Biomedical Engineering Reference
In-Depth Information
Percutaneous connectors (physical wired links between implanted and exterior com-
ponents) raise the risk of infection due to perpetual breach of the skin through which the
wires must pass. In addition, anchor points can be a problem, and movement between the
internal and external components can result in unnecessary stresses to the body and to the
connection itself.
Transcutaneous (wireless) connections are therefore justified in most cases for bidi-
rectional telemetry and to supply power to the embedded portion of the prosthesis. Three
types of telemetry are considered to be appropriate under these circumstances for provid-
ing power and communication: optical, magnetic (inductive), and electromagnetic (radio
frequency). They are discussed in the following sections (Finn and LoPresti, 2003).
Much work is being done in this field to improve the efficiency of radio frequency
identification (RFID) tags, which work on similar principles, albeit in applications where
the tag requires very little power other than to generate a response.
5.7.1.1 Optical Telemetry
Optical telemetry appears most commonly among research groups developing ocular pros-
theses as the cornea and lens are usually transparent. This form of telemetry usually uses
high-energy light from a laser to power the photoelectronics that form part of the implant.
In a prosthesis created at Massachusetts Institute of Technology (MIT) and Harvard Uni-
versity, an array of photodiodes was illuminated by a 30 mW 820 nm laser to produce a
current of 300
μ
Aat7V.
5.7.1.2 Inductive Telemetry
Inductive telemetry has been the standard means of wireless connection to implanted
devices for many years. It is based on the coupling between a pair of coaxial coplanar
coils. The secondary is mounted subcutaneously along with the internal modules, while
the primary coil remains exterior. A low-frequency carrier, usually between 1 and 10 MHz,
supplies the primary coil, and power is coupled into the secondary coil where it is rectified
and filtered to supply power. Additionally, the power carrier can be modulated to transmit
data to the implant.
Problems with this method of power transfer include the fact that, because the coils
have no cores, very little of the energy couples through to the other coil, and most of it
radiates omnidirectionally. This results in a large drain on the batteries and the potential
for electromagnetic interference.
The coupling between coils is dependent on coil loading, the excitation frequency,
and number of turns as well as coil alignment and spacing. Typical values for coil coupling
vary from 0.01 and 0.1. However, this can be improved if the coils are series or parallel
resonant using an appropriate capacitor. If
L
(H) is the self-inductance of the coil, then
the capacitance,
C
(F), should be
1
(
2
π
f
)
C
=
(5.71)
2
L
The resonant coil is driven by a class-E amplifier (usually just a field-effect transistor [FET]
switch) that shunts the supply to ground, as shown in Figure 5-70. As with conventional
switch-mode power supplies, unused energy is not dissipated as heat but is traded back
and forth between electric and magnetic fields in the resonant structure.
The low coupling coefficient to the secondary means that shifts in the load impedance
have very little effect on the primary circuit. However, coil distortion and the proximity
