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Biomedical subjects

J C Barbenel

Publications and source records attributed to J C Barbenel.

At least 37 records · Page 2Linked to original sources
PubMed1994

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 tissue's anisotropic mechanical behaviour.

Animals↗
PubMed1994

External fixation of the distal radius. A biomechanical study.

External fixation is a useful method of treating unstable fractures of the distal radius. There is a lack of information regarding the behaviour of mini-fixation systems, particularly under cyclical loading, which would be expected to occur at the wrist. This laboratory study was designed to investigate the mechanical characteristics of nine current fixation systems. A programme of loading was devised to mimic forces acting on the distal radius. Wooden dowel was used to mount the fixator. Distraction and compression forces were applied in an Instron rig. Displacement was monitored throughout the load programme. No fixator failed at the loads tested. The small Hoffman rectangular frame proved stiffest in compression and extension, but was heavier than other devices. A trend of progressive, permanent deformation became apparent in those fixators which contain plastic or composite materials. This feature has implications for loss of fracture reduction in the clinical setting.

Biomechanical Phenomena↗
PubMed1993

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.

Blood Viscosity↗
PubMed1993

Abnormal thermal hyperaemia in the skin in paraplegia.

Surface insulation, together with laser Doppler flowmetry, was used to assess the skin microcirculation of paraplegic patients. Two control groups of five male and five female subjects were used to establish the response of normals with which to compare the results obtained from six paraplegic subjects. No significant sex related difference was revealed from this study. It was found that in normal subjects, surface insulation resulted in a significant increase in both skin temperature and skin blood flow. In paraplegic patients, the temperature increase was significantly less than in the normal subjects and there was no significant thermally induced hyperaemia after surface insulation.

Adult↗
PubMed1992

Mechanical and optical anisotropy of bovine pericardium.

A Moiré fringe method was used to determine the shape of inflated disks of fresh bovine pericardium. The results suggested that the tissue was anisotropic and allowed the identification of the directions of the axes of elastic symmetry. Quantitative biaxial inflation and simple tension tests confirmed the anisotropy. In the circumferential direction the tissue was more extensible and had greater mean strength (18 MPa against 2.5 MPa) and mean terminal modulus (46 MPa against 14 MPa) than in the root-to-apex direction. The tissue was also optically anisotropic when viewed by transmitted polarised light and the directions of mechanical and optical anisotropy were related.

Animals↗
PubMed1992

Novel quantitative methods for the determination of biomaterial cytotoxicity.

Two novel methods for the determination of biomaterial cytotoxicity using cell culture are presented. The methods combine a standardized protocol for producing extracts from medical devices with either the established MTT assay or a new fluorimetric assay. The suitability of both methods for evaluating the toxicity of candidate materials was demonstrated by resolution of the differences in the toxic effects of serial dilutions of a PVC extract on BHK21 and HT1080 cells. The tests yield highly reproducible, quantitative results and can be applied to materials in the usual physical forms applicable to artificial organs.

Animals↗
PubMed1991

The influence of model parameter values on the prediction of skin surface temperature: II. Contact problems.

A model of heat transfer and temperature distribution in the skin and superficial tissues which is based on a finite difference numerical solution of the one-dimensional multilayer coupled bioheat equation is presented. The model is used to investigate the influence of the values chosen to represent the physiological and thermal properties of the tissues on the skin surface temperature after contact with an external medium. It was found that the skin blood flow and dermal conductivity were the main cutaneous parameters which influence the contact response, but in terms of normalized temperature the response was little influenced by cutaneous metabolic heat generation and deep dermal temperature. For contact with a good conductor, the transient behaviour was sensitive to the heat transfer coefficient on the outer surface and the thickness of the contact material, but insensitive to the conductivity of the material.

Humans↗
PubMed1991

Influence of age on postischaemic reactive hyperaemia.

