Light Emitting Diodes

From OpenOptogenetics.org

Jump to: navigation, search

Principle

Why choose LEDs?

As far as optogenetics applications are concerned, LEDs surpass lasers in almost every respect. They are cheaper, smaller, more reliable, and easier to control. They can be incorporated into implants, allowing untethered light delivery. But their major drawback, and the reason they have not been as widely adopted by the optogenetics community, is the difficulty of coupling their light into a fiber optic cable with high efficiency. Although individual LEDs can emit as much as 5 watts of light, the light is emitted in all directions, rather than in a coherent beam. The amount of light from an LED that eventually makes it to the fiber—whether it travels through coupling lenses or not— has traditionally been on the order of a few milliwatts for a 200 micron fiber (see charts below). For laser beams, coupling efficiency for the same fibers can easily reach 50%, or 50 mW for a 100 mW laser. However, a recent class of fiber-coupled LEDs offers much higher intensities of fiber-coupled light, e.g. >25mW blue light through a 200 micron fiber (Clements2012, white paper.)

Choosing between LEDs and lasers for optogenetic stimulation is not always straightforward. Manufacturers offer "fiber-coupled" LED light sources that are pre-coupled to optical fibers, but it is critical to note the light power that a system couples into an optical fiber of appropriate core diameter (200 microns and below for typical in vivo work.) For many of these commercial systems the achievable light through a 200 micron fiber is a fraction of what competitor systems or a laser might provide. Some systems do offer high fiber-coupled light intensities, along with the typical advantages of LEDs over lasers (cost, size, controllability, output stability, etc.)

A customized solution involving a pigtailed LED, a driver circuit, and a power source can be made from standard components costing a total of $20 or less. Increasing coupling efficiency, either through collimating lenses or tapered fibers, could result in LEDs becoming the light source of choice for optogenetics experiments in the near future.

Principle

The simplest way to pass light from an LED to a fiber optic cable is to align the tip of the fiber with the light-emitting portion of the LED. The fiber can simply be pressed against the LED and glued in place, or mounted a short distance from the LED surface.

You can pigtail your own LED by simply gluing a cleaved or polished fiber onto the LED using an "optical" (transparent) epoxy. You can add a second layer of opaque epoxy if you are concerned with light leaking from the LED/fiber assembly.

Custom solutions

LEDs

  • Philips LUXEON Rebel LEDs cost around $5 each and are some of the brightest on the market. They can be purchased from Future Lighting Solutions, SparkFun Electronics, and Luxeon Star, among others. They are surface-mount style, which means they work best when soldered to a printed circuit board, but they also can be purchased with a solder-free housing.

Epoxy glues

  • 90-87-6 from Epoxy Technologies (UV-cure optical epoxy).
  • EPOTEK 302-3M-black from Epoxy Technologies (opaque epoxy).

UV curing system

Driver circuit

LEDs require a constant current source to emit light consistently. For the LEDs that serve as indicators in consumer electronics, a resistor in series with a regulated power supply is usually sufficient. For the high-powered LEDs used for optogenetics, providing stable currents up to 1 A will require a dedicated driver circuit. Because LEDs are so common in commercial lighting applications, the range of available driver circuits is nearly boundless.

  • The 707 BuckToot LED module is a cost-effective solution for testing LEDs, but probably lacks the flexibility necessary for experiments.
  • The LuxDrive "BuckPuck" offers external modulation and a package that is simple to mount on a breadboard or printed circuit board.

Before you choose a driver for your LED, make sure you know how much current it expects! If your LED is properly matched with a driver circuit, you should be able to use it with the power source of your choice.

LEDs OEM

You can buy your LEDs directly from the original equipment manufacturers (OEM), but the minimum quantity for such orders is usually high. For smaller quantities, it is better to make your purchase from an electronics vendor, such as Mouser Electronics or DigiKey, or from a speciality LED retailer, such as Future Lighting Solutions or Luxeon Star.

Here are some manufacturers of high-power LEDs:

LEDs arrays

  • Bridgelux provides LED arrays. See this JOVE video that explains how to use an LED arrays for photostimulation of neurons in brain slices.

LED controllers

LED-based "patterned illuminators"

Mightex Systems

Polygon400 multiwavelength patterned illuminators from Mightex Systems.

