Harmen Droogendijk

Bio-inspired MEMS

Flow and Inertial Sensors

Promotion date: February 28.

Promotor: Prof.dr.ir. Gijs Krijnen

In biology, mechanosensors, equipped with differing hair-like structures for signal pick-up, are sensitive to a variety of physical quantities like: acceleration, flow, rotational rate, balancing and IR-light. As an example, crickets use filiform hairs for sensing of low-frequency flows to obtain information about the environment and avoid, for example, attacks.

Their filiform hairs are able to sense airflows with velocity amplitudes down to 30 μms−1 and operate around the energy levels of thermal noise. Hair-sensor inspired flow-sensors for measurement of (tiny) ac-airflows using capacitive readout, have been designed and fabricated using technology generally denoted as MEMS (microelectromechanical systems).

An improvement of the hair-sensors is obtained in both an increase in responsivity for frequencies within the sensor’s bandwidth as for lower flow velocity thresholds.

It is demonstrated that electromechanical amplitude modulation (EMAM) can improve the measurement performance at low frequencies. Under certain conditions, noise can be used to increase signal-to-noise ratios by exploiting the concept of stochastic resonance. SR is implemented by controlling the strength of position dependent on capacitive wells.

Further. First, a biomimetic accelerometer has been realized using surface micromachining and SU-8 lithography, inspired by the clavate hair system of the cricket. Second, inspired by the fly’s haltere, a biomimetic gimbal-suspended hair-based gyroscope has been designed, fabricated and partially characterized. Third, an angular accelerometer based on the semicircular channels of the vestibular system, has been developed.

In general, cricket flow sensors perform not only better than the MEMS hair sensors, but are also close to operation at their physical limits. The results emphasize the intriguing research on bio-inspired sensors in order to learn from nature.

Was your research application oriented?

The biomimetic work of the Transducers Science & Technology group was emanated from a Vici-grant and started about ten years ago. The main challenge was to be inspired by nature to build devices using microelectromechnicals systems (MEMS) on dimensions similar as in nature. The goals set by nature, particularly by crickets and flies, proved to be very successful during all these years, leading to a variety of functioning sensors on different areas of research using equally different measurements and technologies.

To these, I added a new signal-to-noise strategy by exploiting the transduction nature of our MEMS flow sensors. It showed that signals were detectable that otherwise would have stayed hidden in noise background signals. By superimposing a carrier wave upon these weak signals a kind of amplitude modulation occurred, making it possible to measure these relevant signals anyway.

Another contribution to the research already done, was studying and characterizing motion tracking principles deployed by flies when using their haltere system. This very accurate biological system can serve as a quick responsive gyroscope by means of which flies are able to perform incredible aerobatic maneuvers. They use the tip of their wings for real-time measurements of Coriolis forces.

Will this type of research continue, you expect?

I truly hope so. A lot of things have become more clear now after these ten years of research. It shows that nature is optimizing time and time again, be it on: selectivity, sensitivity, signal picking thresholds, achievable power and speed of detection and action, just to mention some vital characteristics.

By studying nature in such detail and trying to pursue the goals set, we now possess profound insight in the performance characteristics. So, we now know what makes a good sensor, better as ever before, being aware of the physical levels that nature is so successful in approaching so closely.

Science leaves us with a new set of challenges because of this. In order to approach the physical limits as successful as nature, new materials have to be created, for example possessing combinations of properties never seen before. A good example are the sensitive hairs of crickets: they are stiff, are attached to a surface in a way that allows it to vibrate in a very subtle way, detecting minute changes of air movements. Fabricating thinner structures, longer and equally stiff are hard to accomplish, but this is the only way to try and approach the goals set by nature. The materials used in nature for mechanical suspension also are much more flexible. Here again serious challenges lay ahead of us.

Were your findings published?

Articles appeared in the New Journal of Physics, Applied Physical Letters, the Journal of Micromechanics & Microengineering and Micromachines. Some other articles are still under review, like for Interface.

Did you change as a researcher and scientist during these four years?

Now I know what it takes to bring the research and results to the attention of a wider audience of fellow-researchers and of the public, as I am lucky to say my findings were also published on the site of the University of Twente. When the PhD period proceeded my confidence grew and also I found a way to carefully present the topics of research and describing the results in a good balance between model making and experimental validation results. This led to solid stories that magazines were willing to publish. In essence: it is important to present your results in a way that clarifies your message to the scientific community, explaining why your story is of importance to them.

Also I learned that collaborating with colleagues from other disciplines can be very fruitful. The collaboration with Jérôme Casas of the University of Tours was very informative and interesting. On top of that, it lead to vivid discussions and nice publications as well.

What are your future plans?

I would like to apply for a job in industry, as a career in academics is quite hard these days. My experimental work and multidisciplinary approach are strength points for finding a R&D job, I believe.

What is important, in your view, for Mesa+ to stay successful as an international institute in the future?

As a member of the Transducer Science and Technology group we were a bit of a misfit within Mesa+ foremost because our research is more focused on a micrometer level. Still we made use of highly refined cleanroom techniques, a domain Mesa+ is most famous for. These facilities are very helpful.

I guess, Mesa+ should boost entrepreneurial activities even more. Also it would be favorable when PhD researchers are informed more about career opportunities within Mesa+. It is a shame so many PhD’s leave the Institute after fulfilling their PhD work, meaning a lot of expertise brain drain. Mesa+ can only stay in the top of the world’s rankings when top researchers are linked with the Institute.