This work was carried out to evaluate the response of skin microcirculation to ischaemia in younger and older subjects, to determine if there were functional changes in microcirculation in the older skin. Skin perfusion during reactive hyperaemia was measured at volar sites on the forearms in five younger (mean age: 25.2 years) and five older normal subjects (mean age: 64.6 years). Ischaemia was produced by the application of loads of 22.3 N (24.5 kPa) and 44.5 N (49.0 kPa) applied to the skin surface for 3 min by means of a hollow cylindrical indentor. The skin perfusion within the area indented was continuously monitored by a laser Doppler flowmeter before, during and after loading. In the pre-loaded condition, there was no difference in skin perfusion between the younger and the older groups. Following loading with 22.3 N a significantly lower value of the peak perfusion relative to the pre-loaded value was observed in the older group (P less than 0.05). The perfusion in the older group following loading with 44.5 N was significantly reduced compared to the younger group (P less than 0.05). It is concluded that postischaemic reactive hyperaemia in the older skin was reduced compared with the younger skin.

Adult↗
PubMed1991

Pressure management.

The prevention or minimisation of the occurrence of pressure sores is an important consideration in the rehabilitation of physically disabled people, especially for the wheelchair user with a spinal cord injury. Although there is little definitive information on the cause of pressure sores, several intrinsic and extrinsic factors have been highlighted. Probably the most significant causative factor is the application of force to the skin surface. The relationship between the magnitude of pressure and its duration; the temperature and humidity at the interface; and the physiological effects that this has on the microcirculation and lymphatic drainage are discussed in this article. It is suggested that a rationale for the prevention of pressure sores includes the limitation of the duration of pressure applied to the skin surface and the reduction of the peak pressures particularly at vulnerable sites. In this context the design criteria for a clinical interface pressure measurement system, and the uses and limitations of the commercially available options, are considered. The development of a structured programme of wheelchair and support surface provision, assessment and follow-up is required.

Equipment Design↗
PubMed1990

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.

Biomechanical Phenomena↗
PubMed1990

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.

Equipment Design↗
PubMed1990

The influence of model parameter values on the prediction of skin surface temperature: I. Resting and surface insulation.

A model is presented of heat transfer and temperature distributions in the skin and superficial tissues. It is based on a finite difference numerical solution of the one-dimensional multilayer coupled bioheat equation. In this paper, the model is used to investigate the influence of the values of parameters chosen to represent the physiological and heat transfer processes on the temperature of the skin under resting conditions and after insulation of the skin surface. Equilibrium resting temperatures were strongly influenced by deep body temperature especially at lower heat transfer coefficients on the skin surface, but slightly affected by the values chosen for skin blood flow and metabolic heat generation; both the heat transfer coefficients and environmental temperature strongly influenced the surface temperature. After surface insulation the temperature elevation was strongly influenced by the thermal conductivities of tissues, skin blood flow and deep boundary temperature; metabolic heat generation was only significantly at unphysiologically high values.

Humans↗
PubMed1990

Further evidence that the residual vWf:Ag in porcine FVIII:C induces human platelet aggregation.

Porcine or bovine factor VIII concentrates (FVIII:C) have been used during the past 3 decades to control bleeding in patients who have developed antibodies to human factor VIII. Since current preparations of animal FVIII:C are not known to transmit infectious agents such as hepatitis or human immunodeficiency virus, they are of potential therapeutic interest. A purified porcine FVIII:C (Hyate:C) is now widely used as an alternative to human FVIII:C in patients with inhibitor. Unlike earlier preparations of porcine FVIII:C, thrombocytopaenia is rare with the current preparation. Nonetheless, it causes the aggregation of human platelets in vitro. Our aim was to identify precisely the plasma factor which induces platelet aggregation. The effects of commercial porcine FVIII:C, porcine fibrinogen, porcine fibronectin and the corresponding preparations from human origin on platelet aggregation were studied. Platelet aggregation was quantified by measuring the fall in single platelet count in human whole blood. Of these preparations, only porcine FVIII:C (0.1-1 U/ml) and porcine fibrinogen (80-600 micrograms/ml) induced a fall in single platelet count of up to 85% due to aggregation. The extent of aggregation was directly proportional to the amount (0.007-0.1 U/ml test aliquot) of residual von Willebrand factor antigen (vWf:Ag) in the preparations. A monoclonal antibody to vWf:Ag inhibited the aggregation. We believe that the aggregation of human platelets induced in vitro by porcine FVIII:C is mediated by vWf:Ag which also may be responsible for thrombocytopaenia reported following administration of porcine FVIII:C in vivo.

