J. T. MORTIMER AND P. H. PECKHAM 
1191 
tissue destruction and inadequate healing. In 
the unsuccessful instances, failure occurred 
within three days of implant. 
THE CALDWELL ELECTRODE 
The physical properties of the recording or 
stimulating electrode, to a large extent, dictate 
the success or failure of an experiment. If the 
electrode breaks, the experiment is most likely 
lost. If the properties are such that the elec- 
trodes induce excess tissue trauma, intense 
fibrous encapsulation will result. As the fibrous 
capsule thickens, the physical separation be- 
tween the electrode and active tissues increases. 
The effect of this separation increase is to 
diminish the amplitude of the recorded electri- 
cal signal or to raise the effective threshold to ex- 
cite the surrounding normal tissue. 
The aforementioned difficulties may be mini- 
mized by the following considerations : 
1. using biocompatible conductors, such as 
platinum and stainless steel ; 
2. using a flexible electrode configuration to 
reduce mechanical trauma ; 
3. choosing a material and electrode config- 
uration to avoid material fatigue, result- 
ing in electrode breakage. 
There are perhaps a number of combinations 
of materials and electrode configurations which 
could perform with these points. Based on the 
experience gathered in this laboratory, the 
Caldwell electrode^ has been found to be satis- 
factory for use in both recording and stimula- 
tion applications. 
The electrode is formed by winding a steel 
alloy wire (28)u, diameter) around a mandril 
110ft in diameter. The end result is an electrode 
approximately 180/x in diameter which closely 
resembles a tightly wound helical spring upon 
microscopic examination. Using an insulated 
wire and regulating the amount of wire de- 
insulated, the exposed electrode surface can be 
controlled. Using metals such as Karma®, 304 
stainless steel or 316 stainless steel in this con- 
figuration, an extremely flexible and sufficiently 
strong electrode is obtained. This combination 
with a silicone rubber filler (which adds 
strength) has been successfully implanted in 
animals and man for periods in excess of one 
year without complications.® 
The Caldwell electrode is implanted with the 
aid of a hypodermic needle, usually a 22 gauge. 
The electrode is placed in the needle with the 
deinsulated end at the needle tip, leaving a 
small hook at the deinsulated end. The hook is 
formed by bending the wire backward over the 
coiled section, which limits the inward travel 
and provides a barb to catch in the tissue when 
the needle is withdrawn. In Figure 3 is shown 
a typical electrode in the hypodermic needle 
prior to implant. 
The passive tissue reaction (no external po- 
tential applied) to this electrode has been ex- 
tensively studied by Peckham,'^ Caldwell" and 
Gertler et al.^ The results of these investigators 
indicate that the average fibrous capsule thick- 
ness when implanted in muscle is 90)Lt with a 
standard deviation of AOfi.^ 
The tissue reaction to an active electrode dur- 
ing stimulation of skeletal muscle was studied 
by Gertler et al.^ In this study, a pulsed DC 
current that ranged from 1 ma to 15 ma was ap- 
plied for approximately a twenty-four hour 
period. The pulse duration was 100/u,sec at a 
frequency of 50 Hz. Microscopic examination 
of the muscle sections revealed no statistical 
difference in tissue damage between the pas- 
sive implants and those subjected to the high- 
est current levels in that study. The experience 
of these authors substantiates the Gertler et al.^ 
findings in experiments of up to 31 days of 
continuous stimulation. (The range of stimulus 
amplitudes for most physiologic preparations is 
between 0.5 ma and 10 ma.) As this study was 
limited to the cathode, it is worthwhile to men- 
tion that these results may not hold for an 
anode with similar physical properties. 
CHRONIC IMPLANT TECHNIQUE 
The procedure for implanting the chronic 
stimulating (or recording) system into an ani- 
mal consists of three steps : injection of the elec- 
trode into the muscle, connection of the elec- 
trode to the lead wires, and passing the lead 
wire through the skin interface. 
Experiments performed to date have concen- 
trated on the chronic electrical stimulation of 
