Thermally switchable, crystallizable oil and silicone composite adhesives for skin-interfaced wearable devices

Abstract

Continuous health monitoring is essential for clinical care, especially for patients in neonatal and pediatric intensive care units. Monitoring currently requires wired biosensors affixed to the skin with strong adhesives that can cause irritation and iatrogenic injuries during removal. Emerging wireless alternatives are attractive, but requirements for skin adhesives remain. Here, we present a materials strategy enabling wirelessly triggered reductions in adhesive strength to eliminate the possibility for injury during removal. The materials involve silicone composites loaded with crystallizable oils with melting temperatures close to, but above, surface body temperature. This solid/liquid phase transition occurs upon heating, reducing the adhesion at the skin interface by more than 75%. Experimental and computational studies reveal insights into effects of oil mixed randomly and patterned deterministically into the composite. Demonstrations in skin-integrated sensors that include wirelessly controlled heating and adhesion reduction illustrate the broad utility of these ideas in clinical-grade health monitoring.

Fig. 1.. Design of a thermally switchable adhesive silicone composite for use in skin-mounted devices.

(A) Life-sized neonate doll wearing an electronic device for capturing biosignals. (B) Layer with switchable adhesion strength integrated in a wearable device architecture. Random oil crystallite (C) and patterned oil well (D) composites for thermally switchable adhesives. Scale bar in optical image (D) is 100 μm and applies to optical images in both (C) and (D). (E and F) Cross-sectional schematic illustrations with dimensions of random crystallite and patterned well architectures, respectively. (G) Process flow for use of wearable devices that integrate a thermally switchable adhesive layer.

Fig. 2.. Characterization of random oil crystallite/silicone composites for thermally switchable adhesives.

(A) Schematic illustration of random oil crystallite architecture. (B) Adhesion energy versus amount of 1-pentadecanol in adhesive silicone (silicone gel 1) at 23°, 45°, and 50°C. Solid and dashed lines correspond to an analytic model. (C) Adhesion energy versus temperature for 20 wt % 1-pentadecanol in silicone gel layer 1. (D and E) Adhesion energy versus thickness of silicone gel 1 layer on composites of 20 wt % 1-pentadecanol and n-docosane, respectively, in silicone gel 1 measured at 23° and 45°C. Dashed lines correspond to analytic model. At the right of each plot are optical images of the initial composite (top), after coating with 90 μm of silicone gel 1 (middle), and after heating the gel-coated composite (bottom). Scale bar is 100 μm and applies to all optical images in (D) and (E). Each data point and error bar are calculated from five different samples (n = 5).

Fig. 3.. Characterization of patterned oil well/silicone composites for thermally switchable adhesives.

(A) Schematic illustration of the discontinuous dewetting procedure for filling patterned silicone wells with oil. (B) Optical images of silicone wells (diameters, 100, 50, and 10 μm) filled with 1-pentadecanol. Scale bar is 100 μm and applies to all larger panels. Middle panel inset: Optical image of 50-μm-diameter wells with different spacings (10 and 35 μm). Scale bar is 500 μm. (C) Effect of well spacing at a set well diameter (50 μm) and (D) effect of well diameter at a set well spacing (10 μm) on adhesion energy of 1-pentadecanol composites. Blue squares and open red triangles are data measured at 23° and 50°C, respectively. (E) All data in (C) and (D) combined, in addition to data from n-docosane composites, show that the total oil well area defines the adhesion energy, independent of well diameter, spacing, or oil type (blue squares, 1-pentadecanol; open red triangles, n-docosane). Closed and open data points are measured at 23° and 50°C, respectively. For certain samples, the area covered by oil is the same (e.g., samples with well spacing and a diameter of 100 μm exhibit the same oil area as samples with spacing and a diameter of 50 μm), yielding data points that appear to overlap at the same x value. (F and G) 3D laser confocal reflectance imaging of filled oil wells (diameter, 50 μm; spacing, 10 μm) covered with a layer (100 μm) of silicone gel 1 before (F) and after (G) melting. After heating (50°C for 60 s), the oil diffuses from the wells to the surface of the composite, reducing adhesion. A few wells not fully emptied are outlined in yellow. Scale bar is 100 μm and applies to (F) and (G). Each data point and error bar are calculated from five different samples (n = 5).

Fig. 4.. Skin-interfaced wireless health-monitoring devices using thermally switchable adhesives (patterned oil well/silicone composites) to prevent skin injury during device removal.

(A) NIRS, MA, and ECG devices worn at typical locations and schematic depictions of health signals that can be measured from each device. (B) A NIRS device mounted on the forehead with a thermally switchable adhesive yields photoplethysmogram (PPG) IR and PPG red signals. a.u., arbitrary units. (C) Three-axis accelerometry as representative MA data recorded over a 225-s interval and as an individual engages in various activities including drinking water, talking, walking, and resting. (D and E) Example of cardiac activity and respiration data from overall data in (C). (F) ECG waveforms measured using a thermally switchable adhesive and a standard commercial adhesive (2477P, 3M Inc.).

Fig. 5.. Resistive heating circuit to wirelessly control adhesion.

(A) Schematic illustration of the resistive heating circuit. (B) Image of a fabricated device during bending. Scale bar is 10 mm. (C) Schematic illustration of the electronic design for actuation, data acquisition, and wireless transmission. IC, integrated circuit. MOSFET, metal-oxide-semiconductor field-effect transistor. GPIO, general-purpose input/output. MCU, microcontroller unit. ADC, analog-to-digital converter. (D) Temperature versus power density for a resistive heater (Kapton 200RS100, DuPont) with a heating area of 14 mm by 24 mm (total area including contacts: 21 mm by 24 mm). (E) Temperature versus time at a power density of 0.2 W cm⁻¹ for the system characterized in (D). (F and G) Temperature distribution at the surface of the skin (solid outline shows the area of the heater) (F) and in cross section of the skin (G) generated by 3D FEA. Linear color bar range is 29° to 47°C and applies to (F) and (G). Linear scale bar in (G) is 10 mm and applies to (F) and (G). (H) Temperature from 3D FEA simulation versus depth into the skin.

Fig. 6.. Resistive heating circuit encapsulated with the MA device.

(A to C) The resistive heating circuit folds around the MA device while still enabling data collection from the inertial measurement unit (IMU). Scale bars are 5 mm. (D) Device adhered at the suprasternal notch of a healthy adult. (E) Three-axis accelerometry from the MA device, measured over 200 s, while the individual performed different actions.