Recently, Dr. GONG Xinxin, a member of Prof. FEI Guangtao’s group at the Institute of Solid State Physics, Hefei Institutes of Physical Science designed a novel flexible strain sensor with high gauge factor, which does not need expensive measurement equipment, showing high cost advantage.

Strain sensors have been widely used in many areas such as biomedical detection, medical rehabilitation, intelligent robot, light-weight mobile electronics, and wearable devices, whose performance requires conditions of high strain and flexibility.

Actually, a strain sensor also requires high gauge factor (GF) and low resistance. Sensors with low GF display little variation of resistance resulting errors, and which with high resistance need high accurate and expensive equipment.

In recent years, flexible strain sensors are mainly made of metals or semiconductors. In general, sensors based on metals have low resistance but low GF, otherwise, which based on semiconductors have high GF but high resistance.

This limits the application of both of them in a large certain degree. Therefore, developing a new type of flexible sensor that realize both low resistance and high GF remians as urgent demand.

 Fig. 1 The behaviors of the sensor at different states. (Image by GONG Xinxin) 

To aim at this, Dr. GONG and her coworkers found an excellent candidate material for strain sensor: polyaniline (PANI), one kind of conductive polymer, whose conductivity fall in between metal and semiconductor and can be adjusted by doping with acid.

In addition, infiltrating polydimethylsiloxane (PDMS) elastomers in PANI film would be an effective approach to get a wide toleration to strain and high conductivity.

Hence, they produced a new type of strain sensor with high tolerable strain by infiltrating PDMS into a conductive PANI film. The novel strain sensor based on PANI/PDMS film could be folded, twisted, and bended, displaying flexible properties (Fig.1).

Fig. 2 The properties of PANI/PDMS film strain sensor. (a) The relative change in resistance under variable strains; (b) GFs of the sensor under variable strains; (c) Current response of the strain sensor. The sensor was repeatedly stretched and released under 50% strain at a speed of 400%/s. Insets: enlarged view of the selected areas. (Image by GONG Xinxin) 

The flexible strain sensor could withstand strain up to ~50% with GF of about 54, higher than other materials previously reported, and demonstrated a high durability, fast response, and good performance (Fig.2).

More importantly, a prototype flexible strain sensor which was fixed onto one person’s finger (Fig.3) demonstrated the great potential applications in the monitoring of people’s activities.

This work was published in Organic Electronics entitled Flexible strain sensor with high performance based on PANI/PDMS films. 

Fig. 3 Response current of the strain sensor in monitoring finger bending. The strain sensor was fixed on an index finger with adhesive tapes. Inset: Photographs of finger-bending to the corresponding positions labelled as I, II, III, and IV demonstrate the four finger motion states. (Image by GONG Xinxin)