Scientists at the University of St Andrews have discovered that OLED displays can activate live cells that are genetically programmed to respond to light. By harnessing the ability of OLEDs to "turn on" individual cells in the lab paves the way for cell-specific optogenetic control in cultured neuronal networks, brain slices, and other biomedical research applications.
Working with Dr Gareth Miles from the School of Psychology and Neuroscience, Professor Gather and his team used OLEDs to manipulate individual, live cells from a human embryonic kidney cell line that were tweaked to produce a light-sensitive protein. Upon exposure to blue OLED light from pixels directly underneath the cell, the electric activity of individual targeted cells was stimulated, while neighbouring cells remained in the dark and stayed inactive.
Professor Gather and co-workers at the University of St Andrews used a miniature version of the OLED displays found in modern smartphones to illuminate individual live cells. For cells that are genetically programmed to embed a light-gated ion-channel in their membrane, this facilitates lens-free optical switching of the membrane current with single-cell resolution.
The team used a standard cell line to test their approach, but the goal is to apply the technology to activate individual neurons or groups of neurons, which would facilitate new ways of studying neuronal network function in the lab and ultimately advance knowledge of the neuronal dysfunction that underlies devastating neurodegenerative conditions such as Alzheimer's Disease, Parkinson's Disease and Motor Neurone Disease.
Figure: University of St Andrews - Illustration of optogenetic cell stimulation with OLED microarrays
- A: Schematic of OLED microarray with cells adhered on top of the array (not drawn to scale). The microarray is connected to a high-definition multimedia interface (HDMI) driver with a flexible connector. Each pixel of the array can be turned on and off by the driver and the CMOS backplane, thus providing controlled light exposure of individual cells. Light-induced changes in cell membrane current are measured with a patch clamp electrode (voltage clamp mode, whole-cell configuration). The cross section on the right shows the layer structure of the OLED array.
- B: Optical power density at the surface of the OLED microarray versus applied cathode voltage.
- C: Picture of the experimental setup with an upright microscope equipped with water immersion objective used to direct positioning of the patch electrode housed in a glass pipette. The microarray and the ground electrode are placed in a petri dish filled with salt solution. The flexible connector links the array to the driver located outside the field of view. A flow system constantly renews the salt solution.
The results suggest that OLEDs are an ideal platform technology for investigating and controlling biological processes with single cell resolution.
Professor Malte Gather from the School of Physics and Astronomy, said, "Using a miniature version of the OLED displays used in modern smartphones, we can now expose individual cells to light and thereby activate them in a controlled fashion." Malte added, "We hope to apply OLED technology to study processes in networks of neurons in a new way. Depending on which picture we show to the network, we expect to see different responses and thus better understand how the cells are linked within the network. A principal advantage of the lens-free OLED approach is the capability to perform multiple experiments in parallel and without damage to the cells."
Optogenetics is an emerging technology based on introducing transgenes that code for light-sensitive proteins into cells, and then using light to control cellular behavior. These light-sensitive proteins typically contain or host a chromophore that changes conformation when absorbing light of a certain wavelength. Depending on the function of the light-activated protein, this enables manipulation of cell migration, metabolism, or electrical activity. Optogenetics has been particularly successful in neuroscience, where light-activated ion-channel proteins are now widely used to control the behavior of neuronal cells.
In addition to appropriate gene constructs, precise optogenetic control of cells requires light sources that are spectrally matched to the activation spectrum of the protein of interest and that provide appropriate temporal and spatial control. So far, most optogenetic experiments have used standard arc lamps, lasers, or light-emitting diodes (LEDs). For cells in culture, light is typically delivered through a microscope, whereas in vivo experiments use optical fibers to deliver light to the target cells. For multiple-site stimulation, multiple-point emitting optical fibers have been used. Other developments are based on optrodes (hybrids of light sources and electrophysiological recording electrodes) and arrays of μLEDs. Although these developments are very promising, the spatial resolution of the currently pursued approaches remains limited to dimensions larger than the size of typical cells, and it is unclear whether true cellular or subcellular resolution can be achieved with existing technology. In addition, so far, the number of illumination spots that can be controlled independently is limited.
A potential alternative light source is the organic LED (OLED), a novel type of LED based on π-conjugated, “plastic-type” organic materials. Although most current research on OLEDs is aimed at information displays and solid-state illumination, OLEDs offer a range of highly attractive characteristics for applications in biotechnology and biomedicine. These include simple spectral tuning, mechanical flexibility and low weight, sub-microsecond switching, high brightness, low heating, homogeneous emission, low toxicity, and—most importantly in the context of optogenetics—the potential to provide extremely high spatial resolution. However, bringing OLEDs into contact with an aqueous biological environment requires highly efficient device encapsulation because exposure of the organic materials to as little as a few milligrams of water per square meter of device area leads to catastrophic device failure. Device encapsulation is typically achieved by laminating the OLED between sheets of glass (typical thickness, >100 μm). In this fully enclosed format, OLEDs have been used successfully as light sources for biomedical and sensing applications. However, because of divergence of light, even a micrometer-sized OLED would illuminate a millimeter-scale area at the surface of the encapsulation glass. To date, this issue has prevented researchers from harnessing the high-resolution advantage of OLEDs for lens-free delivery of light to individual cells.
Here, we demonstrate that arrays of microscopic OLEDs can be used as an optogenetic platform to specifically activate the light-sensitive ion channels of individual cells in real time. Using high-performance thin-film encapsulation, we succeeded in culturing eukaryotic cells within less than 2 μm from these arrays without loss of cell viability or damage to the OLED array. The OLEDs allow for lens-free illumination of the individual cells with much higher spatial resolution than existing methods. The light intensity provided by the OLED arrays is sufficient to specifically stimulate electrical activity in light-sensitive, genetically modified human embryonic kidney (HEK)–293 cells, and the achievable shifts in membrane potential are compatible with the levels required to induce action potential firing in neuronal cells.
Source: Introduction section of Arrays of microscopic organic LEDs for high-resolution optogenetics
Anja Steude | Emily C. Witts | Gareth B. Miles | Malte C. Gather
Science Advances 06 May 2016: | Vol. 2, no. 5, e1600061 | DOI: 10.1126/sciadv.1600061
Optogenetics is a paradigm-changing new method to study and manipulate the behavior of cells with light. Following major advances of the used genetic constructs over the last decade, the light sources required for optogenetic control are now receiving increased attention. We report a novel optogenetic illumination platform based on high-density arrays of microscopic organic light-emitting diodes (OLEDs). Because of the small dimensions of each array element (6 × 9 μm2) and the use of ultrathin device encapsulation, these arrays enable illumination of cells with unprecedented spatiotemporal resolution. We show that adherent eukaryotic cells readily proliferate on these arrays, and we demonstrate specific light-induced control of the ionic current across the membrane of individual live cells expressing different optogenetic constructs. Our work paves the way for the use of OLEDs for cell-specific optogenetic control in cultured neuronal networks and for acute brain slices, or as implants in vivo.