Archive for February, 2013

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Viviana Gradinaru, an assistant professor of biology at Caltech, discovered her passion for neuroscience as an undergraduate at Caltech, her alma mater. Viviana did her Ph.D. work with Karl Deisseroth at Stanford University where she played an instrumental role in the early development and applications of optogenetics, a research area concerned with the perturbation of neuronal activity via light-controlled ion channels and pumps. More information on her own lab at Caltech can be found at glab.caltech.edu. Viviana is also interested in entrepreneurship for better human health and has co-founded a company, Circuit Therapeutics, based on optogenetics.

In the spirit of ideas worth spreading, TEDx is a program of local, self-organized events that bring people together to share a TED-like experience. At a TEDx event, TEDTalks video and live speakers combine to spark deep discussion and connection in a small group. These local, self-organized events are branded TEDx, where x = independently organized TED event. The TED Conference provides general guidance for the TEDx program, but individual TEDx events are self-organized.* (*Subject to certain rules and regulations)

On January 18, 2013, Caltech hosted TEDxCaltech: The Brain, a forward-looking celebration of humankind’s quest to understand the brain, by exploring the past, present and future of neuroscience. Visit TEDxCaltech.com for more details.

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This video shows the launch and swimming of a tissue-engineered jellyfish, or “Medusoid,” compared to real jellyfish, and the intermediate design steps. The construct is made from silicone rubber and powered by lab-grown heart tissue. Contraction of the Medusoid, at a frequency of 1-2Hz, can be triggered by external electrical field stimulation. The Medusoid was built in a proof-of-concept study at Caltech and Harvard for designing muscular pumps for biomedical application

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Composite image showing a neurone cell on a SGFET
Figure: Linking neurons with Graphene

Graphene-based transistors that respond to changes in chemical solutions could be used to link electronic devices directly to the human nervous system. That is the claim of researchers in Germany who have built arrays of devices that respond to changes in the electrolytes surrounding living cells. The team hopes that its research could result in retinal implants that could help some visually impaired people see images.

The research focuses on the small voltage that a neuron creates across its cell membrane when it fires, with the potential difference arising from sodium ions moving into the cell and potassium ions moving out into the surrounding solution. Since the 1970s, biophysicists have been trying to detect this sudden change in the electrolytic properties of the liquid surrounding a cell using a type of field-effect transistor (FET). These devices are called Solution-Gated FETs (SGFETs) and much of the initial research was done using silicon. But after graphene was isolated in 2004, some researchers realized that this material – a layer of carbon just one atom thick – could be used to create better SGFETs.

According to Jose Garrido of the Technische Universität München, who has led the work, graphene offers several important advantages over silicon. First, the graphene surface remains clean – unlike silicon, which quickly forms a performance-degrading oxide layer when exposed to the electrolyte. Second, electrons in graphene have an extremely high mobility, which makes the device much more sensitive than silicon SGFETs. Finally, graphene is extremely flexible, which is good because any device implanted within the brain or similar tissue must be bendable.

A SGFET is a different take on a conventional graphene FET, in which the current flowing through its graphene channel can be controlled by changing the voltage applied to a nearby “gate” electrode. In a SGFET, in contrast, the gate voltage is kept constant and the graphene is exposed to the electrolytic environment of the cell. Any shift in the concentration of ions in the solution affects the electronic properties of the graphene  thereby changing its conductivity and the current flowing in the graphene channel. The firing of a neuron is therefore detected as an electronic signal.

In their new work, Garrido and colleagues have created 8 × 8 arrays of SGFETs – with each individual transistor measuring about 10 μm across. These arrays were used to detect firing signals from neuron cells that were cultured on an artificial medium. The researchers have also shown that neuron cells are able to survive for long periods of time in close proximity to graphene layers. They now want to show that the SGFETs work in living tissue – rather than cell cultures – and that neuronal tissue is not adversely affected by the presence of the devices.

According to Garrido, an important application of the graphene SGFETs would be creating retinal implants that could improve the sight of visually impaired people. Indeed, he believes that an array containing about 1000 elements could provide the brain with enough information for a person to be able to perceive an image. Another important application could be as cortical implants to help people control artificial limbs.

Although creating a 1000-element array of graphene SGFETs is a straightforward process, Garrido says that integrating the technology within a person will require a great deal more work.




