A3036 Implantable Light-Emitting Diode (ILED) Manual

Contents

Description

[02-SEP-26] Our A3036IL Implantable Light-Emitting Diodes (ILEDs) are light-emitting diodes (LEDs) designed for implantation in animals. They provide the stimulation for optogenetic experiments. They are accessories to our A3041 Implantable Stimulator-Transponders (ISTs). The A3036IL uses a variety of LEDs: the blue and green EZ500, blue and green EZ290, and deep red LXZ1-PA01. All A3036ILs are equipped with a thin-walled steel tube on the far side from the LED to allow the ILED to be mounted during implantation. The tube is partially cut away near the base so it may be cut more easily after the ILED has been secured with dental cement. Our ILEDs come in three classes: Clear-Epoxy LED (CE-LED), Tapered-Fiber LED (TF-LED), and Blunt-Fiber LED (BF-LED). Each class is named after the element that delivers light to the target tissue.

Figure: Sketches of the Implantable LEDs. Left: Clear-Epoxy LED (CE-LED). Center: Tapered-Fiber LED (TF-LED). Right: Blunt-Fiber LED (BF-LED). In each case we show the LED mounted over a burr hole in a rat's skull.

Each ILED is constructed on its own circuit board. The LED is on the "bottom" side. The sockets and mounting post are on the "top" side. Clear epoxy surrounds the base of the mounting post and the LED. In a CE-LED, the entire LED die and bond wire is covered in clear epoxy. In the TF-LED and BF-LED, the base of the fiber is pressed up against the LED die, but there is a thin layer of clear epoxy between the two, and clear epoxy surrounds the LED die, bond wire, and fiber base, holding the fiber in place. When we mount an ILED on the skull of an animal, we use the mounting post to position the ILED. We connect the lamp power pins from our implantable stimulator to the two sockets. We cover the ILED with dental cement so as to fasten it in place on the skull. To ensure a strong bond, we can include nearby a screw threaded into the skull that we embed in the same dental cement. We take care to surround the lamp power sockets and pins on all sides with cement so as to insulate them from the rest of the animal's body: when we turn on the ILED, we do not want to see artifact in our recordings of other biopotentials. Once our first application of cement is cured, we cut the mounting post, then cover the exposed cut tube with a more cement.

Figure: A3036IL Clear-Epoxy Light-Emitting Diode (CE-LED), Bottom View. An epoxy dome covers the bare, wire-bonded LED die. A steel tube allows us to fix the ILED during surgery. Two sockets allow for connection to an Implantable Stimulator-Transponder (IST).

The Clear-Epoxy Implantable Light-Emitting Diode (CE-LED) is designed for non-intrusive illumination of body tissue. We attach it to the surface of the organ we wish to illuminate. A CE-LED delivers 100% of the light emitted by the LED to the body tissue, while a fiber-coupled ILED loses 60% or more of the light we want to illuminate brain tissue with a CE-LED, we attach the CE-LED to the surface of the skull and illuminate the brain through the bone. Dispersion and absorption of light in bone tissue causes a dramatic reduction in light intensity on the way to the brain. But the skull of a mouse is only 300 μm thick. A CE-LED emitting 10 mW should be able to invoke optogenetic response through the skull of a mouse, but the skull of a rat is 1000 μm thick. A CE-LED should be able to invoke circling and seizures in a rat using 10-ms, 30-mW pulses of light flashing at 10 Hz, but we doubt that it is possible to invoke a spreading depolarization with a 30-s, 30-mW pulse of light using a CE-LED through a rat's skull. If we want to invoke a spreading depolarization with an ILED, we must use a light guide to carry the light through the skull.

Figure: A3036IL Tapered-Fiber LED (TF-LED) For Depth Illumination in Mice. Fiber diameter 270 μm, length 4±0.3 mm. The tip emits 2.8 mW at 10 mA forward current.

Our Tapered-Fiber Light-Emitting Diode (TF-LEDs) use a pointed optical fiber to carry light through the skull and into the brain beneath. Of the light emitted by the LED, roughly one third is captured by the base of the optical fiber. The other two thirds escapes at the base of the fiber and is absorbed by the cement that fastens the TF-LED to the skull. The fraction of light lost at the base of the fiber depends upon the diameter of the fiber. For 450-μm diameter fibers the loss is around 62% and for 270-μm fiber it is around 68%. The sharp end of the TF-LED minimizes damage to the brain as the fiber penetrates the brain to the target region. When illuminated, the tapered tip emits light in all directions, but mostly in the forward direction, so that the optimum placement of the TF-LED is with the tip of the fiber at our target location in the brain. By this means, the target region is illumiunated with the maximum power and minimum trauma.

Figure: A3036 Blunt-Fiber LED. Left: Fiber, post, and Sockets. Right: Light radiated from tip, base, and surface imperfections. Made from 450-μm diameter optical fiber with numerical aperture 0.86.

