Silicon Technology For Medical Application

A team of cardiologists, materials scientists, and bioengineers have created and tested a new type of implantable device for measuring the heart's electrical output that they say is a vast improvement over current devices. The new device represents the first use of flexible silicon technology for a medical application.

This technology may herald a new generation of active, flexible, implantable devices for applications in many areas of the body, commented Brian Litt, an associate professor of Neurology at the University Of Pennsylvania School Of Medicine and also an associate professor of Bioengineering in Penn's School of Engineering and Applied Science.

Implantable silicon-based devices have the potential to serve as tools for mapping and treating epileptic seizures, providing more precise control over deep brain stimulation, as well as other neurological applications, say Story Landis, PhD, director of the National Institute of Neurological Disorders and Stroke, which provided support for the study.

The new devices bring electronic circuits right to the tissue, rather than having them located remotely, inside a sealed can that is placed elsewhere in the body, such as under the collar bone or in the abdomen, explained Litt. This enables the devices to process signals right at the tissues, which allows them to have a much higher number of electrodes for sensing or stimulation than is currently possible in medical devices.

Now, for example, devices for mapping and eliminating life-threatening heart rhythms allow for up to 10 wires in a catheter that is moved in and around the heart, and is connected to rigid silicon circuits distant from the target tissue. This design limits the complexity and resolution of devices since the electronics cannot get wet or touch the target tissue.

The team tested the new devices - made of nanoscale, flexible ribbons of silicon embedded with 288 electrodes, forming a lattice-like array of hundreds of connections - on the heart of a porcine animal model. The tissue-hugging shape allows for measuring electrical activity with greater resolution in time and space. The new device can also operate when immersed in the body's salty fluids. The devices can collect large amounts of data from the body, at high speed. This allowed the researchers to map electrical activity on the heart of the large animal.

In this experiment, the researchers built a device to map waves of electrical activity in the heart of a large animal. The device uses the 288 contacts and more than 2,000 transistors spaced closely together, while standard clinical systems usually use about five to 10 contacts and no active transistors. High-density maps of electrical activity on the heart were recorded from the device, during both natural and paced beats.

Scientists are also planning to design advanced, intelligent pacemakers that can improve the pumping function of hearts weakened by heart attacks and other diseases. For each of these applications, the team is conducting experiments to test flexible devices in animals before starting human trials.

Another focus of ongoing work is to develop similar types of devices that are not only flexible, like a sheet of plastic, but fully stretchable, like a rubber band. The ability to fully conform and wrap around large areas of curved tissues will require stretch ability, as well as flexibility.

About Soft Interference Lithography (SIL)

Researchers at Northwestern University had developed a nano manufacturing technique (2007) which can be used to produce nanostructures measuring tens of square centimeters. This new technique, dubbed ‘soft interference lithography’ (SIL), can lead to nanomaterials with optical properties mimicking some metamaterials in the natural world such as peacock feathers and butterfly wings. As said the researchers, their SIL technique combines the ability of interference lithography to produce wafer-scale nanopatterns with the versatility of soft lithography and used it to create plasmonic metamaterials.

Here is how scientists described SIL. “The optical nanomaterials in this research are called ‘plasmonic metamaterials’ because their unique physical properties originate from shape and structure rather than material composition only. Two examples of metamaterials in the natural world are peacock feathers and butterfly wings. Their brightly colored patterns are due to structural variations at the hundreds of nanometers level, which cause them to absorb or reflect light. Through the development of a new nanomanufacturing technique, researchers succeeded in making gold films with virtually infinite arrays of perforations as small as 100 nanometers — 500-1000 times smaller than a human hair. On a magnified scale, these perforated gold films look like Swiss cheese except the perforations are well-ordered and can spread over macroscale distances. The researchers’ ability to make these optical metamaterials inexpensively and on large wafers or sheets is what sets this work apart from other techniques.”

In the research section of her site, Mrs. Odom describes plasmonic materials and their optical properties. “Plasmonics is an exciting and emerging area that uses metal nanostructures to manipulate light on the nanoscale. Depending on their size, shape, and materials properties, noble metal nanoparticles can scatter and absorb light to produce colors ranging from the ultra-violet to the near-infrared. In addition, significantly more light can be transmitted through metal films perforated with subwavelength hole arrays than is permitted by geometric optics, a phenomena known as enhanced optical transmission.”

And she gives some additional details. “We focus primarily on the optical properties of two different but complementary systems that can control light on the nanometer scale: (i) metallic films of nanohole arrays and (ii) pyramidal nanoparticles. The former have properties dominated by SPPs, and the latter have properties dominated by LSPs. Such nanostructures are easily made by our innovative fabrication scheme, PEEL, for preparing large-area, free-standing films of nanoscale holes and particles. PEEL is a simple procedure which combines Phase-shifting photolithography, Etching, Electron-beam deposition, and Lift-off of the metal film.”

