J.C. Barbenel

Mechanical and failure behaviour of the stratum corneum

The load-deformation-time behaviour of heat-separated human stratum corneum was investigated using a pure shear specimen geometry. The tissue displayed non-linear load-deformation behaviour and stress relaxation, although the extensibility and amount of stress relaxation was considerably less than that shown by other soft connective tissues. Controlled failure tests were carried out after an edge cut had been made in the pure shear specimen. Sources of secondary failure, either at the free edge of the specimen, or due to the presence of inhomogeneities in the tissues, were common. Analysis of the test results suggested that the fracture surface energy of stratum corneum has a mean value of 3.6 kJ m-2 which is comparable with the tougher synthetic polymers.

Mechanics of native bovine pericardium. I. The multiangular behaviour of strength and stiffness of the tissue.

Native bovine pericardial samples have recently been reported to be mechanically anisotropic in both biaxial and uniaxial tests. We present results of multiangular tensile tests conducted on strips from native pericardial samples. These tests were needed in order to establish the nature of the anisotropy, in other words identify the class of the material symmetry. We also examine the possibility of describing the multiangular behaviour of the ultimate tensile stress and stiffness of the tissue by using a theory pertinent to composite structures comprised of linear elastic elements. This application was in general successful and should contribute towards a better understanding of the mechanics and deformations of natural materials with a woven fabric (structure).

Mechanics of native bovine pericardium: II. A structure based model for the anisotropic mechanical behaviour of the tissue

A constitutive law to describe the mechanical behaviour of native bovine pericardial samples is proposed. The model is based upon an idealization of the tissue structure and in particular makes use of the preferential orientation of the collagen and elastin fibres in the plane of the tissue along a mechanically predominant direction. This non-axisymmetric layout of the tissue fibres has been shown in the past to cause a systematic variation of the tensile strength and the stiffness with the angle of orientation. The proposed constitutive relation provided a very good fit for results collected along two orthogonal principal directions in the past in uniaxial and biaxial tensile testing modes. It also allowed a better understanding of the interaction of the various load bearing elements and was able to describe some strange features of the tissues anisotropic mechanical behaviour.

Device for measuring soft tissue interface pressures

This paper describes the construction and performance of a simple pressure sensing device with a continuous electrical output. It was constructed utilizing a commercially available transducer, an electropneumatic sensor capsule and a 1 m long tube. The transducer used was a piezo-resistive pressure-sensitive device producing an output voltage proportional to the applied pressure. This low cost, high accuracy device is temperature compensated and shows good linearity and negligible hysteresis. The sensor cell has a good thickness-to-diameter ratio and is sufficiently flexible to conform to most contours of the body. The tubing that conveys the pressure transmitting fluid also serves as a means of keeping the transducer distant from the measuring site. The device showed a highly satisfactory performance under laboratory conditions and has proven to be robust and reliable when used for clinical studies.

Mouthpiece forces produced while playing the trumpet

A transducer for measuring the force applied to the trumpet mouthpiece during performance is described. The device allowed the players to perform on their own instrument and in their usual manner. The results of tests on 60 subjects showed that during playing the force between the mouthpiece and instrument increased with increasing loudness and ascending pitch but that there was no significant correlation between mouthpiece force and proficiency or style. The maximum force which the players could tolerate was greater for high proficiency players than for medium.

Foot microcirculation and blood rheology in diabetes.

Diabetes mellitus is associated with circulatory abnormalities. The blood flow in the skin of the dorsum of the foot and haemorheological variables were measured in 36 subjects. They were divided into three equal groups of diabetic patients: those with neuropathy, and both age and sex matched non-diabetic subjects; all were characterized by age, duration of diabetes and blood biochemistry. High and low shear rate blood viscosities were measured; aggregation was characterized using a Myrerene Aggregometer. The microcirculation in the skin of the dorsum of the foot was measured using a laser Doppler flowmeter. Measurements were made at room temperature with the subjects supine with the leg horizontal, and then with the lower leg vertical; the measurements were repeated at 42 degrees C. Both diabetic groups had significantly increased low shear whole blood viscosity compared with normal subjects. The aggregation index was significantly greater in diabetic neuropaths than normal subjects. There were significant differences in the depth of vasomotor activity between the three groups, with the diabetic neuropaths commonly showing no motor activity at room temperature. The only significant correlations were between equilibrium laser Doppler values with the limb horizontal and both the low and high shear whole blood viscosities.

A Psychological Investigation of the Role of Mouthpiece Force in Trumpet Performance

Amongst beginning trumpet players, signs of physical stress are frequently observed especially when the player attempts to explore the higher register of the instrument. Preliminary interviews with teachers of trumpet revealed a general concern about development of an easy and relaxed playing style, and with the reduction of mouthpiece-to-lip forces to a low or minimal level. An apocryphal story amongst many trumpet players concerns individuals who can, reputedly, produce double-high Cs on an instrument merely suspended by a couple of strings. The present research programme investigated the physical, psychological and psychophysical manifestations of mouthpiece force amongst groups of trumpet players of different degrees of skill. Our results in fact differ rather sharply from the introspections of trumpet pedagogues. All players, skilled and less skilled, used varying but substantial amounts of force; less-skilled players could not be differentiated from skilled performers in terms of forces used; and skilled players were no better than less skilled (or in some cases unskilled) subjects at the task of judging how much force was being used (a) by themselves, and (b) by other players, all judgements being prone to considerable error.

Transcutaneous Oxygen Tension In Diabetes: Relation To Peripheral Neuropathy And Blood Rheology

Summary Circulatory disturbance and abnormal blood rheology are common in diabetes, and in conjunction with peripheral neuropathy could have a role in the pathogeneses of foot ulceration. We have measured blood flow indirectly on the foot by transcutaneous oxygen tension together with blood viscosity and red cell deformability in a group of diabetic patients with peripheral neuropathy and compared them with patients without neuropathy and non-diabetic control subjects. Transcutaneous oxygen tension (at 44°C) rose significantly between supine and sitting positions (Patients with neuropathy; 58.3 ± 9.6 vs 64.5 ± 8.9 mmHg, p 0.05. Non-diabetic subjects; 61.2 ± 7.7 vs 68.5 ± 10.1 mmHg, p

Changes in the Material Properties and Function of Pericardial Bioprosthetic Heart Valves Invivo

We have studied changes in the function and material properties of pericardial heart valves which had been implanted for between 2 and 7 years and were free from calcification and leaflet tears when explanted. Similar measurements have also been made on the characteristics of pericardial valves which have been cycled in laboratory fatigue tests for 400 million cycles. Explanted valves had restricted leaflet opening and increased pressure differences during forward flow. Fibrous tissue ingrowth, leaflet thickening and reduced tissue extensibility contributed to the change in valve function. Little change was seen in the function of valves after laboratory fatigue tests although there was a significant reduction in tissue extensibility.