Our A3036IL Blunt-Fiber LEDs (BF-LEDs) use a round-tipped optical fiber to carry light through the skull to the surface of the brain. Of the light emitted by the LED, roughly two thirds is lost at the base of the optical fiber and one third reaches the blunt tip. The BF-LED is designed to illuminate the surface of the brain without penetrating the dura. The blunt tip emits light mostly in the downward direction, and so enters the brain. In the case of a rat, the loss of light due to coupling into the fiber is far less than the loss we would suffer if we tried to use an ILED on the top side of the skull to shine light through the skull to the brain. In particular, the dispersion of light through the skull would spread the power to such an extent that the intensity of the light would no longer be capable of invoking of a spreading depolarization. A 30-s, 10-mW pulse of light delivered by a BF-LED to the surface of a rat's brain will invoke a spreading depolarization without fail.

Figure: Fiber Light Guides for ILEDs. Top: Blunt tip on 450-μm diameter optical fiber. Bottom: Tapered tip on 270-μm optical fiber.

The optical fibers we use with our A3036IL Implantable Light-Emitting Diodes are made of high index glass to capture the maximum possible light from the LED die. We polish a fiber blank and create a tapered tip with a machine that heats and stretches the blank. To create a blunt end, we reduce and heat a tapered tip until it assumes a rounded shape at the end. We glue the polished base of the fiber to the surface of an LED die. The fiber carries light to the tip. The fraction of optical power emitted by the LED that is delivered to the fiber tip is what we call the ILED's coupling efficiency. Our 450-μm fibers have a typical coupling efficiency of 38%. For our 270-μm fibers, the typical coupling efficiency is 32%. We make all our fiber-coupled ILEDs using LED die chips wire-bonded to a printed circuit board. We have not yet found a red LED of adequate efficiency to produce a useful fiber-coupled ILED, so we are for now restricted to blue and green wavelengths for our fiber-coupled ILEDs.

Figure: Implantable LED (ILED) Back Side. Near socket: L+ lead. Far socket: L− lead.

The A3036IL Clear-Epoxy Implantable Light-Emitting Diode (CE-LED) has the LED die covered with clear epoxy. We can place it on the surface of an organ or the skull. We can also drill a hole in the skull and place the CE-LED over the hole, as shown in the sketch above. We do not use a mold or a die to cast the clear epoxy on the CE-LED. Instead, we coat the LED die with epoxy and spread the epoxy around under a microscope so that the die is covered by no more than half a millimeter of epoxy. The resulting epoxy encapsulation is flat to ±0.2 mm, but usually has a slight dome shape. We make red, blue, and green CE-LEDs.

Versions

[02-SEP-26] The following versions of the Implantable Light-Emitting Diode (A3036IL, ILED) are in production. The gold-plated pins on the end of the stimulator's leads mate with a pair of sockets on the A3036IL. We use attached and wire-bonded EZ500 chips for our blue and green CE-LEDs, TF-LEDs, and BF-LEDs. We use the deep-red Luxeon Z series LXZ1-PA01 for our red CE-LEDs. We do not make red fiber-coupled LEDs. Our ILED part numbers begin with the "A3036IL", where "3036" is the OSI assembly number and "IL" is for "Implantable Lamp". This is followed by a dash and a letter. If the letter is solitary, the ILED is a CE-LED, which is an LED in clear epoxy with a mounting post and two sockets. The letter tells us which LED the ILED contains: "A" for blue EZ500, "B" for green EZ5600, and "E" for red Luxeon Z. The number after the letter indicates a fiber, and gives the fiber diameter in microns. Another dash is followed by the length of the fiber in millimeters. The length is followed by a letter "T" for a tapered fiber and "B" for a blunt fiber.

We have high-index fiber in stock of all diameters 270 μm to 450 μm. We can make blunt or tapered-tip fibers of any diameter in this range. The shortest fiber we can make easily is 4 mm. For an additional charge, we can reduce the length to 3 mm.

Figure: Three A3036IL-A270-6T TF-LEDs, Two A3036IL-A CE-LEDs, and One A3036IL-A270-3T TF-LED.

The EZ500 data sheet does not provide spectra for light emission. But the Luxeon Z data sheet from Philips Lumileds provide spectra. If we assume the EZ500 blue and green spectra are similar, the half-power spectral width of the EZ500 460-nm blue LED is 20 nm, and of the 525-nm green is 50 nm.

Optical Power

[02-SEP-26] The following table compares the optical power output of at 10 mA forward current for various combinations of fibers, domes, and LEDs. For all these devices, optical power is approximately linear with current. Notice that the EZ290 fiber-coupled LEDs are not as efficient as the EZ500, even though the surface area of the EZ290 is smaller. The central bond wire of the EZ290 interferes with the contact between the fiber and the emitting surface, forcing us to keep the base of the fiber farther from the die, and shadowing a portion of the die as well.

