A force-sensing resistor is a material whose resistance changes when a force, pressure or mechanical stress is applied. They are also known as force-sensitive resistor and are sometimes referred to by the initialism FSR.

The technology of force-sensing resistors was invented and patented in 1977 by Franklin Eventoff. In 1985, Eventoff founded Interlink Electronics, a company based on his force-sensing-resistor (FSR). In 1987, Eventoff received the prestigious International IR 100 award for developing the FSR. In 2001, Eventoff founded a new company, Sensitronics, that he currently runs.

Force-sensing resistors consist of a conductive polymer, which predictably changes resistance following applying force to its surface. They are normally supplied as a polymer sheet or ink that can be applied by screen printing. The sensing film consists of electrically conducting and non-conducting particles suspended in a matrix. The particles are sub-micrometre sizes formulated to reduce temperature dependence, improve mechanical properties and increase surface durability. Applying a force to the surface of the sensing film causes particles to touch the conducting electrodes, changing the film's resistance. As with all resistive-based sensors, force-sensing resistors require a relatively simple interface and can operate satisfactorily in moderately hostile environments. Compared to other force sensors, the advantages of FSRs are their size (thickness typically less than 0.5 mm), low cost, and good shock resistance. A disadvantage is their low precision: measurement results may differ by 10% and more. Force-sensing capacitors offer superior sensitivity and long-term stability, but require more complicated drive electronics.

There are two major operation principles in force-sensing resistors: percolation and quantum tunneling. Although both phenomena co-occur in the conductive polymer, one phenomenon dominates over the other depending on particle concentration. Particle concentration is also referred in the literature as the filler volume fraction ${\displaystyle \phi }$. More recently, new mechanistic explanations have been established to explain the performance of force-sensing resistors; these are based on the property of contact resistance ${\displaystyle R_{C}}$ occurring between the sensor electrodes and the conductive polymer. Specifically the force induced transition from Sharvin contacts to conventional Holm contacts. The contact resistance, ${\displaystyle R_{C}}$, plays an important role in the current conduction of force-sensing resistors in a twofold manner. First, for a given applied stress ${\displaystyle \sigma }$, or force ${\displaystyle F}$, a plastic deformation occurs between the sensor electrodes and the polymer particles thus reducing the contact resistance. Second, the uneven polymer surface is flattened when subjected to incremental forces, and therefore, more contact paths are created; this causes an increment in the effective area for current conduction ${\displaystyle A}$. At a macroscopic scale, the polymer surface is smooth. However, under a scanning electron microscope, the conductive polymer is irregular due to agglomerations of the polymeric binder.

To date, no comprehensive model is capable of predicting all the non-linearities observed in force-sensing resistors. The multiple phenomena occurring in the conductive polymer turn out to be too complex such to embrace them all simultaneously; this condition is typical of systems encompassed within condensed matter physics. However, in most cases, the experimental behavior of force-sensing resistors can be grossly approximated to either the percolation theory or to the equations governing quantum tunneling through a rectangular potential barrier.

The percolation phenomenon dominates in the conductive polymer when the particle concentration is above the percolation threshold ${\displaystyle \phi _{c}}$. A force-sensing resistor operating based on percolation exhibits a positive coefficient of pressure, and therefore, an increment in the applied pressure causes an increment in the electrical resistance ${\displaystyle R}$, For a given applied stress ${\displaystyle \sigma }$, the electrical resistivity ${\displaystyle \rho }$ of the conductive polymer can be computed from:

where ${\displaystyle \rho _{0}}$ matches for a prefactor depending on the transport properties of the conductive polymer, and ${\displaystyle x}$ is the critical conductivity exponent. Under percolation regime, the particles are separated from each other when mechanical stress is applied; this causes a net increment in the device's resistance.

Quantum tunneling is the most common operation mode of force-sensing resistors. A conductive polymer operating on the basis of quantum tunneling exhibits a resistance decrement for incremental values of stress ${\displaystyle \sigma }$. Commercial FSRs such as the FlexiForce, Interlink and Peratech sensors operate based on quantum tunneling. The Peratech sensors are also referred to in the literature as quantum tunnelling composite.