.44 N mm(-1) to 1.45 N mm(-1). All volunteers exhibited similar qualitative anisotropic characteristics. For the anterior right forearm, the stiffest force-displacement response was when the probe displaced along the longitudinal axis of the forearm. The response of the anterior left forearm was stiffest in a direction 20° to the longitudinal axis of the forearm. The posterior upper arm was stiffest in a direction 90° to the longitudinal axis of the arm. The averaged posterior upper arm response was less stiff than the averaged anterior forearm response. The maximum probe force at 1.3mm probe displacement was 0.69N for the posterior upper arm and 1.1N for the right anterior forearm. The average energy loss during the loading-unloading cycle ranged from 11.9% to 34.2%. This data will be very useful for studying the non-linear, anisotropic, and viscoelastic behaviour of skin and also for generating material parameters for appropriate constitutive models.

A unique micromechanocalorimeter for simultaneous measurement of heat rate and force production of cardiac trabeculae carneae

To study cardiac muscle energetics quantitatively, it is of paramount importance to measure, simultaneously, mechanical and thermal performance. Ideally, this should be achieved under conditions that minimize the risk of tissue anoxia, especially under high rates of energy expenditure. In vitro, this consideration necessitates the use of preparations of small radial dimensions. To that end, we have constructed a unique micromechanocalorimeter, consisting of an open-ended flow-through microcalorimeter, a force transducer, and a pair of muscle-length actuators. The device enables the metabolic and mechanical performance of cardiac trabeculae carneae to be investigated for prolonged periods in a continuously replenished oxygen- and nutrient-rich environment.

An innovative work-loop calorimeter for in vitro measurement of the mechanics and energetics of working cardiac trabeculae

We describe a unique work-loop calorimeter with which we can measure, simultaneously, the rate of heat production and force-length work output of isolated cardiac trabeculae. The mechanics of the force-length work-loop contraction mimic those of the pressure-volume work-loops experienced by the heart. Within the measurement chamber of a flow-through microcalorimeter, a trabecula is electrically stimulated to respond, under software control, in one of three modes: fixed-end, isometric, or isotonic. In each mode, software controls the position of a linear motor, with feedback from muscle force, to adjust muscle length in the desired temporal sequence. In the case of a work-loop contraction, the software achieves seamless transitions between phases of length control (isometric contraction, isometric relaxation, and restoration of resting muscle length) and force control (isotonic shortening). The area enclosed by the resulting force-length loop represents the work done by the trabecula. The change of enthalpy expended by the muscle is given by the sum of the work term and the associated amount of evolved heat. With these simultaneous measurements, we provide the first estimation of suprabasal, net mechanical efficiency (ratio of work to change of enthalpy) of mammalian cardiac trabeculae. The maximum efficiency is at the vicinity of 12%.

Trabeculae carneae as models of the ventricular walls: implications for the delivery of oxygen

Trabeculae carneae are the smallest naturally arising collections of linearly arranged myocytes in the heart. They are the preparation of choice for studies of function of intact myocardium in vitro. In vivo, trabeculae are unique in receiving oxygen from two independent sources: the coronary circulation and the surrounding ventricular blood. Because oxygen partial pressure (PO2) in the coronary arterioles is identical in specimens from both ventricles, whereas that of ventricular blood is 2.5-fold higher in the left ventricle than in the right ventricle, trabeculae represent a “natural laboratory” in which to examine the influence of “extravascular” PO2 on the extent of capillarization of myocardial tissue. We exploit this advantage to test four hypotheses. (1) In trabeculae from either ventricle, a peripheral annulus of cells is devoid of capillaries. (2) Hence, sufficiently small trabeculae from either ventricle are totally devoid of capillaries. (3) The capillary-to-myocyte ratios in specimens from either ventricle are identical to those of their respective walls. (4) Capillary-to-myocyte ratios are comparable in specimens from either ventricle, reflecting equivalent energy demands in vivo, driven by identical contractile frequencies and comparable wall stresses. We applied confocal fluorescent imaging to trabeculae in cross section, subsequently using semi-automated segmentation techniques to distinguish capillaries from myocytes. We quantified the capillary-to-myocyte ratios of trabeculae from both ventricles and compared them to those determined for the ventricular free walls and septum. Quantitative interpretation was furthered by mathematical modeling, using both the classical solution to the diffusion equation for elliptical cross sections, and a novel approach applicable to cross sections of arbitrary shape containing arbitrary disposition of capillaries and non-respiring collagen cords.

