Flexible and stretchable force sensor for static and dynamic measurements (FlexFo)

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Engineering


Force sensing plays a key technological role in providing tactile feedback in automated systems thus maximising efficiency in industrial applications (i.e., pick-and-place robots, robotic welding) and enabling novel devices and applications (i.e., video games controllers and smart homes). Now that human-oriented technologies (i.e., electronic and robotic skins, prosthetics, surgical robotic arms and rehabilitative devices, force-sensitive buttons on smartphones) are becoming a ubiquitous part of daily life, the requirement for improved force sensors is self-evident. Sensors that mimic human tactile receptors have been developed. However, the existing devices do not satisfy technological needs of flexibility, stretch-ability, high force and spatial resolution, self-powering and adaptability to measure both static and dynamic forces.
Therefore, new forms of sensor are essential and this research programme aims to tackle this technological need by proposing a new transformative device featuring a force sensitive flexible and stretchable material with embedded well-aligned and ordered nanowires (a smart nanocomposite material). The smart nanocomposite is made using a unique and innovative approach that involves filling a polymer with well-ordered and aligned high aspect ratio nanowires (well-defined geometrical shape with length much greater than width). This approach differentiates substantially from the usual conservative methods where low aspect ratio nanoparticles (imperfect spherical shapes) are randomly dispersed and distributed into polymers. In this way, the transformative strategy of organising the nanowires in well-ordered patterns will overcome the disadvantages and limitations of present sensors such as low area/force/position resolution, limited functionality (measuring either static or dynamic forces) and low adaptability to different applications (flexible but not stretchable).
The intrinsic discrete particle aspect and piezoelectric nature of the nanowires enables sensor operation in a combined resistive and piezoelectric functionality and thus enables both static and dynamics force measurements with the same device. The device will be driven with low DC bias voltage (low power consumption and zero-power when operating in "piezoelectric mode"), and will provide modularity, flexibility and stretch-ability for optimal surface conformability (i.e., adaptability to a wide range of systems and geometries). The sensor prototypes will be tested against commercially available sensors and their force resolution, flexibility, stretch-ability and reliability will be compared under different bending conditions.
In summary, the research programme has three main objectives: to create a combined resistive and piezoelectric smart nanocomposite; use the smart nanocomposite to develop a flexible and stretchable force sensor for both static and dynamic measurements; and to test and compare the developed devices against commercially available sensors.
The research will benefit those fields in which force sensing is needed (static, dynamic, impact force measurements). The first targeted application will involve integration of the devices into robotic arms to provide tactile feedback. However, the proposed approach of developing a smart nanocomposite that will enhance the performance of a sensing device has the potential to revolutionise the force sensing market, greatly improve current applications (i.e. robotics) and target novel applications including force sensing on humans (grippers, hands/feet sensors), smart clothes for healthcare and fashion, sports equipment and gadgets (currently limited solely to position or acceleration sensing).

Planned Impact

The research programme (FlexFo) will develop and optimise a procedure to create a smart nanocomposite material with nanostructures efficiently integrated into polymers, and utilise this material to develop a fully functional proof-of-concept flexible and stretchable force sensor.
In the immediate term, the interdisciplinary nature of the research programme will allow for the project outcomes to benefit and impact onto different engineering disciplines including microsystems, micro and nano fabrication, polymer materials and composites, and sensor technology. One of the main impacts will be the provision of innovative concepts to the research community for the design, fabrication and characterisation of flexible force sensors. Proposing efficient integration of nanostructures into flexible devices, the project takes a highly transformative approach to the design of flexible sensing technology, which is a highly competitive pursuit within the UK and internationally, and of growing interest for the research community.
In the short/medium term, FlexFo has strong potential for providing benefits to sectors where tactile/force feedback is needed, particularly in human-oriented technology where force sensors are already used widely or have been recently introduced (robotics, medicine, structural health monitoring in automotive, aerospace and civil sectors and textiles). The expansive field of application facilitates impact to both academia and industry where research and development activities on human-oriented devices are being undertaken. In the long term, the outcomes of the research programme will have impact on society through downstream applications of the developed technology. In fact, the device prototype will be readily integrated into conventional robotic systems where tactile feedback is needed (i.e., pick-and-place robotic arms, prosthetics, surgical robotic arms and rehabilitative devices) potentially establishing a rapid pathway to impact. The key impact element in the mid-long term comes from the modularity, scalability and potential integrability into CMOS processes of the nanostructures' synthesis procedures and sensing technology.
Beneficiaries involved with force detection and flexible sensor technology have been recognised already. Academic beneficiaries include researchers at University of Edinburgh, Heriot-Watt University, and the University of Sussex. Industrial beneficiaries include Cambridge Display Technology, Sansible Wearables, FlexEnable and Endura. FlexFo will pioneer new integration technologies of polymers and composites into CMOS and MEMS technology, in order to enable new device functionalities and applications (such as high force and spatial resolution sensing or human-like touch sensitive smart skin). This will have impact in the short to long term on the research and development community, as interest is constantly growing in the use of alternative materials to silicon to enable novel flexible smart technologies. This is the setting in which FlexFo will open up a host of novel force sensing schemes and strategies for cutting-edge applications (i.e., high force and spatial resolution sensors for structural health monitoring in aeronautics, for haptic devices in healthcare, and for smart sports equipment).


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Description A nanocomposite material has been developed that incorporates piezoelectric zinc oxide nanowires within a thin (< 0.5 mm) layer of a soft elastomer. Metal electrodes have been created on either side of the material, and when the material is subject to a pressure, a voltage is generated in response. The complete pressure sensing device is created using a simple, bottom-up fabrication process on a flexible substrate of Kapton. The nanowires are grown on a seed layer of silver and encased in polydimethylsiloxane (PDMS), which allows for the deformation of the sensor to be transduced to the nanowires. The device that has been produced using the novel fabrication method has demonstrated superior sensitivity, i.e. voltage produced for a given pressure value, when compared to previously reported devices with a similar sensing mechanism.

The device operates in a zero-power mode, with no external DC bias voltages needed. The device responds to dynamic pressures up to at least 120 kPa, with a sensitivity of 24 mV/kPa, 100 times higher than values reported previously. As a result of this work, the fabrication process for producing flexible pressure sensors incorporating a piezoelectric nanocomposite material has been developed and optimised. The knowledge that has been advanced during this work has the potential to be used in applications such as robotics, smart clothing and electronic skins.
Exploitation Route Integration of the developed sensing technology with advanced electronic devices could be performed for a number of applications. Further research into the growth process for the piezoelectric nanowires could focus on the optimisation of their size and uniformity in order to maximise the sensitivity to deformation, as well as the mechanism of the growth process itself.
Sectors Electronics,Healthcare,Manufacturing, including Industrial Biotechology

Description EPSRC Centre for Advanced Materials for Renewable Energy Generation (CAMREG) - Flexible Fund
Amount £50,000 (GBP)
Organisation University of Edinburgh 
Sector Academic/University
Country United Kingdom
Start 05/2019 
End 08/2019
Description Visiting Scholar, IdEX funding, University of Bordeaux, France
Amount € 15,000 (EUR)
Organisation University of Bordeaux 
Sector Academic/University
Country France
Start 04/2018 
End 06/2018