1188 
MONITORING 
used here to designate any mechanical device 
which is implanted in the skin and through 
which lead wires pass. The purpose of the in- 
terface is to prevent infection and to anchor 
the lead wires. 
Subjective evidence, gathered in this labora- 
tory over the past years, indicates that the in- 
vasion of bacteria, causing infection, takes 
place via two mechanisms : 
1. Bacteria may be carried into the body if 
the lead wire is permitted to slide freely 
in the wound through which it enters the 
skin ; 
2. Bacteria may also gain access through the 
small fissure between the skin and the 
wire of the interface. 
Therefore, bacterial invasion may be mini- 
mized by eliminating wire slippage and keeping 
the skin-interface fissure to the smallest possi- 
ble size. 
Anchoring the wire (with its insulating cov- 
ering) to the interface eliminates wire slippage 
in most cases. The major problem then lies in 
minimizing interface movement or fissure open- 
ing between the skin and the interface. 
To date, most interfaces have a dumbbell 
shape and are formed from polymer materials 
with smooth surfaces such as teflon and silicon 
rubber. The wire or wires are embedded in the 
center portion of the interface and the skin is 
closed in such a way to place one end of the 
dumbbell below the skin and the other on the 
outer surface of the skin. Two major drawbacks 
to this configuration are: 
1. Fissure size is not minimized but made 
larger by placing the wires inside the cen- 
ter section of the dumbbell. The minimum 
diameter of the fissure corresponds to the 
diameter of the stem or shank of the 
dumbbell. 
2. The use of solid materials such as teflon 
and silicon rubber prohibit attachment 
of tissue to their surfaces. This arrange- 
ment then allows a constant mechanical 
irritation of the tissues around the inter- 
face resulting in cellular destruction and 
chronic inflammation. 
Ideally, one would like to use a material to 
which body tissues adhere. To date, there is 
no known inanimate material to which the liv- 
ing cells of the body attach themselves. A sub- 
stitute for the ideal is a biocompatible material 
with a structure which allows tissues to grow 
into it or through it. The ingrowth of connective 
tissue substitutes for the adherence to act in 
anchoring the interface. Materials with these 
characteristics are fabrics and porous compos- 
ites. A number of materials falling in these 
categories have been tested by implantation and 
are found to be biocompatible. Fibers woven 
into meshes and velours that have been exten- 
sively tested are dacron, nylon, and teflon. Po- 
rous composites tested or under test are ceram- 
ics ^ and carbons.- Both show promising results. 
Histological examination of the implant site has 
shown a signiflcantly milder foreign body re- 
action when porous materials are used than 
when similar impervious materials are used.^ 
At the present, about half of the composite ma- 
terials have been tested as percutaneous devices 
with the remainder under consideration. 
The use of a carbon material as an interface 
suggests that this conducting interface might 
also act as an indifferent or reference electrode. 
However, the results obtained by Gibbons, Peck- 
ham, and Martin indicate that severe degrada- 
tion can occur under anodic conditions. There- 
fore, it is not recommended that a carbon 
interface be used as an anode electrode. 
In a modification of the teflon dumbbell shape, 
Miller* has attached a synthetic fiber velour 
to the small diameter section. Epithelium re- 
portedly grew into the velour, forming a satis- 
factory interface. 
An- alternate approach that has been exten- 
sively utilized by the authors was described by 
Gertler et al.^ The sketch shown in Figure 1 
depicts a cut-away of an implanted interface. 
In Figure 2 is shown the actual interface prior 
to implant with the lead wires attached to it. 
This technique, unlike the dumbbell configura- 
tion, bonds the interface to the electrode lead 
exterior to the epidermis. The largest diameter 
of any hole through the skin then becomes the 
diameter of the insulated wire used for signal 
transmission. 
Slippage of the lead wire through the skin 
into the subcutaneous space is minimized by 
placing the silicone rubber ridge (to which the 
lead wires are attached) next to the skin. Out- 
