A3036 Implantable Light-Emitting Diode Manual

Contents

Description

Note: The A3036IL Implantable Light-Emitting Diode (ILED) is an active product, but its partner device of the same assembly number, the A3036 Implantable Stimulator-Transponder, has been replaced by the A3041 Implantable Stimulator-Transponder.

[18-MAR-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 (CE-ILED), Tapered-Fiber (TF-ILED), and Blunt-Fiber (BF-ILED). Each class is named after the element that delivers light to the target tissue.

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

All ILEDs are built on a circuit board. The LED is on the "bottom" side. The connection sockets and mounting post are on the "top" side. Clear epoxy surrounds the base of the mounting post and the LED. In a CE-ILED, the entire LED die and bond wire is covered in clear epoxy. In the TF-ILED and BF-ILED, 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 little more cement.

Figure: A3036IL Tapered-Fiber LED (TF-ILED) 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.

The A3036IL Clear-Epoxy Implantable Light-Emitting Diode (CE-ILED) is designed for non-intrusive illumination of body tissue. We attach it to the surface of the organ we wish to illuminate. If we want to illuminate brain tissue with a CE-ILED, we can attach the CE-ILED to the surface of the skull and hope to illuminate the brain through the thickness of the skull. But the dispersion of light in bone tissue causes a dramatic reduction in light intensity. We may be able to illuminate the brain through the 300-μm thickness of a mouse's skull, but we are going to need a lot of power to provoke optogenetic response through the 1-mm thickness of a rat's skull. In the case of a rat, we can make an aperture in the skull and set the CE-ILED within the aperture to illuminate the brain surface directly without penetrating the dura.

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

Our A3036IL Tapered-Fiber LEDs (TF-ILEDs) are designed to deliver light deeper into body with the highest possible intensity at the target location. The sharp tip of the TF-ILED serves two purposes: it minimizes damage to the tissue the fiber passes through to arrive at its target, and it provides light emission a hemisphere rather than in a downward cone. The Blunt-Fiber ILED (BF-ILED) is designed to carry light through a burr hole to the surface of the skull in cases where we do not want to penetrate the dura or the brain surface. The BF-ILED delivers a higher intensity of light to the brain surface than a CE-ILED, and does not require such a large aperture to accommodate the LED. Furthermore, the BF-ILED allows us to place cement below the ILED circuit board and all around the power sockets and pins with more ease, thus providing better insulation of the lamp power from the body, and minimizing lamp artifact in our recorded biopotentials.

Figure: Tapered Fiber. Diameter 300-μm. (Sam Orphanos)

The glass 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. The fibers are polished at the base and tapered at the tip. The taper allows the fiber to penetrate to the target area with the minimum of trauma to the surrounding tissue. The taper also has the advantage of emitting light in all directions, as opposed to only downwards. We glue the polished base to the surface of an LED and the fiber carries LED light to the tapered tip. We call the combination of fiber and LED a Tapered-Fiber ILEDs (TF-ILEDs). The fraction of optical power emitted by the LED that is delivered to the tapered tip of the fiber is the coupling efficiency of the fiber. Our TF-ILEDs have a typical coupling efficiency of around 38%.

Figure: Blunt Fiber. Made from a 450-μm diameter fiber.

We make A3036IL TF-ILEDs out of bare LED chips wire-bonded to a printed circuit board. We have blue and green TF-ILEDs, but we have not yet found a red LED of adequate efficiency that we can purchase in die form for contact coupling with the fiber base.

Figure: Clear-Epoxy LED (CE-ILED, A3036IL-C) with Epoxy Dome For Surface Illumination. Near socket: orange L+ lead. Far socket: purple L− lead. A steel tube allows us to fix the ILED during surgery. The printed circuit board is 4.0 mm × 3.0 mm, and 0.8 mm thick.

The A3036IL Clear-Epoxy Implantable Light-Emitting Diode (CE-ILED) 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-ILED 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-ILED. 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-ILEDs.

Versions

[11-MAR-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-ILEDs, TF-ILEDs, and BF-ILEDs. We use the deep-red Luxeon Z series LXZ1-PA01 for our red CE-ILEDs. 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-ILED, 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-ILEDs, Two A3036IL-A CE-ILEDs, and One A3036IL-A270-3T TF-ILED.

The EZ500 data sheet does not provide spectra for their light emission. But the Luxeon Z data sheet from Philips Lumileds does provide spectra, which we present below. We assume the EZ500 blue and green spectra are similar.

Figure: Spectra for Luxeon Z LEDs.

Based upon the above spectra, we estimate that 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

[11-MAR-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.

We calibrate our implantable LEDs by measuring their optical power output when the current through its LED is 10 mA. We measure the LED's power output with one of our calibration stands. We have one calibration stand for bare LEDs and another for fiber-coupled LEDs. The LED output power is approximately proportional to current, as shown in the plot below. We get roughly twice as much power at 20 mA and half as much at 5 mA.

