Researchers create tiny, wirelessly powered cardiac device

Engineerblogger
Sept 9, 2012

Ada Poon, assistant professor of electrical engineering, led the research. (Photo: Linda A. Cicero / Stanford News Service)

Stanford electrical engineers overturn existing models to demonstrate the feasibility of a millimeter-sized, wirelessly powered cardiac device. The findings, say the researchers, could dramatically alter the scale of medical devices implanted in the human body.

A team of engineers at Stanford has demonstrated the feasibility of a super-small, implantable cardiac device that gets its power not from batteries but from radio waves transmitted from a small power device on the surface of the body.

The implanted device is contained in a cube just 0.8 millimeter on a side. It could fit on the head of pin.

The findings were published in the journal Applied Physics Letters. In their paper, the researchers demonstrated wireless power transfer to a millimeter-sized device implanted 5 centimeters inside the chest on the surface of the heart – a depth once thought out of reach for wireless power transmission.

The engineers say the research is a major step toward a day when all implants are driven wirelessly. Beyond the heart, they believe such devices might include swallowable endoscopes – so-called "pillcams" that travel the digestive tract – permanent pacemakers and precision brain stimulators – virtually any medical applications where device size and power matter.
A revolution in the body

Implantable medical devices in the human body have revolutionized medicine. Hundreds of thousands if not millions of pacemakers, cochlear implants and drug pumps are today helping patients live relatively normal lives, but these devices are not without engineering challenges.

First, they require power, which means batteries, and batteries are bulky. In a device like a pacemaker, the battery alone accounts for as much as half the volume of the device. Second, batteries have finite lives. New surgery is needed when they wane.

"Wireless power solves both challenges," said Ada Poon, assistant professor of electrical engineering, who headed up the research. She was assisted by Sanghoek Kim and John Ho, both doctoral candidates in her lab.

Last year, Poon made headlines when she demonstrated a wirelessly powered, self-propelled device capable of swimming through the bloodstream. To get there she needed to overturn some long-held assumptions about delivery of wireless power through the human body.

Her latest device works by a combination of inductive and radiative transmission of power. Both are types of electromagnetic transfer in which a transmitter sends radio waves to a coil of wire inside the body. The radio waves produce an electrical current in the coil sufficient to operate a small device.

There is an indirect relationship between the frequency of the transmitted radio waves and the size of the receiving antenna. That is, to deliver a desired level of power, lower frequency waves require bigger coils. Higher frequency waves can work with smaller coils.

"For implantable medical devices, therefore, the goal is a high-frequency transmitter and a small receiver, but there is one big hurdle," Kim said.
Ignoring consensus

Existing mathematical models have held that high-frequency radio waves do not penetrate far enough into human tissue, necessitating the use of low-frequency transmitters and large antennas – too large to be practical for implantable devices.

Ignoring the consensus, Poon proved the models wrong. Human tissues dissipate electric fields quickly, it is true, but radio waves can travel in a different way – as alternating waves of electric and magnetic fields. With the correct equations in hand, she discovered that high-frequency signals travel much deeper than anyone suspected.

"In fact, to achieve greater power efficiency, it is actually advantageous that human tissue is a very poor electrical conductor," said Kim. "If it were a good conductor, it would absorb energy, create heating and prevent sufficient power from reaching the implant."

According to their revised models, the researchers found that the maximum power transfer through human tissue occurs at about 1.7 billion cycles per second, much higher than previously thought.

"In this high-frequency range, we can increase power transfer by about 10 times over earlier devices," said Ho, who honed the mathematical models.

The discovery meant that the team could shrink the receiving antenna by a factor of 10 as well, to a scale that makes wireless implantable devices feasible. At the optimal frequency, a millimeter-radius coil is capable of harvesting more than 50 microwatts of power, well in excess of the needs of a recently demonstrated 8-microwatt pacemaker.
Engineering challenges

With the dimensional challenges solved, the team found itself bound by other engineering constraints. First, electronic medical devices must meet stringent health standards established by IEEE (Institute of Electrical and Electronics Engineers), particularly with regard to tissue heating. Second, the team found that the receiving and transmitting antennas had to be optimally oriented to achieve maximum efficiency. Differences in alignment of just a few degrees could produce troubling drops in power.