Extracted from “http://physicsworld.com/cws/article/news/2013/feb/20/graphene-transistors-give-bioelectronics-a-boost”

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Graphene can function much like a laser when excited with very short femtosecond (fs) light pulses. The team of scientists has shown that the material has two technologically important properties – population inversion of electrons and optical gain. The findings suggest that graphene could be used to make a variety of optoelectronics devices, including broadband optical amplifiers, high-speed modulators, and absorbers for telecommunications and ultra-fast lasers.

Graphene is a sheet of carbon atoms arranged in a honeycomb-like lattice just one atom thick. Graphene has unique electronic and mechanical properties. It could find use in number of technological applications – even replacing silicon as the electronics industry’s material of choice in the future. This is because of the fact that electrons whizz through graphene at extremely high speeds, behaving like “Dirac” particles with no rest mass.

The material could also be an ideal candidate for photonics applications especially optical communications, where speed is all-important. For example, it has an ideal “internal quantum efficiency” because almost every photon absorbed by graphene generates an electron–hole pair that could, in principle, be converted into electric current. Thanks to its Dirac electrons, it can also absorb light of any colour and responds extremely fast to light, which suggests that it could be used to create devices much faster than any employed in optical telecommunications today.

Researchers have already shown that they can make basic devices, such as solar cells, light emitters, touch screens and photodetectors from graphene. However, few studies have looked at what happens when the material is excited with femtosecond (fs) light pulses that create so-called non-equilibrium charge states – particularly the state consisting of extremely dense Dirac electrons. Materials that harbour such states have nonlinear optical properties that are important for making real-world optical devices, such as ultrafast modulators, amplifiers and wavelength converters.

Epitaxially grown Graphene Monolayer with Pump Laser Pulses (t = 35 fs) and photon energy of around 1.55 eV has been demonstrated by researchers. The reflection of light from the samples have been measured because Graphene is just one atom thick and has zero energy electronic bandgap. This measurement provides information on the amount of light absorbed by the material. This in turn depends on the optical conductivity of graphene.

Researchers found that the optical conductivity changes from being positive to negative as the intensity of the pump pulses increases. This means that more light is coming out of the material than going in, something that indicates optical gain.

Researchers also demonstrated that the intense external pump laser pulses excite electrons in graphene so that more of these charge carriers exist in the upper “Dirac cone” – the conduction band of the material – than in the lower cone. Once such a population inversion has occurred, a probe photon then stimulates these excited states to emit infrared light in a coherent cascade. The coherent light emitted shows gain on the order of about 1%, a value that is much greater than those seen in conventional semiconductor optical amplifiers – a surprising result since graphene is merely one-atom thick.

Researchers have also found that this optical gain could be observed over a wide range of energies – up to hundreds of milli-electronvolts (meVs) below the pump photon energy. Such a broad optical gain might be unique to graphene and related to the fact that photoexcited electrons in the material scatter extremely fast among themselves. What is more, an ultrashort pulse just 35 fs long is sufficient to produce this broadband gain – something that has never been seen before in any material.

The population inversion and resulting optical gain in the infrared part of the electromagnetic spectrum confirms graphene’s potential for applications such as broadband optical amplifiers, lasers and in telecommunications. However, there is still much to do before this happens, says Wang, who is now looking at further characterizing the photoexcited graphene states in the near-infrared to the mid- and far-infrared spectral regions.




Extracted and Edited from “http://physicsworld.com/cws/article/news/2012/apr/25/graphene-emits-infrared-light”

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Micro-Electro-Mechanical Systems, or MEMS, is a technology that in its most general form can be defined as miniaturized mechanical and electro-mechanical elements (i.e., devices and structures) that are made using the techniques of microfabrication. The critical physical dimensions of MEMS devices can vary from well below one micron on the lower end of the dimensional spectrum, all the way to several millimeters. Likewise, the types of MEMS devices can vary from relatively simple structures having no moving elements, to extremely complex electromechanical systems with multiple moving elements under the control of integrated microelectronics. The one main criterion of MEMS is that there are at least some elements having some sort of mechanical functionality whether or not these elements can move. The term used to define MEMS varies in different parts of the world. In the United States they are predominantly called MEMS, while in some other parts of the world they are called “Microsystems Technology” or “micromachined devices”.

