Ajay Kottapalli is a scientist who turns to nature for inspiration to build new sensors. He constructs these using soft polymer materials. Two years ago, Kottapalli joined the University of Groningen as a tenure-track Assistant Professor in the ENgineering and TEchnology institute Groningen (ENTEG).
Kottapalli became interested in bio-inspired sensors during his PhD studies, which he conducted at Nanyang Technical University in Singapore, in alliance with Massachusetts Institute of Technology (MIT). His first work was inspired by blind cave fish, who can swim at high speeds without colliding into underwater objects.
Blind cave fish ‘see’ underwater objects through ‘touch at a distance’ flow mapping, generated via the lateral-line system on their bodies. This system of sense organs can detect flows and pressure variations in water via hair cells that are sensitive to water movement. Anything that moves in water will produce disturbances that form cues for the fish to determine the location, shape and velocity of an object.
Based on this natural flow sensing in blind cave fish, Kottapalli developed ultra-sensitive, affordable and miniaturized flow sensors using microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) technology. The potential applications of these miniaturized flow sensors are enormous. Arrays of these sensors can potentially become the ‘eyes and ears’ of underwater and aerial vehicles by mapping objects around them, creating pathways for energy-efficient vehicle manoeuvring.
Over the past two years, Kottapalli started developing more biomimetic sensors in his new lab in Groningen. One of his current projects involves seal whisker-inspired MEMS sensors, and is carried out in collaboration with the Pieterburen seal sanctuary. Seals have the remarkable ability to use their whiskers to track their prey underwater – even around corners. In murky and dark water conditions, where vision is impeded, some seals can still track their prey after it has swum by. Fish leave these vortices in their wake and seals can follow them like a trail of breadcrumbs.
Seal whiskers are interesting because they have a unique undulatory geometry and an elliptical cross-section, which suppresses the vortices that the whiskers generate in water along their length. This helps to minimize whisker vibrations that are generated when the seals are swimming and enables them to be vigilant to the flows generated by the fishes. Kottapalli’s research team is studying seal whiskers to understand the fundamental flow-sensing mechanisms involved in their wake-tracking ability.
Inspired by these seal whiskers, Amar Kamat (a postdoctoral researcher in Kottapalli’s team) and Xingwen Zheng (PhD student in Kottapalli's group) are now developing artificial MEMS flow sensors, which use artificial whiskers made up of 3D-printed polymers. These have the same geometry as seal whiskers. The artificial 3D-printed whiskers, as well as genuine seal whiskers (collected from dead seals that arrived at the seal sanctuary) are tested in flow tanks to gain an understanding of the vibrations and ‘slaloming’ behaviour of the whiskers in tracking wake trails.
The sensors inspired by animals can also be used in humans. Research has shown that the sensors of the lateral line of fishes have an uncanny similarity to sensory hairs inside the mammalian ear: the stereocilia. The human inner ear detects sound, angular velocity and acceleration via the stereocilia, which protrude from the top of sensory cells. The underlying design principles of the stereocilia, which are responsible for an ultrahigh sensitivity and extraordinary sensing performance, are not well understood.
To investigate this system, Kottapalli and his team set out to re-create the mammalian ear cilia using polymer nano-scale 3D printing. The experimental studies that they are currently performing on these artificial stereocilia may explain in what way the morphological design parameters contribute to the remarkable performance of the biological stereocilia. The 3D-printed sensors are highly sensitive to water flow or touch and they are also very flexible and biocompatible. Such sensors could be useful in various biomedical and lab-on-chip applications. One of Kottapalli’s PhD students, Debarun Sengupta, is currently developing an artificial polymer-based cochlea which would be biocompatible and could be incorporated in hearing aids.
Building on their experience, Kottapalli and his team developed other sensors for personalised health monitoring. There is an increasing demand for flexible, ultra-sensitive and wearable sensors in the field of prosthetics, soft robotics and healthcare devices. In 2019, they published a study on ‘squishable’ piezoresistive sensors that change electrical resistance when force is applied to them. These sensors were made from a microporous PDMS polymer ‘sponge’, the pores of which were filled with graphene, a two-dimensional carbon material.
These sensors are able to detect pressure or strain with high sensitivity and repeatability. They demonstrated this strain sensitivity for human gait monitoring, by incorporating the sensors into the sole of a shoe. There, it measured gait characteristics such as walking, leaning, standing or running. Interestingly, the sensors were even able to distinguish between the gait of a normal foot and a flat foot.
As a next step, Kottapalli and his team intend to use these sensors to monitor gait characteristics in people suffering from Parkinson’s disease, multiple sclerosis and other neurological conditions. Such measurements could track disease progression and aid in the development of treatment plans for patients. Abnormal gait may even be used to establish the initial diagnosis. However, these sensors have not yet been commercialized as large-scale manufacturing is still a problem.
A lot of new research in Kottapalli’s lab is focused on generating power via the new generation sensing materials and structures using piezoelectric and triboelectric energy harvesting. Almost all modern gadgets require some sort of power. It will be extremely useful to have self-powered, smart, wearable devices that meet their own energy needs by scavenging mechanical energy produced through physical activities. If it were up to Kottapalli, the future would be filled with self-powered, bio-inspired sensors.
Sensors are developed to be used. This is why Kottapalli is eager to bring his research from the lab to the market. Amar Kamat, a postdoctoral researcher in Kottapalli’s team, is developing low-cost cilia-inspired flow sensors using state-of-the-art 3D printing. The team recently developed a novel processing workflow to manufacture the flow sensors, and a patent application for this device was filed with the European Patent Office last November. The small sensors are sensitive to low flow velocities, which make them suitable for automated flow-sensing applications in the healthcare industry.
Kamat and Kottapalli will launch a start-up company in the near future, to commercialize the sensor technology for applications in intensive care units (ICUs). The company will collaborate with the UMCG, where the technology will be tested. Through the lab-to-market handover of the flow sensor technology, the team aims to address some key challenges faced in ICUs, such as alleviation of nurses’ workload, early diagnosis of injuries and reduction of adverse drug events. And if all goes well, this will be the first of many bio-inspired sensors from the Kottapalli lab that will make it to the market.
Text by Antara Mazumdar
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1. Kottapalli, A. G. P., Asadnia, M., Miao, J. M., & Triantafyllou, M. S. (2015). Harbour seal whisker inspired flow sensors to reduce vortex-induced vibrations. In 2015 28th IEEE International Conference on Micro Electro Mechanical Systems (MEMS) (February ed., Vol. 2015, pp. 889-892). IEEE. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS)
2. Kottapalli, A. G. P., Bora, M., Asadnia, M., Miao, J., Venkatraman, S. S., & Triantafyllou, M. (2016). Nanofibril scaffold assisted MEMS artificial hydrogel neuromasts for enhanced sensitivity flow sensing. Scientific Reports, 6, .
3. Kamat, A. M., Pei, Y. & Kottapalli, A. G. P., 30-Jun-2019, In: Nanomaterials. 9, 7, 14 p., 954.
4. Sengupta, D., Pei, Y. & Kottapalli, A. G. P., 25-Sep-2019, In: ACS Applied Materials & Interfaces. 11, 38, p. 35201-35211 11 p., 9b11776.
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