"This can't happen medical devices," said Poon. "As the human heart and body are in constant motion, solving this issue was critical to the success of our research." The team responded by designing an innovative slotted transmitting antenna structure. It delivers consistent power efficiency regardless of orientation of the two antennas.

The new design serves additionally to focus the radio waves precisely at the point inside the body where the implanted device rests on the surface of the heart – increasing the electric field where it is needed most, but canceling it elsewhere. This helps reduce overall tissue heating to levels well within the IEEE standards. Poon has applied for a patent on the antenna structure.

This research was made possible by funding from the C2S2 Focus Center, one of six research centers funded under the Focus Center Research Program, a Semiconductor Research Corporation entity. Lisa Chen also contributed to this study.

Source: Stanford University

Researchers Develop New, Less Expensive Nanolithography Technique

Engineerblogger
Sept 9, 2012

This technique uses no electronic components to bring the cantilevers into contact with the substrate surface.

Researchers from North Carolina State University have developed a new nanolithography technique that is less expensive than other approaches and can be used to create technologies with biomedical applications.

“Among other things, this type of lithography can be used to manufacture chips for use in biological sensors that can identify target molecules, such as proteins or genetic material associated with specific medical conditions,” says Dr. Albena Ivanisevic, co-author of a paper describing the research. Ivanisevic is an associate professor of materials science and engineering at NC State and associate professor of the joint biomedical engineering program at NC State and the University of North Carolina at Chapel Hill. Nanolithography is a way of printing patterns at the nanoscale.

The new technique relies on cantilevers, which are 150-micron long silicon strips. The cantilevers can be tipped with spheres made of polymer or with naturally occurring spores. The spheres and spores are coated with ink and dried. The spheres and spores are absorbent and will soak up water when exposed to increased humidity.

As a result, when the cantilevers are exposed to humidity in a chamber, the spheres and spores absorb water – making the tips of the cantilevers heavier and dragging them down into contact with any chosen surface.

Users can manipulate the size of the spheres and spores, which allows them to control the patterns created by the cantilevers. For example, at low humidity, a large sphere will absorb more water than a small sphere, and will therefore be dragged down into contact with the substrate surface. The small sphere won’t be lowered into contact with the surface until it is exposed to higher humidity and absorbs more water.

Further, the differing characteristics of sphere polymers and spores mean that they absorb different amounts of water when exposed to the same humidity – giving users even more control of the nanolithography.

“This technique is less expensive than other device-driven lithography techniques used for microfabrication because the cantilevers do not rely on electronic components to bring the cantilevers into contact with the substrate surface,” Ivanisevic says. “Next steps for this work include using this approach to fabricate lithographic patterns onto tissue for use in tissue regeneration efforts.”

The paper, “Parallel Dip-Pen Nanolithography using Spore- and Colloid-Terminated Cantilevers,” was published online Aug. 17 in the journal Small. Lead author of the paper is Dr. Marcus A. Kramer, who did the work at NC State while completing his Ph.D. at Purdue University.


Source: North Carolina State University

Reference Material Could Aid Nanomaterial Toxicity Research

Engineerblogger
Sept 9, 2012

TEM image shows the nanoscale crystalline structure of titanium dioxide in NIST SRM 1898 (color added for clarity.)  Credit: Impellitteri/EPA

The National Institute of Standards and Technology (NIST) has issued a new nanoscale reference material for use in a wide range of environmental, health and safety studies of industrial nanomaterials. The new NIST reference material is a sample of commercial titanium dioxide powder commonly known as “P25.”

NIST Standard Reference Materials® (SRMs) are typically samples of industrially or clinically important materials that have been carefully analyzed by NIST. They are provided with certified values for certain key properties so that they can be used in experiments as a known reference point.

Nanoscale titanium-dioxide powder may well be the most widely manufactured and used nanomaterial in the world, and not coincidentally, it is also one of the most widely studied. In the form of larger particles, titanium dioxide is a common white pigment. As nanoscale particles, the material is widely used as a photocatalyst, a sterilizing agent and an ultraviolet blocker (in sunscreen lotions, for example).

