Microsystems for Bioelectronics

subject to spontaneous degradation. Therefore living organisms must spend some energy to fight accu-

mulating disorder, for example to repair damage by replacing some ‘broken’ macromolecules, etc. [17].

A characteristic measure of the rate of energy consumption, or power, by living organisms is their

metabolic heat rate. Studies across a broad spectrum of the organisms have revealed a striking

observation that their mean metabolic rates are confined in a narrow range between 0.3 and 9 Watts per

kilogram of body mass [12]. Table 1.1 presents examples of mass, size, and power consumption of the

cell, averaged within different taxonomic groups (the numbers were derived from [12]).

It is interesting to estimate the rate of energy consumption of the cellular structure of the human

body. First, note that approximately 70% of the living cell (‘wet mass’) is due to water. Therefore as an

order-of-magnitude estimate, it is reasonable to assume that the cell has a ‘wet’ mass density close to

the density of water (1 gram/cm

3

). Thus, a cell with a size of 10

m

m has a mass of 1 ng or 10

–12

kg.

Now, since a typical weight of a human can range between 50 and 100 kg, an approximate number of

cells in the body is 5  10

13

–10

14

, as is consistent with numbers in the literature. Next, since the

average power consumption per cell in mammals is 4  10

–12

W(Table 1.1), multiplying this rate by

the number of cells results in 200–400 W for human power consumption. By comparison, the average

power per person can be derived from medical studies of daily energy intake as follows. The typical

total energy expenditure of men aged 30–39 years is 14.3 MJ/day ¼ 3400 kcal/day [18]. Thus the

corresponding average power is P ¼ E/t ¼ 14.3 MJ/(24 h  3600 s) ~165 W.s.

The two different estimates for human power consumption differ by about a factor of 2. The difference

is in part due to variation in the size and in energy consumption rates of various human cells. Also, the

number assumed for human cell power in Table 1.1 is an average across many different mammals [12].

1.4 CURRENT STATUS OF TECHNOLOGIES FOR AUTONOMOUS

MICROSYSTEMS

1.4.1 Cardiac pacemakers

This class of devices repres ent s an example of successful commercia l iz at io n fo r impl ant ab le

bioelectronic systems. It also is a clear illustration of how the success of miniaturization in

Table 1.1 Examples of mass, size, and power consumption of the cell, averaged within different

taxonomic groups

Cell average within

different taxonomic

groups Wet mass, kg Volume,

m

m

3

Size,

m

m Power, W

Prokaryote 2  10

–15

2 1.3 10

–14

Cyanobacteria 7  10

–14

70 4 2.5  10

–13

Eukariotic microalgae 6  10

–12

6000 18 5  10

–11

Protozoa 3  10

–11

3  10

4

32 2.5  10

–10

Human cells 10

–12

1000 ~10 4  10

–12

(Source: [12])

1.4 Current status of technologies for autonomous microsystems 9