Mightex's Polygon400 Patterned Illuminators provide programmable multiwavelength patterned illumination, with any user-defined spatial/temporal illumination patterns. They also enable simultaneous illumination of multiple regions of interest with a wide range of available LED wavelengths. Fully compatible with electrophysiology equipment, the Polygon400 can be operated with built-in LEDs, or with an external light sources (e.g. an arc lamp or a laser) through a lightguide or a fiber. Microscope adapters are available for easily mounting the Polygon400 onto a wide range of upright & inverted microscopes including Leica, Nikon, Olympus and Zeiss.

With a USB interface and user-friendly software, Polygon400 is extremely easy to install and to operate. Polygon400 is also supported by Nikon's NIS Elements software and MicroManager.

For a list of customer references, please see here.

LEDs pigtailed modules and integrated illumination systems

Doric Lenses

Mapping of a pigtailed LED from Doric Lenses containing a bright blue LED (Luxeon Rebel) coupled to a 0.48 NA 200 µm diameter multimode fiber. Black: measured power. Light blue: driving current. Each step lasts 4 s. The relation between emitted power and driving current is near to linear.

Doric Lenses builds custom pigtailed LEDs (any color and fiber type).

Mightex Systems

BioLED Optogenetics light delivery system from Mightex Systems.

Mightex's BioLED optogenetics light delivery systems are modularized turn-key solutions for optogenetics, capable of generating precisely-timed and high-intensity light pulses. A proprietary “IntelliPulsing” technology enables one to obtain significantly higher power in pulse mode (e.g. 16mW@470nm from a 200um fiber, or >500mW/mm^2) than in CW mode (e.g. 8mW@470nm from the same fiber). With a Mightex's fiber-coupled LED, multiple wavelengths (e.g. 470nm and 590nm etc) can be combined into one (1) fiber, without any moving parts in the optical path. This will allow fast excitation and fast inhibition of neurons/cells. In addition, the fiber is removable/interchangeable, and therefore one can choose different fibers to suit different applications.

Prizmatix

Prizmatix's "Optogenetic Toolbox": 1. Ultra High Power LED, 2. Beam Combiner, 3. Fiber Coupler Adaptor, 4. Fiber optics, 5. Rotary Joint, 6. Beam Switcher, 7. Filter Wheel, 8. Microscope Adaptors, 9 .Liquid Light Guide Adaptor, 10.Liquid Light, 11. Liquid Light Guide XYZ Collimator, 12. C-Mount Adaptor, 13. Fiber Optic Collimator, 14. Reference Photodiode, 15. Single/Dual Fibers, 16. Optogenetics Implantable Cannulae, 17. Fiber Optics, 18/19. Pulser - USB pulse train generator with user-friendly software.

Prizmatix produces LED systems for In-Vivo and In-Vitro applications: The In-Vitro Optogenetics Toolbox, oriented for work on tissue slice preparate, includes Ultra High Power LEDs, Beam Combiners, fiber couplers and other accessories enabling easy construction of multi-wavelength activation / inhibition systems.

The In-Vivo Optogenetics Toolbox, intended for work with freely moving mammals, includes powerful plug-&-go LED light sources (>10mW from 200um implant after rotary joint and all fiber connections or >300mw/mm^2), flexible (rugged and non-brittle) plastic Optogenetics fibers, extremely low friction Rotary Joints (suitable even for mice), custom Cannulae and a DIY implants kit.

LED options include fast switching single wavelength LEDs in Blue, Green, Red and violet, as well as Dual and Stimulation and Silencing in a single fiber. To design and generate pulse trains in an easy and visual manner, a Pulser is also available.

CAIRN Research

CAIRN Research, in particular their [2] illumination system -The OptoFlash can output >30mW/mm2 from the end of a 200um fibre.

While achieving the 30mW / mm2 from a 200um fibre the Cairn OptoFlash also offers interchangeability & combinations of LED heads.

Scientifica

Scientifica sells LED-based iillumination systems from the following manufacturers:

Neuralynx

The SpectraLynx box.

Neuralynx proposes an integrated LED illumination system with 2 or 4 colors, called SectraLynx.

WT&T Inc.

WT&T Inc. produces fiber coupled LEDs, sLED, turn-key fiber-coupled light sources and optical fibers with integrated micro-lenses, tapers.

Plexon Inc.

Plexon provides super-intensity fiber-coupled LEDs at low cost. For example, Plexon blue LEDs couple >25mW blue 465nm light into a 200um optical fiber. Optogenetics white paper

Diagram showing Plexon's miniaturized fiber-coupled LEDs, mounted on electrical commutators. This figure was taken from a white paper overview of Plexon's optogenetics products.


Other suppliers

LED microarrays

Dense arrays of miniature LED light sources called microLEDs hold promises for patterned illumination of neural tissue. A company called mLED Ltd has spun out of the University of Strathclyde in Glasgow to manufacture microLED arrays.