“Titanium dioxide is not considered highly toxic and, in fact, we don’t certify its toxicity,” observes NIST chemist Vincent Hackley. “But it’s a representative industrial nanopowder that you could include in an environmental or toxicity study. It’s important in such research to include measurements that characterize the nanomaterial you’re studying—properties like morphology, surface area and elemental composition. We’re providing a known benchmark.”

The new titanium-dioxide reference material is a mixed phase, nanocrystalline form of the chemical in a dry powder. To assist in its proper use, NIST also has developed protocols* for properly preparing samples for environmental or toxicological studies.

The new SRM also is particularly well suited for use in calibrating and testing analytical instruments that measure specific surface area of nanomaterials by the widely used Brunauer-Emmet-Teller (BET) gas sorption method.

Additional details and purchasing information on NIST Standard Reference Material 1898, “Titanium Dioxide Nanomaterial” are available at www.nist.gov/srm/index.cfm.

SRMs are among the most widely distributed and used products from NIST. The agency prepares, analyzes and distributes nearly 1,300 different materials that are used throughout the world to check the accuracy of instruments and test procedures used in manufacturing, clinical chemistry, environmental monitoring, electronics, criminal forensics and dozens of other fields.


Source: The National Institute of Standards and Technology (NIST)

Additional Information:

• See “Protocols for Measurement and Dispersion of Nanoparticles” at www.nist.gov/mml/np-measurement-protocols.cfm. 

Researchers Set World Record for Highest Surface Area Material

Engineerblogger
Sept 9, 2012


Northwestern University researchers have broken a world record by creating two new synthetic materials with the greatest amount of surface areas reported to date.

Named NU-109 and NU-110, the materials belong to a class of crystalline nanostructure known as metal-organic frameworks (MOFs) that are promising vessels for natural gas storage for vehicles, catalysts, and other sustainable materials chemistry.

The materials’ promise lies in their vast internal surface area. If the internal surface area of one NU-110 crystal the size of a grain of salt could be unfolded, the surface area would cover a desktop. Put another way, the internal surface area of one gram of NU-110 would cover one-and-a-half football fields.

A paper describing the findings, “Metal-organic Framework Materials with Ultrahigh Surface Areas: Is the Sky the Limit?” was published August 20 in the Journal of the American Chemical Society.

The research team, led by Omar Farha, research associate professor of chemistry in the Weinberg College of Arts and Sciences, has synthesized, characterized, and computationally simulated the behavior of the two MOFs that display the highest experimental Brunauer-Emmett-Teller surface areas of any porous material on record, 7,000 m2/g; that is, one kilogram of the material contains an internal surface area that could cover seven square kilometers. (Brunauer-Emmett-Teller, or BET, is an analysis technique for measuring the surface area of a material.)

The extremely high surface area, which is normally not accessible due to solvent molecules that stay trapped within the pores, was achieved using a carbon dioxide activation technique. As opposed to heating, which can remove the solvent but also damage the MOF material, the carbon dioxide-based technique removes the solvent gently and leaves the pores completely intact.

The development could rapidly lead to further advances. MOFs are composed of organic linkers held together by metal atoms, resulting in a molecular cage-like structure. The researchers believe they may be able to more than double the surface area of the materials by using less bulky linker units in the materials’ design.

The research comes from the labs of Joseph T. Hupp, professor of chemistry in Weinberg, and Randall Q. Snurr, professor of chemical and biological engineering at the McCormick School of Engineering.

Other authors include SonBinh Nguyen, professor of chemistry in Weinberg; Ibrahim Eryazici, Nak Cheon Jeong, Brad G. Hauser, Amy A. Sarjeant, and Christopher E. Wilmer, all of Northwestern; and A. Özgür Yazaydın of the University of Surrey in the United Kingdom.

The MOF-designing and -synthesizing technology is being commercialized by NuMat Technologies, a Northwestern startup that has won more than $1 million in business plan competitions since incorporating in February.

Source:  Northwestern University

Transparent solar cells for windows that generate electricity

Engineerblogger
Aug 3, 2012

Visibly transparent photovoltaic devices can open photovoltaic applications in many areas, such as building-integrated photovoltaics or integrated photovoltaic chargers for portable electronics. We demonstrate high-performance, visibly transparent polymer solar cells fabricated via solution processing. The photoactive layer of these visibly transparent polymer solar cells harvests solar energy from the near-infrared region while being less sensitive to visible photons. The top transparent electrode employs a highly transparent silver nanowire–metal oxide composite conducting film, which is coated through mild solution processes. With this combination, we have achieved 4% power-conversion efficiency for solution-processed and visibly transparent polymer solar cells. The optimized devices have a maximum transparency of 66% at 550 nm.