Figure: Optical Power Output versus Current for Various of Bare LEDs. We are measuring output power with our Bare LED Calibration Stand (see below). An IST with battery voltage 3.7 V and lead resistance 56-Ω will deliver 14 mA to the LED with forward voltage drop 2.9 V.

Our Implantable LEDs (A3036IL) generate light with an LED mounted on a printed circuit board. Our favored LED is the EZ500, which comes in blue and green. The EZ500 light-emitting surface is 480 μm square, with a bond wire to one side, so that we can lower the base of an optical fiber right onto the emitting surface for greater coupling efficiency. For red light, we use the LXZ1-PA01 from the Luxeon Z family, which provides a 1-mm square emitting surface. The LXZ1-PA01 is an efficient source of 650-nm deep-red light for illuminating the surface of organ tissue. But its emitting surface is too large for efficient coupling of light into an optical fiber.

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.

Blue LEDs produce roughly 1 mW/mA (one milliwatt per milliamp forward current), green LEDs roughly 0.5 mW/mA, and the deep-red LED roughly 0.8 mW/mA. We make surface illuminators by covering the LED with a clear epoxy dome. The A3036IL-A is a blue C460EZ500 with clear epoxy dome for surface illumination of brain tissue. We make depth illuminators by gluing a glass fiber with a tapered tip to the LED and inserting the tapered tip to the required depth. Roughly one third of the light emitted by the LED is captured by the polished, flat base of the fiber and transported to its tapered tip, where it radiates in all directions. We make the tapered fibers by polishing one end of a high-index optical fiber, and tapering the other end with the help of our custom tapermaking machine.

Video: The Tapermaker Machine Creating a Tapered Light Guide. We taper a 300-μm diameter TD5 fiber. The base of the taper is 4 mm from the base of the fiber. The taper itself is 1 mm long. This is how we make the light guides for our fiber-coupled LEDs.

The A3036IL-A270-4T is a blue C460EZ500 with a 4-mm long, 270-μm diameter fiber light guide with a tapered tip. The A3036IL-A270-4T's average power output at the fiber tip is 2.8 mW for 10 mA forward current.

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

We measure optical power using an SD445 photodiode and an ammeter. We convert photocurrent into optical power using 0.18 mA/mW for 460-nm blue, 0.25 mA/mW for 527-nm green, and 0.40 mA/mW for 655-nm red light. We have two calibration stands that permit us to make consistent measurements of power for any combination of LED and fiber. The drawing below shows the arrangement of LED or TF-ILED and photodiode in each stand. For more information on the calibration and appearance of the test stands, see our development notes.

Figure: Bare LED Calibration Stand (Left) and Fiber-Coupled LED Calibration Stand (Right). On the left, 7.1% of the light emitted by the LED arrives at the detector. On the right, 39% of the light emitted by the fiber tip arrives at the detector.

When we cap an LED with epoxy, we make sure the surface is convex. We avoid loss by total internal reflection within the epoxy, and we direct the light more in the forward direction, which favors illumination of a surface directly in front of the ILED. But the epoxy dome is never uniform, so measuring the power emitted by the ILED after application of the epoxy dome is difficult. We assume the total power transmitted by the ILED is equal to the total power emitted by the LED die before we apply the dome.

We measure the power emitted by fiber-coupled LEDs directly with our calibration stand. The fraction of LED power reaching the fiber tip depends upon the area of the fiber and the LED, and upon the numerical aperture of the fiber itself. We make our light guides out of optical fiber with numerical aperture 0.86 (core index 1.72, cladding index 1.49). All light within ±60° of the fiber axis will be captured by the fiber and transported to the tip. For a typical LED, this ±60° accounts for 75% of the light emitted. A 450-μm diameter light guide, perfectly polished at the base, and perfectly positioned on a 480-μm square EZ500 LED will couple 51% of the emitted light to the fiber tip. The A3036IL-A450-8T uses a 450-μm fiber on a C460EZ500, and we obtain coupling efficiency of around 40%.

The Implantable Stimulator-Transponder (A3041B) boosts its battery voltage to a fixed 3.3 V. It also provides a current limit we can apply to the stimulus. When equipped with 45 mm long leads, each will have resistance 28 Ω, making a total of 56 Ω. Suppose we turn up the stimulus current to its maximum value of 10 mA. To determine the actual LED current we need to know the approximate forward voltage drop of the LED at 10 mA.

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.

The A3036IL-A450-8T TF-ILED has forward voltage 2.73 V at current 10 mA. The voltage drop across 56 Ω at 10 mA is 0.56 V. Thus 3.29 V is enough to drive 10 mA through the leads and LED. At this current, the tip of the fiber produces 5 mW. Thus the maximum optical power from the TF-ILED when driven by the A3041B is 5 mW.

Design

[09-JUN-25] For design files and development logbook, see the A3036 design and development page at D3036.