Doctors may soon be able to quantify temperature, measure movement, perform more accurate surgical operations and monitor internal organ health — all with the touch of a finger.

John Rogers, materials science and engineering professor, and his team recently showed that it is possible to design sensors that fit more curved surfaces, like a finger, with new capabilities in nanometer-scale mechanical engineering. One day, not only can surgeons have the advantage of a better sense of touch, but they can be used to excise tissue more accurately or even image an ultrasound.

For about one year, Rogers’ group had been working with silicon-integrated circuits embedded in meshlike electrode structures that allow for health and wellness monitoring in or on the human body. These epidermal electronic devices have mechanical properties similar to the human skin, but they are thin enough to be laminated against the skin and stay adhered to it without constraint.

While these planar circuits could be laminated onto the flat surfaces of an arm, forehead, shoulder or back, they could not be formed onto body parts with greater curvatures. Silicon circuits fracture after deforming beyond 1 percent strain, making it difficult to form 3-D shapes because of the required flexibility.

“The question is how do you get a brittle, rigid material like silicon, which you need for high-quality electronic devices, married with a very soft, deformable material like silicone rubber in such a way you can form the thing around without breaking the silicon,” Rogers said. “That’s the key challenge.”

Rogers and his group’s recent breakthrough shows they can now create a 3-D model that includes all the mechanical features and electrical functionality of the planar model throughout its entire body.

To form the 3-D models, Rogers casts a thin layer of silicone rubber over a model of the desired body part, peels the layer off, then flattens it. He then transfers the epidermal electronics onto this flattened membrane and flips it inside out so the electronics will be in contact with the body.

Rogers said the biggest difficulty in going from 2-D to 3-D was trying to configure the silicon in such a way that it can survive the flipping-over process. During this process, the silicon undergoes nearly 100 percent strain, so it was necessary to make many advances in the mechanical engineering behind the integrated circuit, he said.

“We figured out ways to create ultra-thin nanomembranes of silicon and structure them into these kind of snake-y, serpentine shapes embedded in a mesh with electrode structures to make an integrated circuit,” he said. “It’s very, very thin silicon, which is important because it affords a kind of flexibility, which, combined with a serpentine shape and the rubber substrate, allows you to stretch it back and forth just like the skin.”

For example, Rogers and his group have created a sock that can be wrapped around the heart and other internal organs. This thin and flexible membrane’s newly developed mechanical properties allow the heart to beat freely without any restriction. The membrane’s integrated electronics make it possible to monitor, measure and stimulate the entire heart while inside somebody’s still-beating chest.

“It’s sort of the inverse of the way that a pacemaker works,” Rogers said. “A pacemaker is delivering electrical potential to stimulate the contraction, but the reverse thing is also happening, which is that you can monitor the potential to make an assessment of the nature of the contractions that the muscle is naturally undergoing.”

The nanomembrane’s circuits can track the status of a heart or any other body part by reading and interpreting the body’s natural electrical signals. Skeletal and cardiac muscles create variations in electrical potential when they fire, which can be picked up by the silicon circuits. Because the biology behind these physiological processes has been understood for years, Rogers and his group knew what signs to look for when programming their membranes’ circuits.

The membrane’s electronic devices measure and spatially map out the heart’s potential, and its electrodes pace the heart in complex ways — far more complex than a point contact electrode is capable of, Rogers said. In the past, single-point contact wires stimulated and sensed the changes in the body’s electrical potentials.

“What we’re now saying is that we can do that same kind of stimulation and measurement, but at the level of fully integrated circuits,” Rogers said. “So instead of a single point contact with a wire, we can do a million contacts with integrated circuit technology that currently only exists in a rigid, planar, wafer-based format.”

While there is not nearly as much muscle contraction in a fingertip compared with a heart, the finger can be stimulated the same way. Rogers developed a finger cuff model to demonstrate the integration of touch sensors, tactile sensors, electrotactile stimulators, temperature gauges and strain gauges in a 3-D object. These all work to measure deformation, force and temperature but can also relay that information as a touch sensation.

Rogers’ vision for the finger cuff is twofold: to assist surgeons during operations and to equip robots with added sensory functions.

Electrotactile stimulation from the finger cuff — an electrical current injected into the finger through a small shock that creates a touch sensation — and its sensors all work together to provide a heightened sense of touch at the body-tissue scale. This allows surgeons to perform precise measurements and movements more easily during operations.

Additionally, Rogers and his group are exploring ways to develop the finger cuff in order to give robots a sense of touch. Using robots to assist surgeons is nothing new, but this could make them an even more viable option.

Rogers said he thinks this advancement in robot sensory technology would first be used in the medical field because “having an exquisitely precise sense of touch becomes an overriding design consideration in a surgical robot.”

While Rogers and his group have already pushed the boundaries for what is possible with nanometer-scale circuits, he expects it to only escalate. He noted that because this 3-D milestone proved to be compatible with silicon circuits, it is likely that more sophisticated integrated circuits would also be possible in the future.