The optical power emitted by our LEDs is approximately proportional to current, as shown in the plot below. Compared to the power emitted at 10 mA drive current, we get roughly twice as much power for 20 mA drive and half as much power at 5 mA drive. The LED we use for fiber-coupled ILEDs is the EZ500. The EZ500 comes in blue 460-nm and green 527-nm, but no other wavelengths. Its light-emitting surface is a 480 μm square with a bond wire to one side. We can lower the base of an optical fiber right onto the emitting surface without disturbing the bond wire, which increases increases our coupling efficiency. We use the same EZ500 LEDs for our blue and green Clear-Epoxy ILEDs (CE-LEDs), and we also make a red CE-LED out of the 650-nm red LXZ1-PA01 LED. The LXZ1-PA01 provides a 1-mm square, silicone-coated emitting surface that is not suitable for fiber-coupling, but serves very well for an epoxy dome cover. The LXZ1-PA01 is an efficient source of deep-red light that penetrates far into living tissue.

Figure: A3036 Clear-Epoxy Light-Emitting Diode (CE-LED) Optical Power Output versus Current. Blue: 460-nm blue EZ500. Green: 527-nm green EZ500. Red: 650-nm red LXZ1-PA01.

A blue A3036 Clear-Epoxy LED (CE-LED) produces roughly 1 mW/mA of 460-nm light, radiating in an approximate hemisphere, with a slight bias in the direction perpendicular to the CE-LED circuit board. The green CE-LED produces roughly 0.5 mW/mA. A red CE-LED produces roughly 0.8 mW/mA. When we couple the light emitted by the LED into an optical fiber, we lose some light because the fiber does not cover the entire emitting surface of the LED die, and we lose some more light because the fiber is unable to constrain light that enters it at too great and angle to the fiber axis. The fraction of emitted light that is captured by the fiber and transported to its tip, be it a Blune-Fiber LED (BF-LED) or Tapered-Fiber LED (TF-LED) is what we call the ILED's coupling efficiency. The core of one of one of our 450-μm diameter fibers is itself 430 μm in diameter, while the cladding around the core is 10 μm thick. When perfectly polished at the base and perfectly positioned on a 480-μm square EZ500 LED, the core will, in theory, receive 63% of the light emitted by the LED. Our optical fibers are constructed out of high-index glass: the core has refractive index 1.72 while the cladding has index 1.49. The interface between the core and the cladding provides the core with a numerical aperture of 0.86. All light incident upon the core within ±60° of the fiber axis will be captured by the and transported to the tip of the fiber. For a typical light-emitting diode, this ±60° accounts for 75% of the light emitted. Combining reception efficiency and capture efficiency together, we obtain a maximum theoretical coupling efficiency between the LED die and the 450-μm fiber of 63% * 75% = 47%. In practice, we obtain coupling efficiency of roughly 38% for 450-μm diameter fibers.

Figure: Optical Power Ouput versus Current for Two A3036IL-A450-8T Tapered-Fiber LEDs. We are measuring power emitted by the fiber tip using our fiber-coupled LED calibration stand.

We measure the optical power emitted by bare LED dies and Clear-Epoxy LED (CE-LEDs) using an SD445 photodiode and an ammeter. Bare LED dies and CE-LEDs emit roughly the same power. The dome of the CE-LED focuses the LED light more in the direction perpendicular to the circuit board, making it easier to measure the total power output, but in both cases, by holding the die or CE-LED within 1 mm of the photodiode surface, we obtain a consistend measurement of output power from one die or CE-LED to the next. Measuring the total power radiated by TF-LEDs and BF-LEDs is much more difficult. The exact shape of the fiber tip affects the distribution of the emitted light over the spherical surface centered on the fiber tip. The measurement is further complicated by the light lost from the base of the fiber, which we do not want to include in our measurement. We measure the power emitted by our fiber tips using a calibration stand that is accurate to ±20%. For more information on the history and development of our calibration stands see our development notes.

Figure: Optical Power Output at Fiber Tip for 10 mA Forward Current, Various Fiber-Coupled LEDs.

When we consider how to drive sufficient current through an ILED to produce the light power we need to provoke an optogenetic response, we need to know the voltage drop across the ILED at the required power output. We can then look at the specification of the Implantable Stimulator-Transponder (IST) we intend to use to drive the LED and see what voltage and current it must provide, and what leads we must select to carry the LED drive current from the IST to the ILED. These leads will present resistance to the flow of the drive current, so we may find that we need leads made of thicker steel wire if we want to carry a higher current a longer distance from the IST implant to the ILED fixture.

Figure: Current and Power versus Voltage for an A3036IL-A450-8T Implantable LED. Blue C460EZ500 LED coupled to 450-μm, 8-mm fiber, power measured at the tapered tip of fiber.

Heating

[27-MAY-26] All but the smallest of our A3041 Implantable Stimulator-Transponders, when connected to an A3036 Implantable Light Emitting Diode (ILED), are capable of overheating the brain. For a discussion of heat generation by ILEDs and its rate of dissipation in the brain, see the Heating chapter of our Implantable Stimulator Overview.

Design

[02-SEP-26] The A3036IL Implantable Light-Emitting Diode (ILED) is an active product, but its partner device, the A3036 Implantable Stimulator-Transponder (IST), has been replaced by the A3041 IST. For design files and development logbook for both the ILEDs and IST, see the A3036 Design Page at D3036.