|Wearable computers can now merge seamlessly into ordinary clothing. Using various conductive textiles, data and power distribution as well as sensing circuitry can be incorporated directly into wash-and-wear clothing. This paper describes some of the techniques used to build circuits from commercially available fabrics, yarns, fasteners, and components.|
Micrograph of silk organza.
Circuits fabricated on organza only need to be protected from folding contact with themselves, which can be accomplished by coating, supporting or backing the fabric with an insulating layer which can also be cloth. Also, circuits formed in this fashion have many degrees of flexibility (i.e. they can be wadded up), as compared to the single degree of flexibility that conventional substrates can provide. There are also conductive yarns manufactured specifically for producing filters for the processing of fine powders. These yarns have conductive and cloth fibers interspersed throughout. Varying the ratio of the two constituent fibers leads to differences in resistivity. These fibers can be sewn to create conductive traces and resistive elements. While some components such as resistors, capacitors, and coils can be sewn out of fabric, there is still a need to attach other components to the fabric. This can be done by soldering directly onto the metallic yarn. Surface mount LEDs, crystals, piezo transducers, and other surface mount components with pads spaced more than 0.100 inch apart are easy to solder into the fabric. Once components are attached, their connections to the metallic yarn may need to be mechanically strengthened. This can be achieved with an acrylic or other flexible coating. Components with ordinary leads can be sewn directly into circuits on fabric, and specially shaped feet could be developed to facilitate this process. Gripper snaps make excellent connectors between the fabric and electronics. Since the snap pierces the yarn it creates a surprisingly robust electrical contact. It also provides a good surface to solder to. In this way subsystems can be easily snapped into clothing or removed for washing.
Several circuits have been built on and with fabric to date, including
busses to connect various digital devices, microcontroller systems that
sense proximity and touch, and all-fabric keyboards and touchpads. In the
microcontroller circuit shown in Figure 2, a PIC16C84 microcontroller and
its supporting components are soldered directly onto a square of fabric.
The circuit uses the bidirectional I/O pins on the PIC to control LEDs
and to sense touch along the length of the fabric, while providing musical
feedback to reinforce the sense of interaction. Building systems in this
way is easy because components can be soldered directly onto the conductive
yarn. The addressability of conductors in the fabric make it a good material
for prototyping, and it can simply be cut where signals lines are to terminate.
All-fabric switching contact keyboard.
One kind of fabric keyboard uses pieced conductive and nonconductive
fabric, sewn together like a quilt to make a row- and column-addressable
structure. The quilted conductive columns are insulated from the conductive
rows with a soft, thick fabric, like felt, velvet, or quilt batting. Holes
in the insulating fabric layer allow the row and column conductors to make
contact with each other when pressed. This insulation also provides a rewardingly
springy, button-like mechanical effect. Contact is made to each row and
column with a gripper snap, and each snap is soldered to a wire which leads
to the keyboard encoding circuitry. This keyboard can be wadded up, thrown
in the wash, and even used as a potholder if desired. Such row-and-column
structures can also be made by embroidering or silk-screening the contact
All-fabric capacitive keyboard.
Keyboards can also be made in a single layer of fabric using capacitive sensing [Baxter97], where an array of embroidered or silk-screened electrodes make up the points of contact. A finger's contact with an electrode can be sensed by measuring the increase in the electrode's total capacitance. It is worth noting that this can be done with a single bidirectional digital I/O pin per electrode, and a leakage resistor sewn in highly resistive yarn. Capacitive sensing arrays can also be used to tell how well a piece of clothing fits the wearer, because the signal varies with pressure.
The keypad shown here has been mass-produced using ordinary embroidery techniques and mildly conductive thread. The result is a keypad that is flexible, durable, and responsive to touch. A printed circuit board supports the components necessary to do capacitive sensing and output keypress events as a serial data stream. The circuit board makes contact with the electrodes at the circular pads only at the bottom of the electrode pattern. In a test application, 50 denim jackets were embroidered in this pattern. Some of these jackets are equipped with miniature MIDI synthesizers controlled by the keypad. The responsiveness of the keyboard to touch and timing were found by several users to be excellent.
A view of the component side of the circuit board has been superimposed
to show its extent and its connections to the fabric. A flexible
circuit board can be substituted for the rigid one used in this implementation.
Ultimately we hope to do away with the circuit board entirely.