Microsystems for Bioelectronics

Energy in the small: Integrated

micro-scale energy sources

2

CHAPTER OUTLINE

Introduction.......................................................................................................................................... 15

Electrochemical energy: Fundamentals of galvanic cells and supercapacitors .......................................... 17

Energy stored in the galvanic cell .............................................................................................18

Power delivery by a galvanic cell ..............................................................................................19

Current status of miniature galvanic cells..................................................................................19

Miniature biofuel cells.............................................................................................................21

Remarks on biocompatibility ....................................................................................................22

Miniature supercapacitors........................................................................................................23

Miniature supercapacitors: Discussion on the status and research needs ..................................... 27

Energy from radioisotopes ..................................................................................................................... 28

Radioisotope energy sources ....................................................................................................28

Energy conversion ...................................................................................................................30

Current status of miniature radioisotope energy sources..............................................................33

Remarks on energy harvesting ............................................................................................................... 33

Radiation ...............................................................................................................................34

Solar energy ............................................................................................................................... 34

Laser energy............................................................................................................................... 35

RF/microwave energy.................................................................................................................. 36

Kinetic energy............................................................................................................................. 37

Thermal energy .......................................................................................................................... 38

Summary .............................................................................................................................................. 43

Appendix: A kinetic model to assess the limits of heat removal ............................................................... 43

List of symbols...................................................................................................................................... 45

References ........................................................................................................................................... 46

2.1 INTRODUCTION

Embedded energy sources are a key enabler for applications with limited or no physical access to

external energy supplies. For nanomorphic micron-scale systems, such as integrated analytical

microsystems, implantable diagnostics, drug delivery devices, etc., the available volume for on-board

CHAPTER

Microsystems for Bioelectronics the Nanomorphic Cell

Copyright Ó 2011 Elsevier Inc. All rights of reproduction in any form reserved.

15

energy supplies is very limited. Thus, the capacity of an energy supply, both in terms of energy stored

and the rate at which it can deliver energy, can place severe constraints on system design.

In this chapter, fundamental scaling limits for a variety of micron-sized energy sources are

investigated, including the galvanic and fuel cells, the supercapacitor, and radioisotope sources.

Further, it is sometimes possible to directly harvest energy from the surrounding environment when

the delivery of the energy can be either unintentional or directed. In the unintentional category,

energy sources might include ambient electromagnetic, solar, vibr ation, fluid flows, thermal grad-

ients, etc. An overview of the energy available for harvesting from a variety of sources is also

provided.

BOX 2.1 MAIN CONCEPT: ‘ATOMIC FUEL’

All known sources of energy use the released energy from breaking or forming inter-atomic or intra-atomic bonds. It

is important to remember, therefore, that the minimum energy-storing element is the atom or molecule and that

energy release is always accompanied by ‘burning atoms.’ The total energy available is proportional to the number of

energy-releasing atoms, N

at

:

E ¼ 3N

at

Where

3

is the energy released per atom.

BOX 2.2 AVOGADRO’S NUMBER

According to Avogadro’s Law, the number of molecules in one mole of any substance is constant. It is called

Avogadro’s Number N

A

:

N

A

¼ 6:022x10

23

mol

1

The number of atoms, n

m

, in a unit mass of a substance can be found from Avogadro’s Number, N

A

, and the

molar mass, M, which corresponds to the relative atomic mass from the Periodic Table expressed, e.g., in grams.

n

m

¼

N

A

M

For example, for metallic Zn, M ¼ 65 g/mole and n

m

¼ 9.26  10

21

at/g

The atomic density, which is the number of atoms in a unit volume of a substance can be found as:

n

at

¼ r,n

m

¼

rN

A

M

where

r

is the density of the substance (often shown in the Periodic Table).

For metallic Zn,

r

¼ 7.14 g/cm

3

and n

at

¼ 6.6110

22

at/cm

3

.

A

cell of the Periodic

Table representing Zn

M

30

Zn

65

7.14

16 CHAPTER 2 Energy in the small: Integrated micro-scale energy sources