Sensors in Neural Engineering – An Introduction to Electrodes
Henry Lancashire
February 2024
Contact details:
Implanted Devices Group
Malet Place Engineering Building, room 8.03
Phone: 020 3108 1611
Email: h.lancashire@ucl.ac.uk
Students in this class have different backgrounds. Though in my experience, few have much knowledge of biosensing,
if any. This set of lectures, including Sara Ghoreishizadeh’s lecture on biochemical sensing, will therefore act as an
introduction to the topic. You already considered electrode polarisation in the first lab, and there is a second lab entirely
dedicated to the topic. As always, our focus is on neural engineering applications. Consequently, most explanations and
examples I give are derived from medical practice or research. However, the range of applications of these concepts is
much broader.
1 Electrodes
Electrode: a conductor by means of which electricity enters or leaves an electrolyte, gas, or other medium, or a vacuum.
Oxford Dictionary
Vocabulary
• Bioelectrodes are specifically designed to interact with “biological tissues”
• They can be used either to detected a bio-signal (electrical or chemical), or for stimulation.
• A bioelectrode provides an electrical interface between an electron-conductor and a special electrolyte (or ion-
conductor): biological tissue.
• The electrode is the electron-conductor (very often made of metal). A group of several electrodes is an array of
electrodes but we are sloppy and tend to call the array the electrode, and refer to each conductor as a “contact”.
• Several electrodes may be held together within an electrode mount.
• Try to use specific terminology: an electrode cuff; an electrode probe; an electrode array.
Electrodes
2 Electrode types
2.1 Non-implantable electrodes
These electrodes are used for short term recording or stimulation, usually for less than 24 hours. They may be used in
clinic (all types) , or by patients at home (surface and anal/vaginal probes only).
Figure 1: Surface Electrodes, similar to self-adhesive Ag|AgCl electrodes.
Figure 2: Percutaneous Electrodes, the electrode point is a the tip, either a single electrode site, or occasionally two
electrodes which may be arranged concentrically or linearly.
Figure 3: An anal/vaginal probe, used to measure EMG from the pelvic floor muscles for biosignal feedback to treat
incontinence.
Version of January 3, 2024 2
Electrodes
2.2 Implantable electrodes
These electrodes are used for long term recording or stimulation, for months or years. They may be used in clinic
(pacemaker electrodes) , or only in research (other types shown). There is a very wide variety of implantable electrode
designs, at least as many as there are researchers working the field. A few examples are presented below, you will
encounter more as you read about the applications of neural engineering. They have advantages and disadvantages.
The key tradeoff in electrode design is between invasiveness and selectivity, i.e. between damage caused by implantation
and the level of detail which can be achieved in recording or stimulating.
Figure 4: Pacemaker electrodes. The metal contact is visible as a ring on the electrode end and either as a hemisphere
tip or a coil tip. Sprung wires are used to resist the forces due to continual heart and chest movement. Spear design
and the end “corkscrew” are used to hold the electrodes in place.
Figure 5: Probe designs used in research. These arrays have many electrodes, in some recent designs over 1000 electrodes.
Commonly they are arrayed along a linear shaft (left, middle), or a the tip of an array of spikes (right). These designs
are similar to Michigan electrode arrays (left, middle), and Utah electrode arrays (right). Utah arrays are now in use
with patients as BCI connections for patients with high level spinal cord injury and quadraplegia, although as research
devices only.
Version of January 3, 2024 3
