Shape customizable flex sensors

Hello folks,

Last year was super busy with two projects, one of which was building and understanding shape customizable flex sensor designs. We presented and demo’d the work at ACM TEI 2017 in March at Yokohama.

small size images

Flex sensors of various geometries : i) Inchworm ii) Fish fin, iii) Bird Wings, iv) Flower petals

A brief walk through of the work, for all those who missed the talk and the demos:

Motivation: Flexible and deformable user interfaces have been of great interest to the HCI research and interaction design communities. An important question for realizing interactive prototypes is how to sense deformations.

A few approaches have used commercially available flex sensors and strain gauges. These being easy to interface, result in slightly thicker prototypes and are shape restricted. Slimmer thin film flex sensors have been demonstrated by screen printing piezoelectric ink material on various substrates. Such sensors involve multi-material,multi-step fabrication and are not easy to customise for various shapes. Research on printed electronics has showcased building various sensing modalities such as touch and pressure sensing through techniques such as conductive inkjet printing to instantly realize digitally designed sensors. However, continuous bend sensing has not been investigated so far, using such instant fabrication techniques for unique sensor shapes.

Basic Principle of Flex sensing Our work builds on the basic principle of resistive flex sensing. Conductivity of certain materials changes on flexion. Resistance of the material increases when bent in one direction and decreases when bent in the opposite direction. We utilize one such conductive material, silver nanoparticle ink, to print fabricate flex sensors.

Standard design and design primitives The most basic of a flex sensor would just be a single line whose resistance is measured from its two ends. For obtaining a higher change in resistance to bending, a bunch of closely spaced lines can be laid out in parallel, whose resistance is measured from the two ends.

We identify 6 primitives within the design:primitives

  • Line width
  • Line length
  • Line spacing
  • Number of sensing lines
  • Connecting traces
  • Composition

We can customise each of these primitives to obtain flex sensors of highly varied geometries.

Digital design and instant inkjet fabrication The sensors can be designed in any vector graphics application such as Inkscape or Illustrator. They are then fabricated in a single step through conductive inkjet printing. In our case, we fill conductive silver ink in an off-the-shelf inkjet printer. (Alternately, one could directly pattern the designs on a substrate by drawing with a conductive pen.)


An Epson L220 Inkjet busy printing

Voltage divider interfacing Once the design is fabricated, we can quickly interface it to a microcontroller using a breadboard, a known value resistor and a few cables. The known value resistor and the printed flex sensor form a voltage divider and we read out the output voltage on a microcontroller such as Arduino and find out the resistance of the printed design in different flexing scenarios.


An Omni-directional flex sensor connected to a microcontroller through a voltage divider

Deformation Sensing We characterize the sensor response for two types of deformations: static and dynamic. Static deformations are those where the sensor is deformed from state to the other slowly. On the other hand, dynamic deformations are faster and the sensor does not have time to settle at a particular state while being deformed.

We printed 3 sensor samples each for 2 different sizes and flexed them across cylinders of various diameters for finding their static deformation behavior. When the sensor was bent with printed side up, the sensor resistance increased from its flat state value and vice versa. Normalized response for both the sensor sizes was quite similar indicating that uniform scaling did not affect the sensor response for the two sizes.

static convex response.png

We then mounted a sensor on a motor assembly to flex it repetitively. A large sized sensor was flexed from flat state to flexion of 3cm radius 50 times at a speed of ~0.25Hz. For our test, the sensor response was highly repetitive and the sensor did not require re-calibration.


Applications A wide variety of end user applications can be built using such custom deformation sensors. We illustrate a few examples:

Animating virtual entities through direct physical input We can directly animate virtual entities through physical deformation input. Using a two sensor linear array, we can animate an emoticon to various moods.

Animation (2)

Multimodal IO Sheet  We can integrate various other sensing modalities such as touch sensing and output capabilities such as LEDs along with flex sensing on a single printed sheet.