Animals↗
PubMed1989

Dipyridamole inhibits red cell-induced platelet activation.

We have recently shown that red blood cells can induce spontaneous platelet aggregation (SPA) in whole blood ex vivo, which could be inhibited by dipyridamole. Since this drug, at therapeutic doses is not an effective inhibitor of platelet aggregation in platelet rich plasma, the inhibition of platelet interaction with the red cell was thought to be the mechanism of its action. Values for the percentage fall in the single platelet count due to SPA in whole blood after 3 and 6 min rollermixing were: control 15 +/- 2.2 and 42 +/- 2.9; 6 microM dipyridamole 6 +/- 1.1 (P less than 0.001) and 31 +/- 2.6 (P less than 0.01); 12 microM dipyridamole 2 +/- 0.9 (P less than 0.0005) and 22 +/- 2.3 (P less than 0.0005) (mean +/- SEM, n = 10). Electron microscopic observation revealed that the aggregation involves an initial platelet adhesion to the red blood cell; the adherent platelets then become activated and serve as foci for the growing aggregates. Dipyridamole appeared to inhibit the initial platelet adhesion to the red cell (the principal trigger mechanism for SPA) which may mimic the initiation of thrombosis in some situations in vivo. The inhibitory effect of dipyridamole on the platelet-red cell interaction suggests that this drug has antithrombotic potential in situations where red blood cells have a trigger role in platelet activation and may explain why the drug has been more effective in some situations than in others.

Adult↗
PubMed1989

Human whole-blood granulocyte aggregation in vitro.

1. Aggregation assays are a commonly used technique for the study of granulocyte activation. These studies are usually performed using a pure cell suspension in buffer. This necessitates a separation procedure which is time-consuming and may modify the function of the cells. Interaction between different cell types is precluded. 2. To avoid these disadvantages a method was developed which quantifies granulocyte aggregation in whole blood. Samples drawn from an incubated vessel before and after the addition of a chemotactic stimulus were fixed with formaldehyde to prevent disaggregation. Erythrocytes were then removed by chemical lysis and using an electronic cell-sizing device the number of single cells and aggregates could then be easily measured. 3. Results from a group of volunteers showed a rapid and reversible response to a chemotactic tripeptide, with a fall in single granulocyte count and the appearance of doublets and triplets. Lymphocytes were unaffected. Intra-assay reproducibility was better than +/- 5%. 4. Using this assay, a significant elevation in aggregability was observed in blood from patients after acute myocardial infarction. 5. This novel technique, by avoiding the separation step, is faster, simpler and more physiological than previous methods, and as such is useful for both assays of drug action in vitro and the study of cell activation in disease states.

Adult↗
PubMed1989

Modelling transient heat transfer through the skin and a contact material.

Two thermal models of the skin and superficial tissues are described, both of which incorporate the thermal effects of subcutaneous and dermal blood flow through temperature-dependent heat sources. The model which represents the dermal blood flow as the perfusion of a region of finite thickness gives rise to purely numerical solutions for the transient temperatures. Representing the dermal blood flow by a heat generating surface gives rise to a model which yields explicitly analytical expressions. Although these analytical expressions require numerical integration techniques to finally evaluate them, they are of a form which allows temperature values to be derived more efficiently than by the purely numerical method. Both models are used to predict the time-dependent temperature profiles resulting from the interaction between the tissues and thermally passive materials in intimate contact with the skin surface. The predictions of the two models are compared and the limitations of the analytical model highlighted.

Aluminum↗