3D Printed Stretchable Tactile Sensors

Abstract

The development of methods for the 3D printing of multifunctional devices could impact areas ranging from wearable electronics and energy harvesting devices to smart prosthetics and human-machine interfaces. Recently, the development of stretchable electronic devices has accelerated, concomitant with advances in functional materials and fabrication processes. In particular, novel strategies have been developed to enable the intimate biointegration of wearable electronic devices with human skin in ways that bypass the mechanical and thermal restrictions of traditional microfabrication technologies. Here, a multimaterial, multiscale, and multifunctional 3D printing approach is employed to fabricate 3D tactile sensors under ambient conditions conformally onto freeform surfaces. The customized sensor is demonstrated with the capabilities of detecting and differentiating human movements, including pulse monitoring and finger motions. The custom 3D printing of functional materials and devices opens new routes for the biointegration of various sensors in wearable electronics systems, and toward advanced bionic skin applications.

Keywords: 3D printing; bionic skin; stretchable electronics; tactile sensors; wearable devices.

Figure 1

Tactile sensor design principle and 3D printing procedure. (a) Schematic of the tactile sensor consisting of a base layer, top and bottom electrodes, an isolating layer, a sensor layer, and a supporting layer. (b) Side and (c) top view of the tactile sensor. (d) 3D printing process of the sensor on a glass substrate in eight sequential steps. In step I, a 4 × 4 mm² silicone base layer is printed. In step II, a 3 × 3 mm² bottom electrode layer is printed using the 75 wt% Ag/silicone ink. In step III, a 1-mm-tall, 150-μm-thick cylinder wall with a radius of 350 μm is printed using the 68 wt% Ag/silicone ink as the sensor layer. In step IV, a 3 × 3 mm² isolating layer is printed using the silicone ink. In step V, a 3 × 3 mm² supporting layer with a thickness of 0.8 mm is printed using the 40 wt% Pluronic ink. In step VI, a 2 × 2 mm² top electrode layer is printed using the 75 wt% Ag/silicone ink. In step VII, the supporting layer is removed by immersing the sensor in water for three hours. Finally, in step VIII, the sensor is dried for completion.

Figure 2

Mechanical characterization and sensing behavior of various inks. SEM images show the Ag particle distribution in the inks with (a) 68 wt% and (b) 75 wt% Ag loadings. All scale bars are 5 μm. (c) Tensile and (d) compressive strength measurements on the cured inks with different Ag loading contents. (e) Resistance as a function of applied pressure for three inks. The specimens for the 68% Ag ink were cylinders (diameter 1 mm, height 1 mm), and for the 70% and 80% Ag inks were serpentine filaments (length 15 mm, diameter 0.2 mm). (f) Relative current change of 68 wt% Ag/silicone ink upon three different applied cyclic pressures. The red lines show the signals for 60 kPa applied pressure. The yellow lines are the signals for 120 kPa loading. The blue lines are the signals for 250 kPa loading.

Figure 3

Sensing behavior of the 3D printed tactile sensor. (a) Top view SEM image of the printed sensor layer. (b) Side and (c) top view SEM images of the 3D printed tactile sensor. All scale bars are 200 μm. (d) Current-voltage characteristics of a tactile sensor under different applied pressures. (e) Plots showing frequency responses to a dynamic pressure of 200 kPa at an input frequency of 0.125 Hz. (f) Current change for various frequencies of 200 kPa applied pressure. (g) Mean compressive gauge factor for different ink materials and the tactile sensor device. (h) Current change of the tactile sensor upon applying a constant strain with an initial pressure of 500 kPa. (i) Current change of the device when subjected to 100 pressing cycles with a pressure of 100 kPa at a frequency of 0.25 Hz.

Figure 4

Mechanical sensing applications of the 3D printed stretchable tactile sensor. (a) Photograph showing the tactile sensor mounted directly above the radial artery. Measurement signals of radial pulse under (b) sedentary and (c) post-exercise states (running upstairs for 5 min). Plots showing the current change signals in response to dynamic loading and unloading cycles in (d) pressing and (e) bending. (f) Optical image showing the conformally printed 3D tactile sensor on a fingertip. Scale bar = 4 mm. (g) Current change signal of the tactile sensor printed on a fingertip upon pressing by a human finger. (h) Top view of a triangular glass object placed on the surface of the 5 × 5 pixel tactile sensor. Scale bar = 2 mm. (i) Signal mapping of the pressure distribution for a triangular object (0.096 g) with a 50 g weight.