The sample bow shaped sheet has two flex sensors on the side and a touch sensor in between. Each flex sensor is accompanied by a resistor to form a voltage divider. Each of the sensors also have a corresponding led that glows when the sensor is interacted with.


Origami Training 

Making an origami model involves a one-time usage of a particular paper surface. A crease on the paper surface due to a fold would result in permanent increase in the sensor resistance.

We can leverage our bend sensors to detect fold sensing for such single usage scenarios. These can act as an affordable guidance tool for novice users to learn the correct sequence of folds. We can alternatively use plain graphite, drawn with a pencil, to pattern it directly on thinner origami paper.


Sensor lines on front and back of origami sheet for Boat and Plane shapes

For more details, please refer to the paper. It is an interesting read.

ACM DL Author-ize serviceFlexy: Shape-Customizable, Single-Layer, Inkjet Printable Patterns for 1D and 2D Flex Sensing

Nirzaree Vadgama, Jürgen Steimle
TEI ’17 Proceedings of the Eleventh International Conference on Tangible, Embedded, and Embodied Interaction, 2017


Many thanks to my adviser for guiding this work through and also the reviewers at ACM TEI 2017 for their valuable feedback.

For any questions or comments, just send a mail. 🙂

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Setting up conductive inkjet printing

Hey folks!

I recently setup conductive inkjet printing at my place, and here’s the how-to:

What you require:

  1. Conductive ink (in my case silver nanoparticle ink)
  2. Inkjet printer
  3. Syringe and filter (comes with the ink so no need to buy separately)
  4. Printing sheets (available from ink manufacturers)

Selecting the ink:

To my knowledge, two companies make such ink, Mitsubishi & AgIC. As my lab used ink from Mitsubishi, I went ahead with the same ink for my further work.

Selecting the printer:

This is the trickiest part as printer companies keep changing the models and not each of them might be compatible for conductive printing. After a lot of interaction with the ink manufacturers, I could finalize a printer, and it has worked so far. 🙂 The important requirements for the printer are:

  • piezo inkjet head (Brother and Epson have piezo technology, while Canon,HP have thermal. Thermal technology is not preferred because heating up the ink might affect its properties, say the ink manufacturers. Nevertheless, thermal and piezo printheads both work for conductive printing (My lab has a Canon iP100, a super high res mobile photo printer.). Given both, go for a piezo printer.)
  • larger nozzle size for printing (Silver particles might get clogged on smaller sized nozzles, therefore larger sized nozzles are preferred)
  • empty cartridges/ink tanks and unused printing pipeline. It is important that  the printer is not tested before, or filled with CMYK ink already, as it will require massive cleaning of the print head and is not preferred.

The good news is that recent inkjet printers by Brother & Epson consist of ink tanks which are initially empty and the user needs to fill the ink with appropriate ink. Thus, we are not required to look for empty cartridges which is a really really tricky task.

I had to choose between the current Brother models and Epson models available in India. The printing resolution was quite similar between Epson and Brother models. Brother models also have larger number of nozzles per color than Epson. However,Epson printers have a slightly larger nozzle size. It is hard to find out the nozzle size in diameter from the websites of the companies, but the minimun quantity of ink dispersed by the nozzle is a proportional measure of the nozzle size. For Epson, that happened to be 3pl (picoliters) and for Brother models that are currently available, it is 1.5 picoliters.  AgIC recommends recent models from Epson, of which L220 was available locally. Thus, I chose L220 inkjet printer from Epson over a Brother printer majorly due to the larger nozzle sizes.


Epson L220 Inkjet Printer

Printing sheets:

I got a pack of 100 translucent coated PET sheets from Mitsubishi. There are white PET sheets too. AgIC also has printing media available. The media is unfortunately quite expensive.

Setting up printing:

Prior to putting the ink in the printer, refrigerate it, as the recommended storage temperatures are 0-8 degrees Celcius. And once you have the ink and the printer, it’s time to put everything together.