While the functional elements of MEMS are miniaturized structures, sensors, actuators, and microelectronics, the most notable (and perhaps most interesting) elements are the microsensors and microactuators. Microsensors and microactuators are appropriately categorized as “transducers”, which are defined as devices that convert energy from one form to another. In the case of microsensors, the device typically converts a measured mechanical signal into an electrical signal.

Over the past several decades MEMS researchers and developers have demonstrated an extremely large number of microsensors for almost every possible sensing modality including temperature, pressure, inertial forces, chemical species, magnetic fields, radiation, etc. Remarkably, many of these micromachined sensors have demonstrated performances exceeding those of their macroscale counterparts. That is, the micromachined version of, for example, a pressure transducer, usually outperforms a pressure sensor made using the most precise macroscale level machining techniques. Not only is the performance of MEMS devices exceptional, but their method of production leverages the same batch fabrication techniques used in the integrated circuit industry – which can translate into low per-device production costs, as well as many other benefits. Consequently, it is possible to not only achieve stellar device performance, but to do so at a relatively low cost level. Not surprisingly, silicon based discrete microsensors were quickly commercially exploited and the markets for these devices continue to grow at a rapid rate.

More recently, the MEMS research and development community has demonstrated a number of microactuators including: microvalves for control of gas and liquid flows; optical switches and mirrors to redirect or modulate light beams; independently controlled micromirror arrays for displays, microresonators for a number of different applications, micropumps to develop positive fluid pressures, microflaps to modulate airstreams on airfoils, as well as many others. Surprisingly, even though these microactuators are extremely small, they frequently can cause effects at the macroscale level; that is, these tiny actuators can perform mechanical feats far larger than their size would imply. For example, researchers have placed small microactuators on the leading edge of airfoils of an aircraft and have been able to steer the aircraft using only these microminiaturized devices.

A surface micromachined electro-statically-actuated micromotor fabricated by the MNX. This device is an example of a MEMS-based microactuator.

The real potential of MEMS starts to become fulfilled when these miniaturized sensors, actuators, and structures can all be merged onto a common silicon substrate along with integrated circuits (i.e., microelectronics). While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components are fabricated using compatible “micromachining” processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices. It is even more interesting if MEMS can be merged not only with microelectronics, but with other technologies such as photonics, nanotechnology, etc. This is sometimes called “heterogeneous integration.” Clearly, these technologies are filled with numerous commercial market opportunities.

While more complex levels of integration are the future trend of MEMS technology, the present state-of-the-art is more modest and usually involves a single discrete microsensor, a single discrete microactuator, a single microsensor integrated with electronics, a multiplicity of essentially identical microsensors integrated with electronics, a single microactuator integrated with electronics, or a multiplicity of essentially identical microactuators integrated with electronics. Nevertheless, as MEMS fabrication methods advance, the promise is an enormous design freedom wherein any type of microsensor and any type of microactuator can be merged with microelectronics as well as photonics, nanotechnology, etc., onto a single substrate.

A surface micromachined resonator fabricated by the MNX. This device can be used as both a microsensor as well as a microactuator.

This vision of MEMS whereby microsensors, microactuators and microelectronics and other technologies, can be integrated onto a single microchip is expected to be one of the most important technological breakthroughs of the future. This will enable the development of smart products by augmenting the computational ability of microelectronics with the perception and control capabilities of microsensors and microactuators. Microelectronic integrated circuits can be thought of as the “brains” of a system and MEMS augments this decision-making capability with “eyes” and “arms”, to allow microsystems to sense and control the environment. Sensors gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena. The electronics then process the information derived from the sensors and through some decision making capability direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby controlling the environment for some desired outcome or purpose. Furthermore, because MEMS devices are manufactured using batch fabrication techniques, similar to ICs, unprecedented levels of functionality, reliability, and sophistication can be placed on a small silicon chip at a relatively low cost. MEMS technology is extremely diverse and fertile, both in its expected application areas, as well as in how the devices are designed and manufactured. Already, MEMS is revolutionizing many product categories by enabling complete systems-on-a-chip to be realized.

