Inflammation is the body’s first response to tissue damage and a mechanism to combat injury.

The inflammation increases vasodilation and permeability of blood vessels, leading to leakage of plasma and fluid creating localised oedema in the tissue. This is accompanied by recruitment of leukocytes prior to a systemic response.

As the level of tissue damage increases, so does the inflammatory response. Accordingly, the resulting level of localised tissue oedema or water in the skin and tissue, termed sub-epidermal moisture (SEM), increases1.

These inflammatory changes in the skin and underlying tissues with the localised tissue oedema can occur from 3 to 10 days before damage and/or breakdown of skin is visible at the surface2

The SEM Scanner

The SEM Scanner, a wireless handheld device that is indicated for use as an adjunct to the standard of care when assessing patients who are at increased risk for pressure ulcers. The SEM Scanner is the world’s first FDA-authorized device to objectively alert clinicians to specific anatomical areas of a patient’s body at increased risk for developing pressure damage. Patient risk assessments are performed with the SEM Scanner before visible damage manifests at the skin surface.

The SEM Scanner consists of a pair of concentric coplanar electrodes, an integrated pressure sensor, software that computes a “delta” value from a set of readings made around a common location, and a user interface device screen that displays the device status, battery status, the most recent reading, and the calculated delta value.

Courtesy of Prof A Gefen

The device exploits differences between the dielectric constants of materials that make up tissue. Dry tissue has a low dielectric constant while water has a dielectric constant that is 20x higher. When the sensor is pressed against an area on the skin, the SEM Scanner measures the electrical capacitance of the sensor, which is affected by the moisture within the underlying skin tissue to a depth of approximately 4 mm. The device converts this reading into a unitless value and displays this value to the user. The device also computes a delta value from each set of measurements and displays this delta value (Δ) to the user.

A Δ < 0.6 at an anatomical site may suggest the tissue is at lower risk for pressure ulcers

A Δ ≥ 0.6 at an anatomical site may suggest increased risk for pressure ulcers

Current clinical decision making relies on assessing a patient’s overall risk for pressure ulcer development and then completing a subjective skin and tissue assessment. Both methods suffer from not being able to direct clinicians to where the risk is building until damage is visible at the skin’s surface. Once pressure ulcers (also known as pressure injuries, or bed sores) become visible on the skin’s surface, tissue damage has already occurred.

Elevated readings from the SEM Scanner provide objective clinical information that directs clinicians to heels and sacrums at risk of developing pressure ulcers, even before the damage is visible3. These data can facilitate earlier, anatomically specific interventions designed to reverse the damaging effects of pressure and shear and prevent the pressure injuries from breaking through the skin.


  1. Gefen A. (2018). The Sub-Epidermal Moisture Scanner: the principles of pressure injury prevention using novel early detection technology. Wounds International, 9 (3)
  2. Moore Z. et al. (2016). Subepidermal moisture (SEM) and bioimpedance: a literature review of a novel method for early detection of pressure-induced tissue damage (pressure ulcers). International Wound Journal, 14(2), pp.331-337
  3. Okonkwo et al. (2018). Differentiating between Healthy Tissue and Early Stage Pressure Injuries: a Pilot Study of the Effectiveness of the SEM Scanner. Abstract 52 presented to 30th NPUAP Annual Conference, Las Vegas, NV, 2-3 March