Shake the ink well. Fill it through the syringe, insert the filter on the mouth of the syringe, and insert the syringe into the ink tank. Remove the filter, fill the ink in the syringe, put the filter back, dispense it to the ink tank and repeat.

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The filter will become slower as some ink gets trapped in it during each cycle of dispensing. When it becomes too slow and hard to get the ink out of the syringe through the filter, change the filter. Yes it gets slightly messy but we don’t have to pour ink in the tank very often so please be patient.I had fortunately about 4 unused filter, of which I used 3 while filling my ink tanks. I had 100 ml of silver ink, and I put about 22 ml ink in each of the ink tanks. The ink tanks always showed low ink warning, as their capacities are 70 ml each. However, its ok to print with that much of ink within each ink tank. Once the ink is filled, then follow the setup instructions of the printer: where to place the print media, charging the ink for the first usage, etc.


Once the printer is setup, it’s time to get the first prints.Design your circuits/sensors in any circuit design software/vector graphics software. Once done, configure print settings:

  1. For getting more conductive traces, it is recommended that in your design all the colors(C,M,Y,K) are set to 100% (more ink material, more conductivity).
  2. Print media settings:
    1. Size: Size of the respective media, Scaling : actual size
    2. Type of media: Glossy photo print or  closest alternative
    3. Quality & Resolution: Highest possible quality and resolution(printing with these settings can take slightly longer but we require high resolution so this is necessary)

..Aaaand Print! 🙂

Stay tuned for some fun flex sensor designs and if you are around Bangalore, for an upcoming workshop on printed sensors! 🙂


  1. Mitsubishi Silvernanoparticle ink and print media
  2. AgIC beginner prototyping kit
  3. AgIC circuit implementation
  4. Instant Inkjet circuits, UbiComp 2013.

Many thanks to folks at AgIC & Mitsubishi for their support and my friends Temu,Ruchi and Viru for helping me get the ink in the short time window. 🙂

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IISc Open Day 2016

(a slightly delayed post!)

March 5 was Open day at IISc, the day of the year when the entire institute is open to public with interesting events, demos and talks. As a single day is super short to see all the interesting stuff, here’s what we could grab:

We began with Center for Cryogenic Technology (as it always seemed a very fancy place). Cryogenics  deals with the study of production and effects of very low temperatures, (lower than −150 °C (123 K)).  At such low temperatures,molecular motion in materials becomes very low resulting in a highly ordered state of matter. This alters some of the physical properties of materials. Liquid nitrogen and liquid helium are commonly used cryogenic fluids.

The first demo was of a cryo-jet, made from a very simple apparatus: a string tied at two rigid ends (a tree and a wall about 50-60m apart) and a plastic bottle placed at one of the ends using two hooks(string directs the bottle to a defined trajectory).Water was filled first in the bottle followed by liquid nitrogen. As soon as liquid nitrogen comes in contact with water, it starts to evaporate much faster (than if it were alone). A small hole in the bottle cap acts as a nozzle, which is initially blocked and later unblocked. As soon as it is unblocked, the bottle shoots away towards the other end. As soon as nitrogen starts to evaporate, it needs much more volume and therefore rushes out through the nozzle, pulling the bottle away in one direction.

Next  demo was dipping various objects in liquid nitrogen and observing the change in their physical properties. The demonstrating student took a rubber pipe dipped it in liquid nitrogen tank for a few seconds. When it was removed, it was super brittle (until it reaches back to atm. temperature). They also made instant ice-cream by dipping sweet milk mixture for a few seconds in liquid N2.

Later we saw liquefaction of Oxygen using liquid nitrogen. Gaseous Oxygen was filled in a balloon which was in turn connected to a glass bulb. When the bulb was immersed in liquid nitrogen container, Oxygen inside the glass bulb was cooled and liquefied and thus,volume of balloon shrunk. When the bulb was removed away from the liquid nitrogen, the O2 converted back to gas and the balloon volume increased.


liquefaction of O2

Resistance of materials also changes when subjected to ultra low temperatures. We saw a light bulb demo, where the connecting copper wire, when dipped in liquid nitrogen caused the bulb to glow brighter, as wire resistance reduced, than when it wasn’t ultra-cooled. Transmission losses during electricity transfer could be reduced by lowering the resistance of the cables. The challenge is maintaining the very low temperatures.

