5 min read
Updated September 9, 2025
A chronic implant does two things an acute one does not. It stays in place for weeks, and it survives an animal that grooms, climbs and sleeps against the cage. Most chronic failures are mechanical, and most of them are decided during the twenty minutes the cement is curing.
Research use only. NeuroNexus products and the procedures described here are for non-clinical research use, and must not be used in human or veterinary medical procedures.
This guide describes an approach, not a protocol. Adapt it to your species, your preparation and your institutional requirements. All procedures run under IACUC approval.
Before surgery#
Anesthesia and analgesia. Institutional guidelines govern both. Plan multi-day analgesia, not just intra-operative.
Aseptic technique. Chronic surgery is survival surgery. Sterile instruments, sterile field, sterile probe.
Instruments. Fine forceps, scissors, hemostats, scalpels, needle holders; a high-speed drill with burrs; a micropipette or blunt needle for saline.
Implant materials. Dental cement or an equivalent adhesive, bone screws, and a protective housing for the connector. Decide the headcap geometry before surgery — where the connector sits, which way the cable leaves, and how tall the stack is.
Ground and reference. Stainless steel or Ag/AgCl wire, anchored to bone screws.
Rig. Stereotaxic frame with micromanipulator, and a dissecting microscope with a cold light source.
Surgery#
Site preparation#
Secure the subject in the frame. Incise the scalp, expose the skull, and remove the periosteum with a cotton swab. Rinse with saline and dry.
A dry, clean skull is what the cement bonds to. Residual periosteum is the most common reason a headcap lifts at week three.
Craniotomy and screws#
Mark the craniotomy and the screw positions together, so the screws do not end up where the cement needs to flow. Drill in short passes with saline cooling.
Place bone screws to anchor both the ground and reference wires and the headcap itself. Screws carry the mechanical load; the cement only distributes it.
Durotomy#
Remove the dura where the shank width or dural toughness makes penetration uncertain. Leave it where you can, since an intact dura limits brain movement over the following weeks.
Insertion#
Mount the probe on the micromanipulator and advance slowly, watching for dimpling. Wet the shank with saline before entry.
Allow the tissue to settle at depth before fixing anything in place. Cementing a probe that is still under load builds the load into the implant.
Securing the probe#
Cement in thin, even layers, allowing each to cure. A single thick pour generates heat as it sets and shrinks unevenly as it cools.
Seal the craniotomy before cementing — silicone elastomer over the exposed brain, then cement over that. Cement in contact with cortex causes a lesion.
Position the connector where it clears the animal's normal movement and where a cable can be attached without levering on the implant. Think about how the animal will be handled, not only about how the surgery ends.
The probe must be firmly anchored, and it must be anchored without transmitting force into the tissue. Those two requirements pull against each other, and resolving them is the craft of a chronic implant.
Reference and ground wiring#
16-channel probes. With a separate reference, place the reference wire in a saline-filled craniotomy. Without one, connect the probe reference input to a bone screw near the site.
32-channel probes. With the internal reference, connect the designated wire to a bone screw. To disable it, modify the connector and reference externally.
64-channel probes. As for 32-channel, with additional reference wires. Confirm the wiring for your specific package before surgery.
Recovery#
Analgesia and monitoring per institutional guidelines. Watch the first 48 hours closely.
House individually, with nesting material chosen so it cannot catch on the implant.
Do not record on day one. Signal in the first days reflects the acute injury response, not the steady state, and pushing for early data risks the implant for results you will not use.
Maintenance#
Impedance, on a schedule. Measure at implant and at every session. The trend matters more than the value: a channel rising steadily is failing, a channel that jumps has a mechanical fault.
Connector care. Keep it capped and clean. Most late-stage signal loss is connector contamination, not tissue.
Cable strain relief. Every connect and disconnect loads the implant. Support the cable so the force goes to the tether, not to the headcap.
Inspect the site at each handling for cement lifting, screw loosening, or signs of infection.
Timeline#
| Period | What to expect | What to do |
|---|---|---|
| Days 0–7 | Acute injury response; unstable units | Monitor, manage pain, do not push for data |
| Weeks 1–4 | Signal stabilizes; tissue heals | Begin recording; establish the impedance baseline |
| Beyond 1 month | Steady state | Longitudinal recording; track impedance and connector condition |
Troubleshooting#
Signal degrades across all channels at once — look at the connector and the ground wire before you look at the tissue. Simultaneous failure is almost always electrical or mechanical.
One channel fails — usually a site or a trace. Note it and continue.
Headcap loosens — screws or skull preparation, decided at surgery. It is rarely recoverable in place; consider ending the implant rather than risking infection.
Behavioural change — assess implant weight and balance, and consult veterinary staff.
Support#
Technical questions: support@neuronexus.com · +1.734.913.8858 Full protocol: Chronic Penetrating Arrays ↗