The overall goal of this procedure is to implant flexible thin film neural probes into a tissue using a temporary stiffener. This is accomplished by first designing and fabricating custom silicon stiffeners that match the probe and have a channel filled with dissolvable polyethylene glycol. The second step is to use a flip chip bonder to assemble the stiffener to the probe.
Next, the stiffened probe assembly is inserted into the target tissue. The final step is to allow the polyethylene glycol to dissolve and extract the shank. Ultimately, the demonstrated method can be used to implant flexible micro electrode probes into neural tissue for in vivo recording and stimulation.
The implication of this technique extend towards clinical treatment of neurological disorders because the removable stiffeners enable flexible implanted devices that can reduce adverse tissue reactions. The main advantage of this technique over existing methods, such as softening materials, is that it can enable precise insertion of arbitrarily long and thin probes. Fabricate a silicon stiffener with a wicking channel from a silicon on insulator wafer with the device layer thickness equal to the desired thickness of the stiffener.
Dry etch the wicking channels in a plasma etcher using the standard Bosch process. Then define the stiffener outline by a longer edge that stops on the buried oxide layer. Now wet etch the buried oxide layer in 49%hydrofluoric acid to release the stiffeners.
Then soak the release stiffeners in deionized water for 15 minutes. Next place a pellet of polyethylene glycol into the reservoir. On the tab of the stiffener.
Heat the stiffener to 65 degrees Celsius so that the polyethylene glycol melts and wicks into the channel by capillary action. Then cool to room temperature to solidify. Place the stiffener upside down on the base stage of the flip chip bonder.
Then pick up the stiffener with the tool head. Next, place the probe upside down on the base stage using the flip chip bonder. Align the stiffener and the probe.
Then lower the stiffener onto the probe. Heat the assembly to 65 degrees Celsius for one minute so that the polyethylene glycol remelt and tags the stiffener to the probe. Then cool to solidify, turn the assembly over and inspect from the top reheat to 65 degrees Celsius to allow the polyethylene glycol to completely fill the interface between the probe and the stiffener.
With probe side up melt one to three extra pellets of polyethylene glycol onto the tab to provide additional reinforcement in this region. Finally, allow the assembly to cool so that the polyethylene glycol solidifies and the assembly is ready for surgical insertion. In an acrylic box, prepare 0.6%aros gel in phosphate buffered saline or PBS saturate the gel with PBS and heat to 37 degrees Celsius.
Then arrange the microm manipulator box of agros gel and microscopic camera system. Insert a glass reference fiducial into the box of gel. Next, use a dental pick to square the features on the reference fiducial to the field of view of the digital microscope.
To mount the probe Stiffener assembly to the micro manipulator, adhere the back of the stiffener tab to a fixture that can be clamped to the micro manipulator. Taking care not to contact the probe with adhesive. Temporarily secure the connector end of the probe to the fixture with a small piece of adhesive putty such that it can be easily removed with low force position the probe assembly over the gel, about one millimeter behind the reference fiducial.
Then insert the probe into the gel using the camera to guide it to a desired depth in the field of view. Immediately remove the connector end of the probe from the micro manipulator and secure it on a nearby surface such as a second manipulator arm. This must be done before the polyethylene glycol on the tab begins to dissolve.
To avoid displacing the probe, take a snapshot of the probe location. Note that the reference fiducial features may be slightly out of focus. Apply saline near the tab to dissolve polyethylene glycol that is above the aros gel, and on the tab start video capture if desired.
Now, begin extraction of the stiffener by applying a displacement of 100 micrometers at a speed of five millimeters per second. Then complete this stiffener extraction at a slower speed of approximately 0.1 millimeter per second. When extraction is complete, take a final snapshot of the prob location with image processing tools.
Compare the images before and after stiffener extraction. Use the features on the reference fiducial that are visible in the field of view to accurately overlay the images. Calibrate the scale of the image based on the size of known features on the probe.
Now measure the distance of probe displacement on the overlay image. This insertion technique was used in conjunction with LLNL Thin film Polyamide probes, which have passed ISO 1 0 9 9 3 biocompatibility standards and are intended for chronic implantation. The AROS gel test was used to quantify average probe displacement for a given combination of probe stiffener and adhesive.
For example, these before and after snapshots demonstrate probe displacement of 29 micrometers. These single neuron spikes were obtained from a flexible micro electrode probe implanted with a removable stiffener into a rat cortex. Following this procedure, we have been able to implant probes with a variety of configurations, including dual-sided devices and probes with multiple shanks After its development.
This technique paved the way for researchers in the field of neuroscience to explore extended neural activity in behaving animal models.