Magnetic levitation demos are always magical. A trail of permanent magnets was constructed and a superconductor (a special alloy dipped in cryogenic fluid for a few seconds) was put on the trail to navigate. The superconductor maintains its orientation due to pinning effect, and orbits around the trail.


levitation demo

Next to Cryogenics were demos by IISc undergraduates and we stopped to glance.

One of the fun demos was understanding a Faraday cage using soap bubbles. A closed conducting surface shields its interior from external electric fields, and is called a Faraday cage. A soap bubble was blown on a smooth surface and then a static charged plastic bottle was brought closer. The bubble moved towards the bottle. Then another bubble was blown inside the first bubble and then when the bottle was brought closer, the inner bubble didn’t react, only the outer one did.

Brachistochrone (Least time) problem demo: The setup here consisted of two different paths between two points, a linear one and  the other an inverted cycloid. The challenge was to guess which of the two paths would be faster for an object starting at rest from the top point to reach the bottom. The linear path has the shortest distance but the inverted cycloid takes less time for an object. It is because the object accelerates much faster due to rapid descent in the beginning than the linear path and then covers the rest of the path due to the built momentum.


interesting visuals at the UG Exhibit Hall

We later saw samples of various types of rocks, demo on ground water conversation and various chemistry demos, math puzzles and more..



Then we went to Center for Product Design & Manufacturing (CPDM).Lots of models and prototypes of novel concepts are always a visual treat. Among the prototypes we saw were those of foldable helmet to minimize storage space, design of a Surgical 4 state chair using a single actuator, a light weight and affordable segway  and others.

At 1pm was one of the air-show slots so we went to the Aerospace department launchpad to watch in action the planes and quadcopters built by hobbyists and the department. It was super hot but the crowd was still very enthusiastic.


After lunch and a bit of rest, we caught up the last set of demos at Electrical Communication Engineering (ECE) department.

Intruder detection with PIR sensors : The goal of the project is to detect and classify various intruders (human,animal) in outdoor scenarios, using low cost sensor network (array of Passive Infrared Sensors (PIR). The motivation of the work comes from the undesirable conflicts that occur occasionally in regions close to forests, either due to human intrusion in the wild or wild-life intrusion in human habitations. An array of 8 PIR sensors is used to classify clutter (movement of vegetation) from various intrusions (animals and humans) using machine learning.

Open shoe project: This project is being developed in collaboration with KTH Sweden, and consists of a foot mounted inertial navigation system. Each module consists of several Inertial Measurement Units (IMUs) along with Bluetooth and the module sends heading and displacement data,to another device, say a phone on which the position of the wearer can be tracked. One of the applications of such technology would be to improve safety of folks like firefighters, by accurately tracking and guiding their indoor navigation as GPS signals are not available accurately.  The project is completely open source, check out more details here.


Open Shoe project (Left) and a smartphone based robot model

The demos ended by 5pm and it was a fun filled Saturday. We also missed a lot of departments demos (Neuroscience, CSA, CEDT, NanoTech..), some other time later..


Spring around!

some of the perks of living in Bangalore (and close to IISc). =)

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Story of Aster as it goes live!

Yo folks!

The startup I currently work with is launching a new product, a backpack with integrated lights, for improving visibility of cyclists on roads. The product is called Aster. (whose meaning in earlier times was ‘a star’)

The backpack features rear lights,front lights, indicator lights and automatic brake lights. It comes with a handlebar control unit for controlling the indicator
lights and the unit also doubles as a theft deterrent for your bike, by generating a motion alert on your parked bike.

