Chapter 9 – Emerging Biophysics Techniques  413

wing as a positive mold—​this can be first coated in a glass material called chalcogenide to

wrap closely around the butterfly structure to a precision of a few nanometers. A procedure

called plasma ashing (exposing the sample to a high-​energy ion plasma beam) can then be

used to destroy the original wing but leaves the glass wrap intact. This can then be fur­

ther coated using metal vapor deposition as required. The key optical feature of butterfly

wings is their ability to function as very efficient photonic bandgap devices, that is, they

can select very precisely which regions of the spectrum of light to transit from a broad-​

spectrum source. This is true not only for visible wavelength, but infrared and ultraviolet also.

Biomimetic butterfly wings share these properties but have the added advantage of not being

attached to a butterfly.

Another class of biomimetic materials crossover is in the field of biocompatible materials.

These are novel, artificial materials used in tissue engineering and regenerative medicine usu­

ally to replace native damaged structures in the human body to improve health. As such, we

discuss these in this chapter in the section on personalizing healthcare.

9.3.7  HYBRID BIO/​BIO–​BIO DEVICES

A number of artificial devices are being fabricated, which incorporate both biological and

nonbiological components. For example, it is possible to use the swimming of bacteria to

make a 20 μm diameter silicon-​based rotor, machined with a submicron precision, and thus

an example of genuine nanotechnology, rotate (Hiratsuka et al., 2006). It was the first “engine”

that combined living bacteria with nonbiological, inorganic components. The bacterium

used was Mycoplasma mobile that normally swims on the surface of soil at a few microns

per second. These bacteria follow the curvature of a surface and so will on average follow the

curved track around the rotor. By chemically modifying the surface of the bacteria with biotin

tags, and then conjugating the rotor surface with streptavidin that has a very strong binding

affinity to biotin, the cells stick to the rotor and make it spin around as they swim around its

perimeter, at a rotation speed of ~2 Hz that could generate a torque of ~10⁻¹⁵ Nm.

This level of torque is roughly four orders of magnitude smaller than purely inorganic

mechanical microscopic motors. However, this should also be contrasted with being five

orders of magnitude larger than the torque generated by the power stroke action of the

molecular motor myosin on an F-​actin filament (see Chapter 8). Microorganisms, such as

these bacteria, have had millions of years to evolve ingenious strategies to explore and move

around. By harnessing this capability and fusing it with the high-​precision technology of

silicon at micro-​ and nanoscale fabrication, one can start to design useful hybrid devices

(Figure 9.4).

Another emerging area of hybridizing bio-​ and nonbiocomponents is bioelectronics. Here,

perhaps the most exciting developments involve attempts to design biological transistors.

The transistor was invented in 1947 by physicists John Bardeen, Walter Brattain, and

William Shockley, made possible through advances in our understanding of the physics of

electron mobility in semiconductor materials. Moore’s law is a heuristic relation, relating

time with the size of key integrated circuitry, most especially the transistor, suggesting that

technological progress is resulting in a decrease in the size of the transistor by roughly a factor

of two every two years. With the enormous developments into shrinking of the effective size

of a transistor to less than ~40 nm of the present day, this has increased speed and efficiency

in modern computing technology to unprecedented levels, which has affected most areas of

biophysical tools.

Interestingly, the size of the modern transistor is now approaching that of assemblages of

the larger biological molecular complexes, which drive many of the key processes in biology.

Bioengineers have created the first biological transistor from the nucleic acids DNA and

RNA, denoted as a transcriptor (Bonnet et al., 2013) in reference to the natural cellular

process of transcribing the genetic code embodied in the DNA sequence into molecules of

mRNA (see Chapter 2). The transcriptor can be viewed as a biological analog of a solid-​

state digital transistor in electronic circuitry. For example, transistors control electron flow,

whereas transcriptors regulate the flux of RNA polymerase enzymes as they translocate