As complex robots replace hand-held scalpels, an increasing number of today's surgeries are being performed from behind a computer console. Researchers at Harvard University's John A. Paulson School of Engineering and Applied Science and the Wyss Institute for Biologically Inspired Engineering have built low-cost, millimeter-scale force sensors that can be used on the medical robotic tools as they snake through hard-to-reach areas of the body.

Inspired by origami and pop-up books, the technique fabricates complex micromachines by layering laser-cut materials into thin, flat plates that pop up into a complete electromechanical devices. The sensor consists of four layers of laser-machined stainless steel sandwiched together and laminated with a flexible polyimide layer to facilitate self-assembly; copper provides the electrical contacts. When agitated in an ultrasonic bath, the 2D structure "pops up" into a boxy 3D sensor via an integrated spring. With a footprint of 2.7 mm, the sensors are small enough to pass through the working port of an 8.6-mm endoscope.

Using a principle called light intensity modulation (LIM), the sensor detects force at the millinewton level. LIM connects a light emitter and a light detector with an elastic element. When force is applied, the elastic element deforms, bringing the emitter and detector closer together. The change in irradiance sensed by the detector can be translated into applied force.

The researchers will next work to make the sensor even smaller and more robust, potentially providing flexible surgical robots with a sense of "touch."