Development and Performance of a Controllable Autoloading Needle-Free Jet Injector

A jet injector platform technology that provides improved performance over existing jet injectors through the use of a controllable linear Lorentz-force actuator and software-based control system has been developed. Injectors designed on this platform are capable of delivering injections using arbitrary pressure pulse shaping. Pulse shaping has been shown to allow a wide degree of control over the depth to which the injection is delivered. A software-based injector control system improves repeatability and allows for automatic reloading of the injector, a task that would be difficult to implement using existing jet injector platforms. A design for a prototype autoloading controllable jet injector (cJI) based on this platform is detailed. The injection capability of this cJI was evaluated both in-vitro and in-vivo using a tissue analog, excised porcine tissue, and ovine tissue. An analysis of the cJIs performance indicates that this design is capable of delivering a controllable volume of fluid to a controllable depth based entirely on the parameters input into the control software.

Characterization of a flow-through microcalorimeter for measuring the heat production of cardiac trabeculae

The energy consumption of isolated cardiac trabeculae can be inferred from measurements of their heat production. Once excised from the heart, to remain viable, trabeculae require continuous superfusion with an oxygen- and nutrient-rich solution. Flow-through calorimeters enable trabeculae to be maintained in a stable and controlled environment for many hours at a time. In this paper we describe and characterize a flow-through microcalorimeter, with sensitivity in the 1μW range, for measuring the heat output of 10μg cardiac trabeculae. The device uses infrared-sensitive, thin-film thermopile sensors to provide a noncontact method for measuring temperature differences. The sensors are capable of resolving 5μK temperature differences within the superfusing fluid. The microcalorimeter has a sensitivity of 2.56V∕W at a flow rate of 1μl∕s, with a time constant of approximately 3.5 s. The sensitivity and time constant are strongly dependent upon the flow rate. Predictions of a finite-element model of the calori...

A fully implantable telemetry system for the chronic monitoring of brain tissue oxygen in freely moving rats

The ability to monitor tissue oxygen concentration in a specific region of the brain in a freely moving animal could provide a new paradigm in neuroscience research. We have developed a fully implantable telemetry system for the continuous and chronic recording of brain tissue oxygen (PO(2,BR)) in conscious animals. A telemetry system with a sampling rate of 2kHz was combined with a miniaturized potentiostat to amperiometrically detect oxygen concentration with carbon paste electrodes. Wireless power was employed to recharge the telemeter battery transcutaneously for potential lifetime monitoring. Rats were implanted with the telemeter in the peritoneal cavity and electrodes stereotaxically implanted into the brain (striatum or medulla oblongata). While the animals were living in their home cages the sensitivity to changes in oxygen was validated by repeatedly altering the inspired oxygen (10%, 100%, respectively) or a pharmacological stimulus (carbonic anhydrase inhibitor: acetazolamide 50mg/kg IP). Basal level of PO(2,BR) was monitored for 3weeks and showed good overall stability and good correlation to movement such as grooming. During hypoxia, PO(2,BR) decreased significantly by -51%±2% from baseline, whereas it increased by 34%±3% during hyperoxia. Following the systemic administration of acetazolamide, PO(2,BR) increased by 38%±4%. We propose this new technology provides a robust method to measure changes in oxygen concentration in specific areas of the brain, in conscious freely moving rats. The ability to track long term changes with disease progression or drug treatment may be enabled.

Design and testing of an MRI-compatible cycle ergometer for non-invasive cardiac assessments during exercise

BackgroundMagnetic resonance imaging (MRI) is an important tool for cardiac research, and it is frequently used for resting cardiac assessments. However, research into non-pharmacological stress cardiac evaluation is limited.MethodsWe aimed to design a portable and relatively inexpensive MRI cycle ergometer capable of continuously measuring pedalling workload while patients exercise to maintain target heart rates.ResultsWe constructed and tested an MRI-compatible cycle ergometer for a 1.5 T MRI scanner. Resting and sub-maximal exercise images (at 110 beats per minute) were successfully obtained in 8 healthy adults.ConclusionsThe MRI-compatible cycle ergometer constructed by our research group enabled cardiac assessments at fixed heart rates, while continuously recording power output by directly measuring pedal force and crank rotation.