The design of the bag itself is unique and has lots of cool features, such as it opens up completely flat for easy packing, has easy access laptop compartment, quick access bottle holder, a minimalist helmet holder, U-lock holder,separate dirty laundry compartment,  and lots of space for whatever you would like to stuff.

Coming to the lights part, the bag is equipped with a total of 54 LEDs (a number which might change by (+-)1,2 units as we fine tune it for the final run) which light up various areas of the backpack at various instances.  Rear lights are for normal visibility, are red in color, and can be set to steady or blinking mode. Front lights are much brighter, are white and help the cyclist be visible in the rear view mirror of the motorists ahead. Profile lights are on left and ride sides of the bag, are steady and red, and help judge the distance and width of the cyclist ahead. Indicator lights are amber in color and they turn on/off when you press the respective button on the handlebar unit. There is also reflective print on the bag.

The backpack also comes with a USB rechargeable battery. The handlebar unit is powered with a coin cell battery. There will be a whole bunch of customizations of the lights possible with the mobile app.

My contribution has been in developing algorithms for braking detection and theft alarm with sensors on the handlebar unit (so if you have any related questions there, just shoot).

We are a super small team of engineers, designers and biz guys based in Bangalore, India and we have designed and manufactured the product completely locally and are very proud of this.

More details about the product at the crowdfunding page. The campaign will be on for two months from today.  (And we are completely funded. Thanks everyone for the support! =)) If you find Aster of utility, then don’t forget to support it and spread the word. 🙂

A big shoutout to everyone who gave us valuable feedback at various stages of development!



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Light of humanity

This incident happened a couple of months back, when I had just moved to Bangalore. Sometimes, I would commute with the city buses to work. I would have to change a  bus in between the route to reach work and similarly two buses for the way back home.

It was a rainy evening and the final bus to home was quite packed with passengers.I was returning after a long day, taking accidental short naps in between.  As the bus arrived at the stop, I got down hurriedly,crossed the road and started towards home.

Just then, I realized my phone was missing from the pocket. Thinking it might be in the jacket or the bag, I searched quickly but couldn’t find it. I thought to go home, and check thoroughly first, so ran to home.  After all the search, couldn’t find it anywhere. I was sure I left it in the bus, as I remembered using it there. With another phone , I called my number and it was ringing but no one responded. I realized that it was on vibrate mode, and in the crowded bus would have been hard to hear for anyone.

In the next minutes, we took a rickshaw to the bus’ final stop hoping we could catch the bus somewhere and by some chance find the phone. Given that it was recently bought and relatively expensive, I couldn’t think of losing it.

We kept calling and around the third attempt, a lady responded. It was a huge relief… I asked her where she was and that I was coming to collect the belonging. She said she is at some bus stop with the phone and that I should reach there soon. It took us about fifteen minutes to reach the exact place, there she was..waiting in the rain. It was my irresponsibility of having left it at the seat and I couldn’t have blamed if I didn’t get it back. The lady said she saw the phone on the seat after I left, and thought if unattended could be claimed by someone else (which nearly happened too). So she stepped in to return it back. I got my well-deserved bit of thrashing for the carelessness. I hugged her and thanked her for the efforts and returned home happily.

To all the beautiful people everywhere.. unknown, unsung.. for their little deeds of goodness & righteousness..

to all the little goodness around...

what is Diwali after all.. if not spreading some goodness.. =)  




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Mini Maker Faire, Bengaluru

On October 15th was the first edition of  mini maker faire in Bangalore, and a few friends and I happened to visit it. It was a nice event showcasing fun projects built by children and adults as individuals or teams.

One of the first projects I saw was by a team of 2 students,who were part of a club ‘Idea Lab’ in a local science museum (VITM).  Their idea was to use Turmeric  as a bio-pesticide instead of the usual chemical one, for several plant varieties. They had conducted studies on a few plants (Pea,Rice etc.) and found that spraying a liquid mix of water and turmeric was repelling for the pests. The newer leaves that sprouted after spraying the mix were less affected by the pests. The solution was cost effective and organic too.