RAPID: A NEW NANOFABRICATION PROCESS

For exploring the full potentiality of nanotechnology and its sea like vast application in elctronics and hardware industry, the ability to create tiny patterns is important. They are extremely important for fabrication of computer chips and many other application. Yet, creating ever smaller features, through a widely-used process called photolithography, has required the use of ultraviolet light, which is difficult and expensive to work with. John Fourkas, Professor of Chemistry and Biochemistry in the University of Maryland College of Chemical and Life Sciences, and his research group have developed a new, table-top technique called RAPID (Resolution Augmentation through Photo-Induced Deactivation) lithography that makes it possible to create small features without the use of ultraviolet light. Photolithography uses light to deposit or remove material and create patterns on a surface. There is usually a direct relationship between the wavelength of light used and the feature size created. Therefore, nanofabrication has depended on short wavelength ultraviolet light to generate ever smaller features.

The RAPID lithography technique have been developed to create patterns twenty times smaller than the wavelength of light employed; by this process it streamlines the nanofabrication process. That’s how RAPID can be used in many applications in areas such as electronics, optics, and biomedical devices.

In this process, two laser lights of same color have been used for controlling the operation. First, short burst of light used to harden the material and secondly, a constant light source used to prevent it. The technique has been highly appreciated for its easiness to implement, as there is no need to control the timing between two different pulsed lasers.

Cold atoms could replace hot gallium in focused ion beams

Scientists at the National Institute of Standards and Technology (NIST) have developed a radical new method of focusing a stream of ions into a point as small as one nanometer (one billionth of a meter). Because of the versatility of their approach—it can be used with a wide range of ions tailored to the task at hand—it is expected to have broad application in nanotechnology both for carving smaller features on semiconductors than now are possible and for nondestructive imaging of nanoscale structures with finer resolution than currently possible with electron microscopes. Researchers and manufacturers routinely use intense, focused beams of ions to carve nanometer-sized features into a wide variety of targets. In principle, ion beams also could produce better images of nanoscale surface features than conventional electron microscopy. But the current technology for both applications is problematic. In the most widely used method, a metal-coated needle generates a narrowly focused beam of gallium ions. The high energies needed to focus gallium for milling tasks end up burying small amounts in the sample, contaminating the material. And because gallium ions are so heavy, if used to collect images they inadvertently damage the sample, blasting away some of its surface while it is being observed. Researchers have tried using other types of ions but were unable to produce the brightness or intensity necessary for the ion beam to cut into most materials.

The NIST team took a completely different approach to generating a focused ion beam that opens up the possibility for use of non-contaminating elements. Instead of starting with a sharp metal point, they generate a small "cloud" of atoms and then combine magnetic fields with laser light to trap and cool these atoms to extremely low temperatures. Another laser is used to ionize the atoms, and the charged particles are accelerated through a small hole to create a small but energetic beam of ions. Researchers have named the groundbreaking device "MOTIS," for Magneto-Optical Trap Ion Source.

Because the lasers cool the atoms to a very low temperature, they're not moving around in random directions very much. As a result, when ions are accelerated, they travel in a highly parallel beam, which is necessary for focusing them down to a very small spot, explains Jabez McClelland of the NIST Center for Nanoscale Science and Technology. The team was able to measure the tiny spread of the beam and show that it was indeed small enough to allow the beam to be focused to a spot size less than 1 nanometer. The initial demonstration used chromium atoms, establishing that other elements besides gallium can achieve the brightness and intensity to work as a focused ion beam .

JUNCTIONLESS TRANSISTORS!

From the early ages of transistors i.e. in 1940s when they were over 1 centimeter size to the modern age where transistors are less than 30 nanometers long – transistors are shrinking their size over three thousands times. The outcome is that more transistor based circuits are integrated into a single chip. But this development cannot continue for much longer. One of the increasingly difficult problems that chip designers are facing is that the high density of components packed on a chip makes interconnections increasingly difficult; and, as conventional chip structures continue to shrink, Moore's Law is on a collision course with the laws of physics.

All existing transistors are based on junctions – obtained by changing the polarity of silicon from positive to negative. It is a little bit like changing the color of silicon from black to white, explains Jean-Pierre Colinge, a professor at Tyndall National Institute, whose team has just reported a breakthrough in nanoelectronics by demonstrating the world's first junctionless transistor.

In modern transistors, a negative-positive-negative structure needs to be created, where the width of the positive region is only a few dozens of atoms wide.

In a junctionless gated transistor the doping concentration in the channel is identical to that in the source and drain. Because the gradient of the doping concentration between source and channel or drain and channel is zero, no diffusion can take place, which eliminates the need for costly ultrafast annealing techniques and allows one to fabricate devices with shorter channels.

The devices have full CMOS functionality, but they contain no junctions or doping gradients and are, therefore, much less sensitive to thermal budget issues than regular CMOS devices.

The key to fabricating a junctionless gated transistor is the formation of a semiconductor layer that is thin and narrow enough to allow for full depletion of carriers when the device is turned off. The semiconductor also needs to be heavily doped to allow for a reasonable amount of current flow when the device is turned on. Putting these two constraints together imposes the use of nanoscale dimensions and high doping concentrations.

The electrical current flows in this silicon nanowire, and the flow of current is perfectly controlled by a wedding ring structure that electrically squeezes the silicon wire. These structures are easy to fabricate even on a miniature scale which leads to the major breakthrough in potential cost reduction.