Multi-site optical stimulation based on a micro-LEDs array

Neural cells expressing ChR2 (green) are covered by a 64 × 64 matrix of bright small light spots (blue dots), with individual control of their intensity and timing. Courtesy: N. Grossman, [1].
Illustration of the micro-LED structure showing the different micro-layers. n: n-type GaN; p: p-type GaN; MQW: multiple quantum wells; SiO2: silicon dioxide; en: n-contact; ep: p-contact. Courtesy: N. Grossman, [1].

A multi-site optical stimulation system based on a micro-LEDs array (MLA) was shown by Grossman et al. [1]. The MLA can generate an arbitrary two-dimensional excitation patterns with micrometre and sub-millisecond resolutions and sufficient irradiance to generate ChR2-evoked spiking in neurons.

The MLA shown in [1] was a 4x4mm2 chip consisting of 64x64 matrix of 20 µm diameter micro-LEDs with 50 µm centre-to-centre spacing. The MLA was fabricated by processing Gallium Nitride semiconductor wafers into micro-structured arrays of emitters. Each micro-LED is individually controlled and can be switched on/off in millisecond and can produce output emission power of 70 µW, corresponding to on-emitter irradiance of 250 mW/mm2. The LEDs are driven in a matrix fashion in which a single emitter is controlled by which a single emitter is controlled by applying a positive bias voltage across its row electrode and sinking a constant current from its column electrode.

The LEDs are images on the neural samples using lens configuration. Grossman et al. described two optical configurations for neural network and single cells experiments. The first configuration images the micro-LEDs with their original size (1:1 imaging) and was designed for stimulation of neural networks. The design consisted of two S5LPJ2851 (Sill Optics GmbH) AR (anti-reflective) coated 50 mm NA=0.14 triplet lenses, i.e. consists of three single lenses, arranged in a 4f relay configuration. It had a large focal length which allowed long (40 mm) working distance that was sufficient to accommodate for electrophysiology recording apparatus such as patch clamp or MEA. The 1:1 images resulting in a large 3x3 mm2 illumination field, in which a single light spot has 30 µm FWHM and up to 3 mW/mm2 irradiance. The second optical configuration de-magnifies the micro-LEDs by approximately a factor of ten (10:1 imaging) and was designed for detailed examination of a single neuron or a small scale neural network. The design consisted of one S5LPJ2851 (Sill Optics GmbH) 50 mm triplet lens and one LUMPlanFI 40X water immersion objective (Olympus) with 4.5 mm focal length and NA=0.8. The single cell setup de-magnifies the micro-LEDs 10:1, resulting in a small 0.3x0.3 mm2 field, in which a single light spot has 3 µm FWHM and up to 300 mW/mm2 irradiance.

In comparison to other light patterning techniques based on spatial light modulators, using digital micro-mirror devices (DMDs) [2] or nematic liquid crystal spatial light modulators (LC-SLMs) [3], the MLA has the advantage that individual light spots can operate independently with almost nanosecond resolution and it is a compact solution that is simple to operate. However, the MLA has a smaller fill factor (~16%). The fill factor of the illumination is being improved by integrating arrays of micro-lenses on top of the micro-LEDs to collimate the emitted light.

[a potentially interesting route to follow]

Publications

  • Clements et al. describes high intensity fiber-coupled LEDs, miniaturized for commutator mounting.
  • White paper from Plexon describing intensities of LED-based fiber-coupled light of different wavelengths currently achievable through 110um and 200um fibers.
  • Huber et al. mounted blue LEDs above the surface of the cortex to stimulate ChR2-positive cells in behaving mice.
  • Grossman et al. [1] designed a 2D micro-LED array (15 µm diameter LEDs with a 20 µm spacing) for stimulating cultures neurons or slices with any sort of spatiotemporal pattern (see LED microarrays).
  • White Paper from Prizmatix comparing lasers and LED light sources for Optogenetics applications [3]
  • Optogenetic stimulation guide from Plexon detailing industry leading LED-sourced fiber-coupled light intensities acheivable through optical fibers of 100um and 200um diameter. [4]

References

Error fetching PMID 20075504:
Error fetching PMID 18003478:
Error fetching PMID 19160517:
  1. Error fetching PMID 20075504: [Grossman2010]
  2. Error fetching PMID 18003478: [Farah2007]
  3. Error fetching PMID 19160517: [Lutz2008]
All Medline abstracts: PubMed HubMed