Turmeric as Bio-pesticide

Turmeric as Bio-pesticide

Quite a good idea; definitely something to investigate further into.

Another fun project was called ‘Digital Reconstruction of Dussasana Vadham‘. The idea was to animate a physical object by projecting information onto it. In this case, animating a 3D model of a Kathakali Dancer’s face to tell a story through expressions.

The motivation behind the work as told by the maker was that not as many people might have seen a live Kathakali performance. Through this digital storytelling, he could reach out to a lot more folks.

Animate any structure to tell a story!

(L) The 3D structure of face of a Kathakali Dancer (R) Projected Image on the structure

We could animate so many things around like this!

Another interesting work was quite high-tech and of much more utility too. Built by a team of researchers at IISc, Bangalore was a low cost, portable nano-particle synthesizer. The setup utilized microfluidics to instantaneously synthesize metal nanoparticles, which could be used in a variety of sensing scenarios.



An example scenario was melamine detection in milk, for which the existing prototype costs a lot and therefore is not prevalent. The proposed solution would be super cheap and accurate too.

Scaled versions of Rail locomotives

Some highly industrious work was done by a Railway locomotive fan from the city. He has been building scaled models of railway locomotive out of recycled paper since many many years. His models are complete physical replicas of their original counterparts, containing the exact same number of components too.


Patience poured in..

Just do it! :D

Just do it! 😀

Among other popular projects were a rope fountain demo and boombox karaoke by a team of designers from the town, some cool prototypes of pinhole camera and an led display watch.

Accompanying them were the usual clan of 3D printers, quadcopters, electronics prototyping kits and others…

Clockwise from top: Karaoke on Solar powered music player, Pin-hole cameras and LED matrix display Watch

Clockwise from top:
Karaoke on Solar powered music player, Pin-hole cameras and LED matrix display Watch

Maker Faire2

Some interesting 3D prints

Was a fun afternoon.

Kudos to all makers! 🙂

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Lake Constance and around

After a long interval, there was a little bit of travel a month ago..  happened to be around the region around Lake Constance in Austria and Germany. The entire region is quite beautiful, especially it was Spring then, when everything comes to life. 🙂

The trip began with Austria, in the towns of Dornbirn and short trip to Bregenz and Feldkirch followed by the German side of the lake, in the towns Lindau, Kressbronn, Markdorf and Friedrichshafen..

Beginning from Austria..  Bregenz is a nice town by the lakeside, and is capital of the Vorarlberg state of Austria. It has a nice art festival each year (Bregenzefestspiele) around July-August when people from around the region come to enjoy the music and theater plays.  I stayed in the bigger town Dornbirn, in the same state, which was also quite nice..


Bregenz, Austria


Dornbirn, Austria

Bregenz town..

the text translates to ” From apprentice to Master” (somewhere in Bregenz town)

Now coming to Germany, Lindau has the lakeside just outside the main train station and so a nice place to sit or take a stroll. At a short distance to Lindau is the village of Kressbronn, where I was hosted by a wonderful couple. We went to a concert of a Polish all-girls band in the neighboring town and had dinner together.  Such friendly and kind people they were!

The next day was at Markdorf and Friedrichshafen. Friedsrichshafen is a totally amazing place , with again lakeside just outside the main train station. Some old friends came to meet so it was even more fun. We took a boat ride (‘Hello Kitty’ was our boat :D, as chosen by the boys ) in the Lake, and saw (for the first time!) airships (Zeppelin) floating across the sky and spent the rest of the time just sitting at the Lakeside..

I would say Europe is a place to be in Spring, even more than Summer (sometimes it gets too hot :P).

To happy travels! Enjoy!




at some shop in Lindau..



hello Beautiful!

hello Beautiful!

Friedrichshafen Lakeside

Friedrichshafen Lakeside

Sea shells!


See you again!

See you again, Lake Constance!

[Thanks Viru for your camera!]

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