Marion Koelle and Marius Hoggenmüller


The bionic paradigm?

Birds have a sophisticated technique to survive the frigid temperatures of winter. They possess a dense coat of feathers which can be puffed out to trap little pockets of air close to the bird’s body. These airpockets insulate the bird’s body and protect it from excessive heat loss. With this technology birds can sustain their body temperature of approx. 40° C even at extreme frost! At chilly temperatures they therefore often look like small fuzzy featherballs (see figure 1).

sparrow in winter

Figure 1: Sparrow with puffed up feathers

[Daniel Lingehöhl: Vogelwelt im Wandel – Trends und Perspektiven. 2010. Wiley VCH Verlag GmbH.]

[Image source:]

What is it?

For our project we adopted the birds’ anti-freezing technique and integrated it in an environment-aware interactive headpiece – at first glance: a simple, every-day bobble hat.

The basic principle is very simple: if the ambient temperature drops below a certain value or the user feels cold the heat-insulating functionality of the hat is activated and the ‘feathers’ of the hat are puffed out. The hat’s control system involves two modes, one based on the sensing of the ambient temperature and another one which is based on user-input via a touch-slider located at the hat’s brim .  It is possible to switch between modes by turning the hat’s bobble.

How does it work?


The skeletal structure of the headpiece consists of two rows of inflatable ‘feathers’ made from a customary air mattress, which are comprised by a cover made from felt  (see figure 2).  If the compressor is turned on the air takes its way through a system of flexible plastic tubes located at the hat’s back and bloats the inflatable pieces which causes the hat to puff up.

Build-up 1Build-up

Figure 2: Inflatable 'feathers' and felt cover


To provide the barometric pressure which is required to inflate the hat, a small (air brush) compressor is used. As the compressor’s internal control can’t be easily accessed due to its direct connection to line voltage, a workaround had to be created. The toggle switch which turns the compressor on respectively off is operated by a servo motor (modelcraft RS-2) equipped with a 3-sided screw.


Figure 3: Servo motor attached to the compressor's toggle switch

Arduino’s Servo Library allows to determine a maximum and a minimum tilt angle between which the servo motor alternates. The compressor’s air duct is affilated to the system of plastic tubes that bloats the inflatable parts when the compressor is turned on. Once turned off, the elusion of the air is induced by the self-weight of the inflatable regions.

Tips & Tricks: Pneumatics

  • Reduce the length of the plastic tubes where possible: unnecessary long connections cause loss of air pressure!
  • Make sure all points of contact are airtight: tape is good for testing as it is removable, but it’s sealing is generally less tight than a permanent bond e.g. with hot adhesive or hot glue.

Sensor System

The system involves two different types of sensors: a temperature sensor and an array of capacitive foils that form the touch-slider which can be controlled by user input.

To determine the ambient temperature a LilyPad Sensor MCP9701A, which is a small thermistor type temperature sensor, is used. The analog output of this sensor is 19,8 mV per degree. The conversion of the sensor output to degrees depending on the input voltage (3,3V  or 5,5V) and model is described in [Post 4]. If the temperature obtained from the sensor drops below a certain threshold – for the presentation we simulated a sudden decrease of temperature with customary ice spray – the compressor is activated and the hat puffs up.

LilyPad Sensor

Figure 4: LiliPad Sensor MCP9701A and ice spray can

To enable the user to control the hat manually, according to his individual perception of temperature, a capacitive touch-slider is fit to the hat’s brim. The advantage of capacitive input is that only low to no force is required for the sensing, which makes it comfortable to control. The touch-slider consists of an array of four capacitive foils. In our prototype we used stripes of aluminium foil, for further analogical use-cases it is also possible to use copper strips or conductive laquer. For the implementation on the Arduino’s side we use the Capsense Sensing Library. The electrical capacitance of the human body can be sensed, when the aluminium stripe is touched with the finger. Therefore each stripe is connected to GND (ground), a high value resistor (1 MΩ),  a capacitor (10 pF) and one of the Arduino’s digital pins. The capacitor is not mandatory, but it improves the stability and repeatability of the obtained values. The exact connection diagram is shown in figure 5.

circuit layout

Figure 5: Circuit layout


Figure 6: Touch-slider

Tips & Tricks: Sensors

  • If the data retrieved from the sensor is jittering you can anticipate this effect by inserting a capacitor to smooth the output values. An example of such a circuit is shown in figure 7.


    Figure 7: How to use a capacitor to smooth a sensor's (here: LilyPad Temp. Sensor) output

  • The output values of sensors may vary according to the model no. The sensor’s datasheets can give information about the mapping of analog output values and sensed variables.

  • For some use-cases you might want to cover the capacitive foil of a capsense setup with a thin insulating material (e.g. paper or plastic) to increase usability and to improve stability of the output values.
  • You’ll need to reset the capsense variables at an interval of 100-300 loops to prevent it from accumulating tension and returning false positives. The code snippet below shows how to do it.

    //Touch input
    CapSense cs_2 = CapSense(7,2); // 10M resistor between pins 7 & 2, pin 2 is sensor pin
    CapSense cs_3 = CapSense(7,3); // 10M resistor between pins 7 & 3, pin 3 is sensor pin
    int counter = 0;

    void setup()
    // your set up
    void loop ()


    //CapSense Input

    long total2 = cs_2.capSense(30);
    long total3 = cs_3.capSense(30);
    currentMax = max (total2, total3)
    if (currentMax > 5000) {

    // do something, e.g. check which foil has been touched and trigger some activity


    if (counter > 100) {

    cs_2 = CapSense(7,2);
    cs_3 = CapSense(7,3);
    counter = 0;


[Capsense Sensing Library,]

[Servo Library,]

Next steps?

The current prototype cannot regulate the air pressure due to the compressor’s two-staged control (ON/OFF)  and the construction’s inability to detain the insufflated air. As a consequence the hat’s insulating functionality only has two stages, whereas for a real-world application a stage-less regulation would be desirable. The prototype could be enhanced by the use of  seamless produced inflatable areas (e.g. made from silicone) in combination with two electromagnetic valves. This enhancement would allow to control the amount of air comprised in the hat continuously at any time according to the temperature sensor’s analog output or the user’s preferences.

A further drawback is the weight of the compressor. At the current stage the compressor has to be carried in a backpack. To make the prototype more user-friendly the compressor would have to be reduced to a minimum of weight. As previous tests proved, commercially available smaller devices, such as USB-hovers, do not have sufficient strength for this use-case. Therefore a device which combines both qualities – low weight and sufficient air pressure- would cause a significant improvement of the prototype’s usability and marketability.

Values and potentials?

In fact the idea to use air to insulate clothing is not new, duvet jackets use the air comprised in it’s feather filling to keep the owner warm. Nevertheless our idea is unique due to its ability to interact. In contrast to a duvet jacket, which is always warming, our headpiece is environment-aware and can be adjusted manually according to the ambient situation. By copying the bird’s anti-freezing technology we are able to create a cloth which is suitable for a wide range of situations. Our  concept can be extended to various kinds of clothing such as jackets or trousers. To determine the technology’s market value it would have to be proved if the puffing up of the cloth actually increases it’s ability to insulate. If that is the case we could think of possible application areas in polar research, astronomy or nautics.

A further aspect to be considered is the design factor. Though the headpiece contains a fully developed technologic system it still has to meet design aspects such as aesthetics and convenience. We tried to meet those requirements by wrapping the inflatable areas in a felt cover, which is cushy on the user’s skin and gives the headpiece a familiar hat-like look. Nevertheless it would be a challenge for designers to create new types of inflatable and attractive clothing.