Imperceptible magnetoelectronics

Abstract

Future electronic skin aims to mimic nature's original both in functionality and appearance. Although some of the multifaceted properties of human skin may remain exclusive to the biological system, electronics opens a unique path that leads beyond imitation and could equip us with unfamiliar senses. Here we demonstrate giant magnetoresistive sensor foils with high sensitivity, unmatched flexibility and mechanical endurance. They are <2 μm thick, extremely flexible (bending radii <3 μm), lightweight (≈3 g m(-2)) and wearable as imperceptible magneto-sensitive skin that enables proximity detection, navigation and touchless control. On elastomeric supports, they can be stretched uniaxially or biaxially, reaching strains of >270% and endure over 1,000 cycles without fatigue. These ultrathin magnetic field sensors readily conform to ubiquitous objects including human skin and offer a new sense for soft robotics, safety and healthcare monitoring, consumer electronics and electronic skin devices.

Figure 1. GMR multilayers on ultrathin PET.

(a) Diagram of an imperceptible magnetic sensor foil. Cross-section of the GMR layer system (inset). (b) Free-standing array of five Co/Cu multilayer elements on 1.4-μm-thick PET foil. (c) Ultralight (3 g m⁻²) sensor array floating on a soap bubble and (d) crumpled between fingertips. Scale bars, 10 mm. (e) GMR characteristics of one Co/Cu first maximum element as prepared (green curve, R=16.9 Ω), after attaching to the palm and folding (blue curve, R=17.0 Ω), and after crumpling as shown in d (red curve, R=17.9 Ω). Comparison with a reference sample on rigid silicon (dashed grey curve, R=16.2 Ω). Corresponding measurements on the other elements in the array gave similar results. (f,g,h) Imperceptible GMR sensor array on a human palm with one element connected to a readout circuit during rest, moving the hand and in proximity to a permanent magnet. Scale bars, 20 mm. (i) Recorded resistance of the sensor element for f through h and Supplementary Movie 3. (j,k,l) On-skin magnetic proximity sensor using a Co/Cu second maximum GMR element on the fingertip connected to a linear array of light-emitting diodes (LEDs) (dashed red frame) as touchless interface. Scale bars, 10 mm (see Supplementary Movie 4).

Figure 2. Stretchable GMR sensors.

(a) Illustration of stretchable magnetoelectronics. A multilayer GMR element on ultrathin PET is laminated face down onto a prestretched stripe of sticky rubber tape. Four contact pads are reaching beyond the tape (top). Relaxing the elastomer results in out-of-plane wrinkling of the sensor foil and enables re-stretching (bottom). (b) Py/Cu second maximum sample mounted to the in situ stretching stage fully elongated (top) and compressed by 50% (bottom). The pink arrow indicates the axis of the applied magnetic field. Scale bars, 5 mm. (c) Optical microscopy (scale bar, 200 μm) and SEM. (scale bar, 100 μm) top-view images reveal the wrinkle structure of the sensor surface in its compressed state. (d) Cross-sectional SEM images of the sensor foil laminated to the rubber tape. The GMR nanomembrane is encapsulated between the ultrathin PET and the stretchable adhesive tape. Some parts of the magnetoresistive foil on the tip of the buckles are bent into radii of curvature of <3 μm (right). Scale bars, 1 μm (left), 2 μm (right).

Figure 3. Results of stretching experiments.

(a) GMR curves recorded for strains from 0% to 250% in increments of 50%, plus 270%. (b) GMR magnitude (red dots) and resistance change normalized to 0% strain (black squares, R₀=9.7 Ω) as a function of applied strain. The shaded region indicates overstretching with plastic deformation of the sensor foil. Reliability on cyclic loading. (c) GMR curves of a Py/Cu second maximum element at 50% strain (blue) and 100% strain (red); first cycle in light shades, cycle 1,000 in strong shades. The characteristic of the as-prepared sample is plotted in dashed grey. (d) GMR magnitude (red dots) and resistance change normalized to the as-prepared sample (black squares, R₀=10.0 Ω) at 50% strain (open symbols) and 100% strain (closed symbols) as a function of cycle number.

Figure 4. Biaxial stretching on a soft diaphragm.

(a,b) Biaxially stretchable Co/Cu second maximum GMR sensor on a VHB membrane spanned over a plastic tray to create a sealed chamber. Water is pumped through the inlet to inflate the VHB membrane and stretch the sensor along both lateral directions. A permanent magnet was fixed inside the water chamber, to enable the dynamic detection of diaphragm inflation/deflation cycles. The flat and fully inflated states are shown in the left and right inset, respectively. (c) GMR curves recorded for different areal strains, as stated in the legend. The left and right insets show the biaxially wrinkled sensor at 0% and 175% strain. (d) GMR magnitude (red dots) and sensor resistance (black squares) as a function of applied areal strain. The strain was estimated from side-view photographs of each inflated state (Supplementary Fig. 11). (e) Sensor signal for a pulsating diaphragm for the dynamic magnetic detection of its inflation/deflation (see Supplementary Movie 6). The dashed line is a smoothed graph to guide the eye.