- Open Access
Polymer-based flexible and multi-directional tactile sensor with multiple NiCr piezoresistors
© The Author(s) 2019
- Received: 3 April 2019
- Accepted: 23 May 2019
- Published: 27 May 2019
A polymer-based tactile sensor with flexibility and multi-directional sensing capability is presented. The proposed sensor consists of a polydimethylsiloxane (PDMS) bump, a polyimide (PI) substrate, Cr/Au electrode lines for electrical connection, NiCr piezoresistors, and an SU-8 support structure. The sensing mechanism is based on piezoresistive effect, in which the resistance of NiCr changes under mechanical load. The PMDS bump positioned at the center of the sensor transfers an applied force to the PI film, and the piezoresistors are differently deformed depending on the magnitude and direction of the force. A diaphragm structure formed by the SU-8 support with a trench allows the piezoresistor to be effectively deformed. Simulation and experimental results confirm that magnitude and direction can be obtained from an arbitrarily applied force by comparing the change in resistance of each sensing element. Based on its compatibility with conventional microfabrication, the proposed sensor may be a promising candidate for a low-cost tactile sensing solution for human‒machine interfaces.
- Tactile sensor
- Strain gauge
- Multi-directional force sensor
- Polymer micromachining
Tactile interfaces are attracting significant attention in the field of robotics because they can realize the force feedback control of robots . A tactile sensor that acquires information via physical contact is a key element of a tactile interface. In particular, miniaturized tactile sensors have been actively developed owing to their potential for use in medical applications, such as robot-assisted surgery systems and cancer diagnosis [2, 3]. For such applications, tactile sensors are supposed to consist of soft and flexible biocompatible materials in order to prevent organ damage and to allow the sensors to be easily mounted on a complex-shaped robot hand. Multi-directional sensing capability over a wide range of forces is also an essential feature for precise and stable measurement. To meet these requirements, a variety of flexible tactile sensors based on various sensing mechanisms have been developed, including resistive , capacitive , and piezoelectric  types.
When compared with other sensing mechanisms, resistive sensors have advantages such as simple device structure and ease of signal processing on the acquired data . In recent years, coupled with progress in the scalable production of nanomaterials, flexible resistive tactile sensors utilizing mainly conductive-nanomaterial-polymer composites have been studied [8–10]. To date, however, most previous works have focused on the development of tactile sensors only for normal force detection, with very few studies on multi-directional force sensing [11–13]. Moreover, nanomaterials tend to agglomerate in uncured viscous polymers, making it difficult to uniformly disperse them in the polymer matrix. This leads to performance deviation between fabricated devices, which limits the reproducibility of the sensor. One promising alternative is to integrate a thin-film piezoresistor into a polymer substrate [14, 15]. For example, Hwang et al.  demonstrated a flexible tactile sensor based on thin metal strain gauges that were patterned on a polymer substrate, and both normal/shear load detection and reproducibility were achieved. Nevertheless, the extraction of magnitude and direction from an arbitrarily applied force is challenging because the output signal of the sensor is asymmetrical to the direction of the shear force. In addition, an elastomer substrate with energy dissipation through material damping limits the deformations of the piezoresistors, which results in sensitivity reduction .
In this study, we report a diaphragm-like tactile sensor utilizing multiple piezoresistor configurations that are capable of detecting multi-directional forces. The proposed polymer-based sensor is batch-fabricated on a 4-in. SiO2/Si wafer via conventional microfabrication to demonstrate its practical use. The dependence of the deformation of piezoresistors on the direction of the applied force was thoroughly investigated by finite element analysis (FEA). To verify the sensing performance of the fabricated device, changes in the resistances of the piezoresistors for normal and shear forces were measured. Based on the piezoresistive characteristics, both the magnitude and direction of the applied force were successfully detected.
It is noteworthy that tactile sensors must be designed to produce linear signals to obtain the magnitudes and directions from multi-directional forces using Eqs. (10‒12). Design improvements for linearity over a wide force range and its experimental evaluation are underway and will be provided in a future publication.
We have demonstrated a flexible and multi-directional tactile sensor composed of polymers, NiCr piezoresistors, and thin metal electrodes. The proposed sensor array was batch-fabricated through conventional microfabrication including photolithography, evaporation, plasma etching, and molding. The strains generated on the sensing elements under different loading conditions were investigated by FEA, which confirmed that the strain values of each sensing element depended on the direction of the applied force. We also measured the changes in resistance of the piezoresistors for normal and shear forces. The experimental results validated the multi-directional sensing capability of our sensor based on piezoresistive characteristics. Our polymer-based sensor can be used for low-cost tactile sensor applications that require multi-directional sensing and mechanical flexibility.
