Elevated levels of mechanical stress reduce perfusion and induce structural damage in tissue. The purpose of this invention is to provide a method to determine the normal component of tissue stress by measuring the stress it applies to surrounding tissue. The contact stress* sensor consists of a flexible bag containing a polyelectrolyte gel and an NMR pH indicator. Normal stress applied to the bag is transmitted to the spongy gel, squeezing interstitial fluid into the surrounding bath. Compressing the gel increases its charge density, forcing a new intracellular and extracellular equilibrium pH distribution. The applied stress is inferred from a non-invasive NMR spectroscopic measurement of pH. Since no external connections are required, it is expected that a sterile field can be maintained. In addition, a potential exists for long- term stability. The sensor contains no cytotoxic substances or non-toxic chemicals in non-physiologic concentrations whose release would also present a threat to 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.
