Microsystems for Bioelectronics

For example, DNA memory uses molecular fragments (nucleotides) as information carriers, each

consisting of more than 10 atoms. The molecular information carriers are densely packe d in a linear

array with distance between nucleotides of only 0.34 nm [19]. By comparison, the critical dimension

of electron memory is ~5 nm, i.e. more than ten times more that of DNA. This explains the 1000

difference in volumetric memory density between electronic and DNA memor y, i.e. 10

16

bit/cm

3

of

electronic memory vs. 10

19

bit/cm

3

for DNA memory.

Utilization of ambient thermal energy

For in silico systems thermal energy (~ k

B

T) must be managed as it may destroy the state or divert the

information carrier from its intended trajectory; for example in communication between several logic

elements. In order to overcome the deleterious effects of thermal energy each logic or memory element

must contain a barrier E

b

> k

B

T. Moreover, in the communication with other elements, N carriers must

be sent to the recipient elements, each of which must have kinetic energy E

k

> k

B

T. As a result the total

energy of device operation, as it was derived in Chapter 3, becomes

E

SW

¼ 2E

b

þ N,E

k

(6.41)

and it can be significantly large, usually >100 k

B

T.

In contrast, the in carbo systems utilize thermal energy to effect data exchange/transmission

between, e.g., logic-to-logic or memory-to-logic elements. All computational molecules move within

the cell’s space by thermally excited random walk with no extra energy required, and thus the second

term in (6.41) is eliminated. In carbo systems actually uses thermal energy in the transmission of

information!

Flexible/on-demand 3D connections/routing

Once more, referring to Eq. (6.41), in in silico systems most energy is consumed by interconnect. This

is due to the need to pump a large number, N, of carriers (electrons) into the interconnecting wire for

reliable communications. As was shown in Chapter 3, for reliable communication, N must dramati-

cally increase for longer path lengths and/ or more receiving devices (fan out). In electrical circuits the

connection paths are pre-determined and in many instances, the electron travels a long distance from,

e.g., point A to point B. Devices of in carbo systems are usually free to travel in all three dimensions

within the cell and they don’t follow a fixed path.

6.11 SUMMARY

In this chapter, the essential units of the nanomorphic cell (energy, control, communication and

sensing) were combined within the 1000

m

m

3

volume (e.g.10

m

m10

m

m10

m

m cube). The corre-

sponding trade-offs that must be made in allocating volume resources for each of these units were

discussed. It was concluded that the computational capability of the nanomorphic cell that could be

sufficient to enable the sense-analyze-announce function of the cell.

Also, in this chapter an effort has been made to compare the projected performance of the nano-

morphic cell (in silico system) with that of the living cell (in carbo system). The approach was to adopt

the view of the living cell as a ‘universal construc tor’, a type of computer that makes copies of itself,

and that was first suggested by von Neumann. In order to provide a common framework for

180 CHAPTER 6 Micron-sized systems: In carbo vs. in silico