Crafting Material Interfaces

Group research by Sara Hendren, Sam Jacoby, & Woong Ki Sung

I. Introduction.

“Elastomers,” from the combined terms “elastic” and “polymer,” encompass a broad range of rubber and rubber-like materials, with conductive particles embedded inside—making them conductive composites. Since they’re highly mold-able and naturally highly resistant, elastomers are widely used for their insulating or dissipative properties.  However, precisely because they’re so pliable—they can be shaped and extruded into almost any desired form, where metals can be limited and/or pricey—they’ve become desirable as conductive materials as well. Elastomers are used for their conductivity properties in four ways:

Insulating (e.g. wire coating)

Dissipative (“anti-static” polymers)

Conductive (materials capable of conducting modest amounts of electrical current)

Highly Conductive or Shielding (materials capable of conducting significant amounts of electrical current)¹

The range of conductivity is pretty wide, requiring different materials for different conductivity rates:

For modest conductivity (surface resistivity 10³ to 10⁷ ohms/square) “conductive carbon” additives are generally used very cost effectively. For highly conducting thermoplastics  (surface resistivities of 10² ohms/square and higher) carbon or stainless steel fibers are often necessary.²

II. How does it work?

Elastomers act as a “film” against a conductive “filler”; with enough filler present, elastomers transform from insulating materials to conductive ones. Filler material can be nickel, silver, aluminum, glass, or other materials.

Even with highly conductive filler material, however, conductive composites always maintain resistivity several orders of magnitude higher than the pure filler material.  So refinements in the research have sought to understand all influences on conductivity—beyond the intrinsic conductive properties of the filler material itself. Influences on conductivity include the size of the filler particles, their relative hardness, and the rate and intensity of contact among particles:

Resistivity increases with increasing filler hardness and/or elastic modulus and insulating film thickness, while resistivity decreases with increasing particle size and intrinsic stress.³

Resistance occurs in three different forms: the base-line, intrinsic resistance of the filler material and polymer; the constriction resistance (when small areas of passage constrain the flow of electrons); and tunneling resistance at the point of contact, where particles of filler meet and must “tunnel over” the film of polymer.

III. Types & Examples

The most common uses for these elastomers are for connecting and shielding mechanical parts—for example, with a gasket as a buffer between two working elements—while retaining enough desired conductivity. Some common use areas include:

Electronic Enclosures
Computer Enclosures
Cell Phones
Hand Held Devices
Network Routers
Medical Diagnostic & Analytical Equipment
Aerospace & Automotive
Electronic Systems
Indoor & Outdoor Enclosures

Such wide usage means you can find all shapes and sizes on the market, and there are custom-order services available. Gaskets, shields, tubing, cords, O-rings, sheets—all of these forms are readily available. (A sample kit from EMI costs $79 and quotes are available for orders; do note, however, that most items require a minimum of $250 per order.)

You can also get conductive foam ($30 for a 6-inch square), conductive elastic fabric ($30 for a piece 12 x 52 inches); and stretch-sensing rubber, narrow cording that’s embedded with carbon particles ($13 for a 10 cm length, 2 mm in diameter). Note, too, that some tech-fabrics contain polymers and can be threaded with conductive materials.

And this conductive foam sensor ($8 for a 12 x 12 x 1/8″ sheet) is modestly conductive, but its resistivity is decreased when compressed—so it can detect changes in pressure, easily measured by simple electronics.

There are ambitious projects in the works for conductive elastomers as well. Stanford researchers have created an electronic synthetic skin, using organic electronics with an elastic polymer stretched over it:

Researcher Zhenan Bao, chemistry professor at Stanford, and her team “used organic electronics – with an elastic polymer called polydimethylsiloxane (PDMS) – to make their electric skin, which is 1,000 times more sensitive than human skin:

Bao took a piece of PDMS measuring six centimetres square with pyramid-shaped chunks cut out of it at regular intervals. When the PDMS is squashed, the pyramid-shaped holes that were previously filled with air become filled with PDMS, changing the device’s capacitance, or its ability to hold an electric charge.

To make it easier to detect the changes in capacitance, Bao stuck the PDMS capacitor onto an organic transistor, which can read out the differences as a change in current. The team used a grid of transistors to track pressure changes at different points across the material.³

Here’s more on this project, as well as a second prototype in development by a team at UC Berkeley. And here’s the video with the skin’s extraordinary sensitivity demonstrated by researchers:

In addition, there are ongoing research initiatives using conductive elastomers: graphene composites, which present great promise for computing applications, and metal rubber, a highly conductive and flexible elastomer that can be mechanically strained to 1000% of its original proportions while remaining conductive.

V. Safety issues

VI. And a last speculative thought:

There’s been a lot of interest in Sugru, a durable silicone rubber that molds to any shape for any purpose.

It “cures” at room temperature, but it maintains some flexibility, so it can hold together things that still need to move, like wires, cables, etc. In its natural state it’s not conductive, but this is such a DIY material goldmine that it’s worth putting out there: What might be ways to make this conductive, and for what uses? You can buy 12 minipacks, 5 grams apiece on the Sugru site.)

Sources:

1. & 2. 3M on conductive polymers

3. Ruschau, G. R., Yoshikawa, S., and R. E. Newnham, “Resistivities of Conductive Composites.” Journal of Applied Physics. 72:3, 1 August 1992. Materials Research Laboratory, Pennsylvania State University, University Park, PA.

Link to the conductive elastomers presentation slides (PDF).