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIT) (2018R1A4A1025986).
SP, JIL, HKL and JK developed the idea. SP and JIL carried out fabrication, measurement, and analysis of the results, and wrote the manuscript. MOK performed finite element analysis and supported fabrication process and measurement. HKL suggested fabrication method and material selection. JK supervised the research and reviewed the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- Pacchierotti C, Sinclair S, Solazzi M, Frisoli A, Hayward V, Prattichizzo D (2017) Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives. IEEE Trans Haptics 10:580–600View ArticleGoogle Scholar
- Tiwana MI, Redmond SJ, Lovell NH (2012) A review of tactile sensing technologies with applications in biomedical engineering. Sens Actuat A Phys 179:17–31View ArticleGoogle Scholar
- Pacchierotti C, Prattichizzo D, Kuchenbecker KJ (2016) Cutaneous feedback of fingertip deformation and vibration for palpation on robotic surgery. IEEE Trans Biomed Eng 63:278–287View ArticleGoogle Scholar
- Lee JI, Pyo S, Kim MO, Kim J (2018) Multidirectional flexible force sensors based on confined, self-adjusting carbon nanotube arrays. Nanotechnology 29:055501View ArticleGoogle Scholar
- Lee HK, Chung J, Chang SI, Yoon E (2011) Real-time measurement of the three-axis contact force distribution using a flexible capacitive polymer tactile sensor. J Micromech Microeng 21:035010View ArticleGoogle Scholar
- Kim MO, Pyo S, Oh Y, Kang Y, Cho KH, Choi J, Kim J (2018) Flexible and multi-directional piezoelectric energy harvester for self-powered human motion sensor. Smart Mater Struct 27:035001View ArticleGoogle Scholar
- Maheshwari W, Saraf R (2008) Tactile devices to sense touch on a par with a human finger. Angew Chem Int Ed 47:7808–7826View ArticleGoogle Scholar
- Pang Y, Tian H, Tao L, Li Y, Wang X, Deng N, Yang Y, Ren TL (2016) Flexible, highly sensitive, and wearable pressure and strain sensors with graphene porous network structure. ACS Appl Mater Interfaces 8:26458–26462View ArticleGoogle Scholar
- Wang T, Zhang Y, Liu Q, Cheng W, Wang X, Pan L, Xu B, Xu H (2018) A self-healable, highly stretchable, and solution processable conductive and polymer composite for ultrasensitive strain and pressure sensing. Adv Funct Mater 28:1705551View ArticleGoogle Scholar
- Zhao X, Xu W, Yi W, Peng Y (2019) A flexible and highly pressure-sensitive PDMS sponge based on silver nanoparticles decorated reduced graphene oxide composite. Sens Actuat A Phys 291:23–31View ArticleGoogle Scholar
- Hu CF, Su WS, Fang W (2011) Development of patterned carbo nanotubes on a 3D polymer substrate for the flexible tactile sensor application. J Micromech Microeng 21:115012View ArticleGoogle Scholar
- Han JE, Kim D, Yun KS (2012) All-polymer hair structure with embedded three-dimensional piezoresistive force sensors. Sens Actuat A Phys 188:89–94View ArticleGoogle Scholar
- Pyo S, Lee JI, Kim MO, Chung T, Oh Y, Lim SC, Park J, Kim J (2014) Development of a flexible three-axis tactile sensor based on screen-printed carbon nanotube-polymer composite. J Micromech Microeng 24:075012View ArticleGoogle Scholar
- Hwang ES, Seo JH, Kim YJ (2007) A polymer-based flexible tactile sensor for both normal and shear load detections and its application for robotics. J Microelectromech Syst 16:556–563View ArticleGoogle Scholar
- Ahmed M, Chitteboyina MM, Butler DP, Celik-Butler Z (2013) MEMS force sensor in a flexible substrate using nichrome piezoresistors. IEEE Sens J 13:4081–4089View ArticleGoogle Scholar
- Pyo S, Choi J, Kim J (2018) Flexible, transparent, sensitive, and crosstalk-free capacitive tactile sensor array based on graphene electrodes and air dielectric. Adv Electron Mater 4:1700427View ArticleGoogle Scholar
- Jung HI, Kwon DS, Kim J (2017) Fabrication and characterization of monolithic piezoresistive high-g three-axis accelerometer. Micro Nano Syst Lett 5:7View ArticleGoogle Scholar
- Eltaib MEH, Hewit JR (2003) Tactile sensing technology for minimal access surgery-a review. Mechatronics 13:1163–1177View ArticleGoogle Scholar