H. SANDLER, T. B, FRYER AND H. L..STONE 
1201 
SENSOR (PRESSURE CELL) 
WAX COATING 
TYGON DIP WITH 
SILASTIC OUTER 
COATING 
MAGNETIC SWITCH 
SILASTIC CABLE (AFTER WIRES ARE IN 
PLACE THE CABLE IS FILLED WITH 
LIQUID SILASTIC AND CURED. THE CABLE 
IS THEN SOLID SILASTIC WITH WIRES 
IMBEDDED IN IT) 
WIRES ANCHORED 
WITH EPOXY 
BATTERY 
METAL ENCLOSED HOUSING 
FILLED WITH WAX 
TYGON COATING 
WITH SILASTIC 
OUTER COATING 
BRIDGE BALANCE 
RESISTORS 
HERMETIC ANTENNA 
CASE WAX SEAL 
Figure 3. — Schematic of single channel pressure transmitter. 
500 grams and have a volume of 250 cm^. A 
typical recording of ^ow including pressure and 
EGG is shown in Figure 5. This system has 
been used to measure cardiac output and flov^ 
distribution in unrestrained animals in the lab- 
oratory and in the field. Ten hours of continu- 
ous operation are provided by two rechargeable 
nickel cadmium batteries. An implantable re- 
charging system has been developed for such 
systems utilizing transfer of electromagnetic 
energy through a closed chest wall." The sys- 
tem consists of an implanted coil of teflon 
coated wire 4" in diameter. Energy is trans- 
ferred through the skin from an external coil of 
the same diameter 3-5 cm away. The recharg- 
ing system has been used successfully for up to 
six months with the multichannel pressure te- 
lemetry units described above. It is presently 
being evaluated for powering a single channel 
completely implanted EMF unit. 
DISCUSSION 
The selection of proper instrumentation to 
measure various physiological parameters is an 
important aspect of any experimental research 
program. Measurements can be made either by 
direct connection to a given recording system or 
by means of radiotelemetry. For most acute ex- 
periments, direct connection is least costly, 
most easily implemented and usually most satis- 
factory. For long term experiments requiring 
data from awake unrestrained animals the use 
of a telemetry link is essential. Recent develop- 
ments in microelectronics have allowed for the 
development of useful chronically implantable 
telemetry units. The requirements of small size 
and reliable operation in the hostile saline envi- 
ronment of the body adds complications to any 
experiment. These problems are rapidly offset 
by the advantages of their use when multiple 
parameters are to be measured over long peri- 
