Silicon probes can be equipped with light guides. The light guide can simply be an optical fiber glued onto the probe shank, or can be built in the shank using various techniques (e.g. by depositing a photoresist material). To preserve good tissue penetration of the probe, the light guide is usually made relatively thin (< or = 105 µm core diameter for optical fibers).
Silicon probe and optical fiber
Royer et al. (2010) explain how to attach a tapered optical fiber to a silicon-probe. Briefly, the fiber end if made conical using the wet etching technique [NOTE: SHOULD INCLUDE A POST ON WET ETCHING], which consists in dissolving silica fibers in hydrofluoric acid. Only pure silica fibers (= both the core and the cladding are made of silica) can be etched this way. Other fibers (like the popular BFL multimode fibers series from Thorlabs) are not pure silica and incorporate other materials which are resistant to acids (such as fluoropolymers).
Silicon probe and non-fiberoptic waveguide
The optical fiber can be replaced with a thin layer of an epoxy-based photoresist. This can significantly reduce the final size of the optoelectrode. This design was proposed by Cho et al.
Commercial Silicon Optrodes
neuronexus.com Neuronexus has a series of optoelectrodes called the O-Series, for acute and chronic experiments. For acute recordings, the optoelectrode consists in an A-Style multichannel (16 or 32) probe combined with an optical fiber terminated by an LC connector. For chronic recordings, the solutions consist in a Z-Style or CM-Style multichannel (16 or 32) probe combined with an optical fiber terminated by an NNC connector. The NNC connector (NNC = NeuroNexus Coupler) is a small (4.6 mm long, 3 mm diameter) custom fiberoptic connector which incorporates a rotary joint for strain relief.
- Fiber: 105/125 µm core/cladding fused silica multimode fiber.
- Fiber location: terminated 200 µm above the most proximal site unless specified.
- Transmission through the NNC connector: 80% (± 2% variation during a single rotation).
"Diode" Silicon Optrodes
The above solutions consist in adding a light guide to a silicon probe in order to deliver light in close proximity to the sites of electrical recording. The first of these prototypes of silicon optrodes had a small fiberoptic connector (such as an LC connector, or one of the connectors described in the page Fiberoptic Guides and Connectors) in order to be connected to a fiber-coupled light source. Another interesting possibility is to add the light source to the probe itself, so that only wires (and not optical fibers) have to be connected to the animal. The light source has to be compact, light-weight and powerful enough. Diodes (light emitting diodes and laser diodes) are the best candidates. Light still has to travel through a waveguide to reach the site of stimulation/inhibition. The waveguide (e.g. an optical fiber) has to be coupled to the light source: we say that the source is "pigtailed" (see page Pigtailed LEDs).
The Buzsaki lab published a methods paper explaining how to build and use silicon probes bearing a pigtailed LED or laser diode ("diode" silicon optrode) .
Lu et al.  have designed custom optrodes consisting of an array of formvar-coated nickel chromium wires (36 mm, California fine wire, USA) positioned near the tip of a 200 μm multimode optical fiber. Eight microelectrodes in an optrode array were arranged in two parallel rows, each containing four wires, and the spacing between neighboring microelectrodes was 200 um.
[See the blog post about the multimodal microimplant].
In a paper published in “Lab on a Chip”, the Stieglitz and Lüthi labs introduced a novel optrode which comprises an SU-8 waveguide, several electrical recording sites and a microfluidic channel for liquid delivery at the tip of the probe. This liquid channel can be used for example to inject a solution containing a virus right under the waveguide tip and around the electrical recording sites. The detailed specifications of this “multimodal” implant can be found below. The different components of the implant were tested separately. Their characterization of the microfluidic channel proved the ability to deliver fluid through the channel without delamination of the channel from the polyimide substrate. The hybrid channel assembly was stable at pressures up to 1.4 bar, resulting in flow rates of about 240 µl/min. They then connected an optical fiber to the implant and measured the transmission loss through the SU-8 waveguide. At 473 nm, the output power of the waveguide was 1 to 2 mW while the output power of the optical fibre was 35 mW. Finally the authors recorded single unit activity using this implant in a mouse expressing ChR2 in the neocortex (ChR2 was delivered using a AAV, which was not injected through the implant but with a conventional pressure ejection system and a glass pipette). Units recorded throughout the cortical region expressing ChR2 showed light-evoked modulation of their firing as expected.
Specifications: the implant has a polyimide-based shaft including 9 platinum electrode sites with a diameter of 30 µm and a centre-to-centre distance of 50 µm. The shaft comprises a 300 nm thick platinum thin-film sandwiched between two 5 µm thick polyimide (PI) layers. Within the PI foil, the electrode sites are connected via conductor paths on a flexible cable to solder pads designed to contact SMD Omnetics connectors. An SU-8 waveguide is placed on top of the PI shaft separated from the PI by a 200 nm thick tungsten–titanium layer (tungsten–titanium has a good adhesion to SU-8). This metal cladding is needed to prevent the light from coupling into the PI substrate which has a higher refractive index than the SU-8. The waveguide is glued into a custom made optical adapter with epoxy (the waveguide is covered with a 200 nm gold layer to prevent the light from leaving the waveguide towards the epoxy glue – gold was chosen as it can easily be removed with a drop of potassium iodide–iodine solution). The gold and tungsten–titanium cladding layers have no electrical contact to the platinum leads and electrode sites as the platinum structures are insulated by the second layer of polyimide. Thus, the electrode sites are not short cut by the cladding. The waveguide ends in front of the electrode sites thus, neurons which are stimulated by the light exiting the waveguide will be closest to the electrode sites for recording. The channel is implemented by attaching a U-profile with an outer cross-section of 190 mm by 65 mm, an inner cross-section of 50 mm by 45 mm and a length of 7 mm to the rear side of the PI shaft. The U-profile was made of SU-8 and was processed on a separate wafer. The channel outlet at the tip of the shaft is a hole in the PI between two electrode sites. Thus, the fluid can be applied to the same tissue volume which is also electrically and optically interfaced.
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