Load cells are essentially transducers that convert force or weight into an electrical signal. They have been widely used for measuring and sensing applications in virtually every industry for decades. At the heart of most load cells is a strain gauge. This element changes resistance when pulled or pushed (placed under tension or compression). Foil strain gauges are the most common and are created from an ultra thin heat-treated metallic foil, which is either chemically etched on a thin dielectric layer or attached using vacuum deposition or sputtering techniques to bond the materials on a molecular level. The latter technique is commonly known as thin film. Desirable strain gauges are small in size, low in cost, very sensitive to strain in the load direction, and insensitive to surrounding environment temperature changes.

To measure strain with a strain gauge, an electric circuit is used that is capable of measuring extremely low resistance changes from induced microstrain. Strain gauge transducers typically employ four strain gauge elements that are electrically connected to form a Wheatstone bridge circuit. The optimal choice for strain measurement, a Wheatstone bridge circuit is a four-leg parallel divided bridge circuit that measures electrical changes resulting from resistance changes. Its output voltage is expressed in millivolts per volt of input (mV/V). A Wheatstone bridge is also well suited for temperature compensation.

Transducers typically employ four strain gauge elements that are electrically connected to form a Wheatstone bridge circuit. The optimal choice for strain measurement, a Wheatstone bridge circuit is a four-leg parallel divided bridge circuit that measures electrical changes resulting from resistance changes. Its output voltage is expressed in millivolts per volt of input (mV/V). A Wheatstone bridge is also well suited for temperature compensation.

In most cases, size and cost are the foremost issues when making the transition from standard industrial to medical applications. The basic technology does not change in terms of such capabilities as range and reliability. Medical applications typically require measurement of loads in ounces, grams, and milligrams, whereas in industrial settings, the load is typically in pounds, kilonewtons, or tons. The only exception to this rule is in physical rehabilitation devices, where standard-sized load cells are used.

All medical load cells must be highly precise and packaged to be portable and lightweight, particularly when they need to be attached directly to patients. If the cell is used inside a machine integrated with another medical device for monitoring, standard packaging materials such as stainless steel and anodized aluminum are used. If it is in contact with the human body or with fluids, special autoclave stainless-steel or disposable sensors can be used.

Early medical load cell applications included mechanical measurements such as bed-weight monitoring. Until the early 1980s, nurses had to physically monitor patients to track critical weight fluctuations. By affixing load cells to hospital beds, the beds could effectively transmit accurate patient weight to a handheld instrument. Typically four load cells, one under each leg of the bed, fed data to a junction box that was connected to a related instrument or controller.

Today, load cells in medical devices range in size from 3–4 in. in diameter for physical therapy applications down to smaller than a dime. Measurement ranges run from milligrams to hundreds of pounds and are not affected by the physical dimensions of load cells. The smallest load cell offers the same range, accuracy, and repeatability of its larger cousins. At some point, however, size does begin to limit the capacity of the sensors, but most medical applications do not require the weight range of a large load cell.