Biomedical

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Interfacial Pressures

Pressure arising between pressure applying devices such as tourniquets, bandages and medical instruments and supporting tissue.

The Calibration Challenge

Commonly available pressure sensors are designed to measure pressures in fluids, i.e. liquids or gases, where shear forces are negligible. Pressure measurement where connective entities are involved, present a particular challenge. This is often the situation of interest in biomedical engineering where pressure applying devices come into contact with living tissue. One particular challenge is the so called hammocking effect where a device (pressure sensor) interposed between a bandage or tourniquet and a support limb in order to estimate the local applied pressure lifts the bandage or tourniquet away from the tissue while at the same time deforming the tissue underneath. The hammocking effect has been studied experimentally [1] and modeled for a rigid disc on a cylinder [2]. The hammocking effect results in an increase in the effective active area of the sensor, i.e. the area over which it integrates the applied pressure, see figure. In simple terms, the tourniquet/bandage device applies pressure to the plate area (now the calibration area) where it is in conformal contact with it. However, the pressure acting on the tourniquet membrane in the lift-off area is also coupled to the edge of the plate by the cuff-membrane producing an increase in indicated pressure which can be multiples of the actual pressure – membrane shear forces are coupled to the sensor. The indicated pressures will therefore be larger than the actual pressure. While the rigid model used here is likely to overstate the hammocking effect, it nevertheless shows that the lift-off area will change with applied pressure in a non-linear and poorly defined way and so both excess pressures and calibration difficulties are to be expected for this sensor configuration. The calibration difficulties arise since the operation of load-cell type sensors requires a well defined constant contact area.

NumericalModel

Contour model of a membrane stretched over a rigid disc supported on a solid cylinder – an extreme case of hammocking.

Towards Solutions to Hammocking

A range of sensing solutions for interfacial pressure measurement has been developed in recent years. These include force sensing resistors, membrane switches, and miniature load cells. Fiber optic based devices have an inherent appeal from a safety perspective [3, 4] although the signal interpretation can be complex.

Miniaturization of solid state pressure sensors driven largely by the automotives industry has resulted in a wide range of microelectromechanical system (MEMS) devices. Their small size makes them less intrusive than traditional metal diaphragm devices and load cell systems. MEMS modifications to adapt them to the interfacial pressure measurement environment has resulted in significant reduction in hammocking, so much so that they can now be used to reliably reflect extant pressures at biomedical interfaces such as bandage-tissue and tourniquet-tissue which arise in everyday clinical settings [5,6].

Hammocking

Cartoon of hammocking for (a) a bandage and (b) a pneumatic tourniquet.

 

  1. Casey V, Griffin S and O’Brien SBG. An investigation of the hammocking effect associated with interface pressure measurements using pneumatic tourniquet cuffs. Medical Engineering and Physics, 2001;23:511-517
  2. O’Brien SBG, Casey V. Numerical and asymptotic solutions for hammocking. Quart J Mech App Math 2002;55:409-420.
  3. Casey, V., S. O’Sullivan, and J.A. McEwen, Interface pressure sensor for IVRA and other biomedical applications. Medical Engineering & Physics, 2004. 26(2): p. 177-182.
  4. Casey, V., S. O’Sullivan, and R. Nagle, Planar transducer for measuring biomedical pressures  issue on October 21, 2003. 2003, Abatis Medical Technologies Ltd, Limerick: World Patent,WO0001296 European Patent, EP1094747, Canadian Patent, CN1299247T, Australian Patent, AU8238898.
  5. Vincent Casey, Pierce Grace and Mary Clarke-Moloney (2011). Pressure Measurement at Biomedical Interfaces, Applied Biomedical Engineering, Dr. Gaetano Gargiulo (Ed.), InTech, DOI: 10.5772/21855. Available from: https://www.intechopen.com/books/applied-biomedical-engineering/pressure-measurement-at-biomedical-interfaces.
  6. Casey, V., Biomedical Interface Pressure Transducer for Medical Tourniquets, US20120330192A1, Editor. Dec 2012, Abatis Medical Technologies Limited – Killaloe, IE.

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