Increased interstitial pressure can reduce tissue perfusion and may lead to edema and increase contact stress* between different tissues. A device to measure and monitor contact stress is under development in BEIP. It consists of a flexible bag containing a polyelectrolyte gel and an NMR pH indicator. Normal stress applied to the bag is felt by the spongy gel, squeezing fluid from its interstitium into the surrounding bath. Compression increases the gel's charge density, altering the existing intracellular and extracellular equilibrium pH distribution. The applied stress is inferred from a noninvasive NMR spectroscopic measurement of pH. Since no external connections are required, a sterile field can be maintained. In addition, long-term stability is expected. The sensor contains no cytotoxic substances or non-toxic chemicals in non-physiologic concentrations whose release would *also present a threat to the surrounding biological tissue. The stiffness of the gel can be changed by altering its polymer content, allowing the operating range and sensitivity of the sensor to be controlled. *N.B. Contact pressure is defined as the normal component of the interfacial stress between two bodies in contact with one another; it is not the ambient isotropic normal stress known as hydrostatic pressure.