Nanotechnology is the ability to manipulate matter at the atomic or molecular level to make something useful at the nano-dimensional scale. Basically, there are two approaches in implementation: the top-down and the bottom-up. In the top-down approach, devices and structures are made using many of the same techniques as used in MEMS except they are made smaller in size, usually by employing more advanced photolithography and etching methods. The bottom-up approach typically involves deposition, growing, or self-assembly technologies. The advantages of nano-dimensional devices over MEMS involve benefits mostly derived from the scaling laws, which can also present some challenges as well.

An array of sub-micron posts made using top-down nanotechnology fabrication methods.

Some experts believe that nanotechnology promises to:

a). allow us to put essentially every atom or molecule in the place and position desired – that is, exact positional control for assembly,

b). allow us to make almost any structure or material consistent with the laws of physics that can be specified at the atomic or molecular level;

c). allow us to have manufacturing costs not greatly exceeding the cost of the required raw materials and energy used in fabrication (i.e., massive parallelism).

A colorized image of a scanning-tunneling microscope image of a surface, which is a common imaging technique used in nanotechnology.

Although MEMS and Nanotechnology are sometimes cited as separate and distinct technologies, in reality the distinction between the two is not so clear-cut. In fact, these two technologies are highly dependent on one another. The well-known scanning tunneling-tip microscope (STM) which is used to detect individual atoms and molecules on the nanometer scale is a MEMS device. Similarly the atomic force microscope (AFM) which is used to manipulate the placement and position of individual atoms and molecules on the surface of a substrate is a MEMS device as well. In fact, a variety of MEMS technologies are required in order to interface with the nano-scale domain.

Likewise, many MEMS technologies are becoming dependent on nanotechnologies for successful new products. For example, the crash airbag accelerometers that are manufactured using MEMS technology can have their long-term reliability degraded due to dynamic in-use stiction effects between the proof mass and the substrate. A nanotechnology called Self-Assembled Monolayers (SAM) coatings are now routinely used to treat the surfaces of the moving MEMS elements so as to prevent stiction effects from occurring over the product’s life.

Many experts have concluded that MEMS and nanotechnology are two different labels for what is essentially a technology encompassing highly miniaturized things that cannot be seen with the human eye. Note that a similar broad definition exists in the integrated circuits domain which is frequently referred to as microelectronics technology even though state-of-the-art IC technologies typically have devices with dimensions of tens of nanometers. Whether or not MEMS and nanotechnology are one in the same, it is unquestioned that there are overwhelming mutual dependencies between these two technologies that will only increase in time. Perhaps what is most important are the common benefits afforded by these technologies, including: increased information capabilities; miniaturization of systems; new materials resulting from new science at miniature dimensional scales; and increased functionality and autonomy for systems.

“Taken from https://www.memsnet.org/mems/what_is.html”

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Microelectromechanical systems (MEMS) (also written as micro-electro-mechanicalMicroElectroMechanical or microelectronic and microelectromechanical systems) is the technology of very small devices; it merges at the nano-scale into nanoelectromechanical systems (NEMS) and nanotechnology.

MEMS are also referred to as micromachines (in Japan), or micro systems technology – MST (in Europe).

MEMS are separate and distinct from the hypothetical vision of molecular nanotechnology or molecular electronics. MEMS are made up of components between 1 to 100 micrometres in size (i.e. 0.001 to 0.1 mm), and MEMS devices generally range in size from 20 micrometres (20 millionths of a metre) to a millimetre (i.e. 0.02 to 1.0 mm). They usually consist of a central unit that processes data (the microprocessor) and several components that interact with the outside such as microsensors. At these size scales, the standard constructs of classical physics are not always useful. Because of the large surface area to volume ratio of MEMS, surface effects such as electrostatics and wetting dominate over volume effects such as inertia or thermal mass.

The potential of very small machines was appreciated before the technology existed that could make them—see, for example, Richard Feynman’s famous 1959 lecture “There’s Plenty of Room at the Bottom”.

MEMS became practical once they could be fabricated using modified semiconductor device fabrication technologies, normally used to make electronics. These include molding and plating, wet etching (KOH, TMAH) and dry etching (RIE and DRIE), electro discharge machining (EDM), and other technologies capable of manufacturing small devices. An early example of a MEMS device is the resonistor – an electromechanical monolithic resonator.

“According to Wikipedia”

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