Cyberwood Temperature Sensing

May 18, 2015

Just about any material can be used as a sensor for some physical quantity. In my early years in the laboratory, we would use carbon composition resistors as inexpensive temperature sensors in cryogenic experiments. These resistors are less than a penny, each, and carbon has a large temperature coefficient of resistivity.

This coefficient is about -0.0005 per °C near room temperature, the negative sign indicates that the resistance decreases as the temperature is increased. We would calibrate our particular resistors against another thermometer. The temperature coefficient of resistivity TCR is given by the equation,

in which ∂R/∂T is the partial derivative of resistance R with respect to temperature. This can be approximated by measuring the resistance values at an upper temperature T2 and a lower temperature T1, as shown. The partial derivative indicates that all other variables are held constant, an important point, since the resistance of some materials is affected by other things, such as magnetic fields, or exposure to light.

While I found a large TCR to be useful, most electronic circuit designers would rather have their resistors maintain the same value over a useful operating range. Fortunately, some alloys with low TCR have been developed. The first, constantan (Cu55Ni45) and manganin (Cu86Mn12Ni2), were invented more than a hundred years ago by the chemist, Edward Weston. Weston was a prolific inventor, having obtained 334 US patents in his lifetime.

An 1890 portrait of Edward Weston (1850-1936)

Weston was president of the American Institute of Electrical Engineers, a predecessor of the Institute of Electrical and Electronics Engineers, from 1888-1889.

(Via Wikimedia Commons.)

Often, when you want to sense a certain physical parameter, the best materials exhibit a high temperature coefficient. The resistance of manganin is also strain-dependent, and it's used in some strain sensors, including pressure sensors in which the strain on a diaphragm is measured. In this case, temperature errors are small, but there are materials with a much greater sensitivity to strain, and also a greater temperature coefficient. How can these be used?

For these materials, we arrange the resistors in a bridge circuit in which two of the resistors are compressed, the other two resistors are stretched, and all resistors see the same temperature (see figure). The bridge cancels out the temperature effect, while doubling the strain effect. In the figure, the arrows alongside the resistors indicate the strain response, either increasing, or decreasing, with strain.

A current source produces a voltage from an imbalance of a resistance bridge.

(Illustration by the author using Inkscape.)

Of course, when sensitive thermometry is needed, a material with a large temperature coefficient of resistivity would be beneficial. One important application of thermometry is calorimetry, but measuring the small amount of heat associated with things like the metabolism of bacterial cells would require an extremely sensitive thermometer.

Scientists from the Swiss Federal Institute of Technology (ETH Zürich), the California Institute of Technology (Pasadena, California), and the University of Salerno (Fisciano, Italy), have developed a bionic composite material made of plant cells and carbon nanotubes (CNTs) that exhibits a temperature sensitivity about two orders of magnitude greater than the best available materials.[1-2]

Since nature has perfected her mechanisms by evolution over the course of millions of years, we've always looked at ways in which we can mimic nature to improve our materials. I wrote about biomimicry in many previous articles, one of which is Insect Velcro, April 22, 2013.

This research team took their inspiration from temperature-sensitive plants, but they didn't directly mimic the temperature response mechanism of the plants; instead, they developed a hybrid material containing plant cells. Significantly, while the temperature response in plants does not persist after cell death, the cells in this material are functional when dead.[2]

Some plants are capable of responding to very small temperature changes, and such a response is evident in the change in the electrical conductivity of their cells.[2] In researching tobacco cells, ETH scientist Raffaele Di Giacomo recalls, "We asked ourselves how we might transfer these cells into a lifeless, dry material in such a way that their temperature-sensitive properties are preserved."[2]

They cultured the cells in a growth medium containing carbon nanotubes, and the nanotubes formed a conducting network between the cells by penetrating the cell walls. These cells were removed from the medium, and they called the resulting lifeless product, "cyberwood." Unlike wood, the cellular-CNT hybrid material is electrically conductive, and the conductivity is extremely temperature sensitive, just as it is in living tobacco cells.[1-2]

Scanning electron micrograph of cyberwood, showing its wood-like structure

(ETH Zürich image by R. Di Giacomo, et al.)

The temperature sensitivity is such that the material can detect the warmth of a human body at significant distances; for example, a hand a few tens of centimeters away.[2] The variable conductivity comes from metal ions in the egg-carton-like structure of pectin located between cellulose microfibrils.[1] Pectins are sugar molecules in plant cell walls, and it forms temperature-dependent cross-links to form a gel.[2]

Increased temperature breaks the pectin cross-links, allowing contained calcium and magnesium ions to move about more freely and increase the electrical conductivity.[2] The CNTs not only allow the bioelectrical property to persist after cell death, but they also increase the baseline conductivity to make the material more easily applied in sensors.[1]

A patent application has been filed on this invention. The research team is extending its work to develop an equivalent material based on pectin and ions, alone.[2] One significant application would be imagers that operate in the far infrared.[2]

Structure of the bio-polymer, pectin. (Via Wikimedia Commons.)

References:

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