Stretchable and Washable Strain Sensor Based on Cracking Structure for Human Motion Monitoring

Abstract

Stretchable and wearable strain sensors have been intensively studied in recent years for applications in human motion monitoring. However, achieving a high-performance strain sensor with high stretchability, ultra-sensitivity, and functionality, such as tunable sensing ranges and sensitivity to various stimuli, has not yet been reported, even though such sensors have great importance for the future applications of wearable electronics. Herein, a novel and versatile strain sensor based on a cracking (silver ink patterned silicone elastomer)-(silver plated nylon structure) (Ag-DS/CF) has been designed and fabricated. The unique structure combined precisely shaped stretchable conductive fabrics and wrinkled Ag-ink pattern to achieve an excellent electrical performance. The Ag-DS/CF could be used to detect both large and subtle human motions and activities, pressure changes, and physical vibrations by achieving high stretchability up to 75%, ultrahigh sensitivity (gauge factor >10⁴-10⁶), tunable sensing ranges (from 7 to 75%). Excellent durability was demonstrated for human motion monitoring with machine washability. The extremely versatile Ag-DS/CF showed outstanding potential for the future of wearable electronics in real-time monitoring of human health, sports performance, etc.

Figure 1

Fabrication, working mechanism, and photograph of Ag-DS/CF strain sensor. Fabrication processes of Ag-DS/CF strain sensors (a). The working mechanism of Ag-DS/CF strain sensor (b). The Ag-DS/CF form cracks perpendicular to direction of applied strain, whereas the fabric stretches parallel to the applied strain. The sensor configuration is expressed as [length of the over-molded part (L_DS)]-[length of the CFs on both sides (L_CF1 and L_CF2) of the over-molded part] in their percentages for the total length of the sensor (L_T = L_DS + L_T-CF). Photograph of the fabricated sample of Ag-DS/CF (c).

Figure 2

Optical microscopy images of Ag-DS/CF. The optical microscopy images of the knitted structure of CFs in Ag-DS/CF (a), interface between protrusion of CF and Ag-ink in Ag-DS/CF (b), and wrinkled Ag-ink pattern in pre-stretching direction in Ag-DS/CF (c), with scale bars of 200 µm and when unstretched.

Figure 3

Measurement results for strain. Relative resistance change versus applied strain of Ag/DS-CF with different sensor configurations (L_DS% − L_T-CF%) (a). The GF versus applied strain of Ag-DS/CF (R₀ = 263–300) (b). Recorded hysteresis curve of Ag-DS/CF during loading- unloading cycles (R₀ = 263) (c). The time-dependent response of Ag-DS/CF 80 sensor configuration under dynamic loadings with strains of 5–20% (R₀ = 263) (d). Step and hold test for Ag-DS/CF (R₀ = 300) (e). The relative resistance change of Ag-DS/CF 80 versus applied strain of 20% for long-term stability test at 3000 repeated cycles (R₀ = 300) (f).

Figure 4

Measurement results for pressure, bending, and vibration. The relative change to the applied pressure of 0–10 kPa for Ag-DS/CF when it is distributed evenly over the area of DS without the pre-stress (a). The dynamic response of Ag-DS/CF to applied pressure of finger touch (b). The relative resistance change versus applied pressure of 10 kPa for over 1000 repeated cycles with the pre-stress. The response was measured after each 100 repeated cycles (c). The Ag-DS/CF response to bending between fingers to approximately bending angle of 180 degrees (d). The time-dependent response of Ag-DS/CF to oscillating steel ruler (e). (R₀ = 300 Ω).

Figure 5

Measurements results for subtle signals and large scale motions associated with the human body. Photographs showing the Ag-DS/CF strain sensor fixed onto the back of the hand (a), gastrocnemius muscle (b), vastus lateralis (c), wrist (d), neck (e,f) cheek (g₁), forehead (g₂), and lower neck (g₃), and chest (h). The corresponding time dependent signals of opening and closing fist (a), muscle tension of gastrocnemius muscle when doing one legged seating calf raises (b₁) and jumping with two feet (b₂), muscle tension of vastus lateralis when standing up from seated position and tensing the muscle after standing up (c), wrist pulse (d), swallowing of saliva (e₁), coughing (e₂), sniffing (e₃), and phonation of words one (f₁), three (f₂), and nine (f₃), chewing (g₁), motion of forehead when lifting the eyebrows (g₂), and head movement when looking down (g₃), and chest movement during normal (h₁) and deep breathing (h₂). (R₀ = 300 Ω).