Scientists are reporting development of a new transparent solar cell, an advance toward giving windows in homes and other buildings the ability to generate electricity while still allowing people to see outside. Their report appears in the journal ACS Nano.

Yang Yang, Rui Zhu, Paul S. Weiss and colleagues explain that there has been intense world-wide interest in so-called polymer solar cells (PSCs), which are made from plastic-like materials. PSCs are lightweight and flexible and can be produced in high volume at low cost. That interest extends to producing transparent PSCs. However, previous versions of transparent PSCs have had many disadvantages, which the team set out to correct.

They describe a new kind of PSC that produces energy by absorbing mainly infrared light, not visible light, making the cells 66 percent transparent to the human eye. They made the device from a photoactive plastic that converts infrared light into an electrical current. Another breakthrough is the transparent conductor made of a mixture of silver nanowire and titanium dioxide nanoparticles, which was able to replace the opaque metal electrode used in the past. This composite electrode also allowed the solar cell to be fabricated economically by solution processing. The authors suggest the panels could be used in smart windows or portable electronics.

The authors acknowledge funding from the Engineering School of UCLA, the Office of Naval Research and the Kavli Foundation.

Source: American Chemical Society 

Additional Information:

• Visibly Transparent Polymer Solar Cells Produced by Solution Processing

The first robot that mimics the water striders’ jumping abilities

Engineerblogger

Aug 3, 2012

Credit: American Chemical Society

The first bio-inspired microrobot capable of not just walking on water like the water strider – but continuously jumping up and down like a real water strider – now is a reality. Scientists reported development of the agile microrobot, which could use its jumping ability to avoid obstacles on reconnaissance or other missions, in ACS Applied Materials & Interfaces.

Qinmin Pan and colleagues explain that scientists have reported a number of advances toward tiny robots that can walk on water. Such robots could skim across lakes and other bodies of water to monitor water quality or act as tiny spies. However, even the most advanced designs – including one from Pan’s team last year – can only walk on water. Pan notes that real water striders actually leap. Making a jumping robot is difficult because the downward force needed to propel it into the air usually pushes the legs through the water’s surface. Pan’s group looked for novel mechanisms and materials to build a true water-striding robot.

Using porous, super water-repellant nickel foam to fabricate the three supporting and two jumping legs, the group made a robot that could leap more than 5.5 inches, despite weighing as much as 1,100 water striders. In experiments, the robot could jump nearly 14 inches forward – more than twice its own length – leaving the water at about 3.6 miles per hour. The authors report that the ability to leap will make the bio-inspired microrobot more agile and better able to avoid obstacles it encounters on the water’s surface.

The authors acknowledge funding from the State Key Laboratory of Robotics and System of Harbin Institute of Technology and the National Natural Science Foundation of China.

Source:  American Chemical Society

Additional Information:

• Why Superhydrophobicity Is Crucial for a Water−Jumping Microrobot? Experimental and Theoretical Investigations”

Printing 3D Blood Vessel Networks out of Sugar

Engineerblogger
Aug 3, 2012

3D Printing Blood Vessel Networks

Scientists can already grow thin layers of cells, so one proposed solution to the vasculature problem is to “print” the cells layer by layer, leaving openings for blood vessels as necessary. But this method leaves seams, and when blood is pumped through the vessels, it pushes those seams apart.

Bioengineers from the University of Pennsylvania have turned the problem inside out by using a 3D printer called a RepRap to make templates of blood vessel networks out of sugar. Once the networks are encased in a block of cells, the sugar can be dissolved, leaving a functional vascular network behind.

“I got the first hint of this solution when I visited a Body Worlds exhibit, where you can see plastic casts of free-standing, whole organ vasculature,” says Bioengineering postdoc Jordan Miller.

Miller, along with Christopher Chen, the Skirkanich Professor of Innovation in the Department of Bioengineering, other members of Chen’s lab, and colleagues from MIT, set out to show that this method of developing sugar vascular networks helps keep interior cells alive and functioning.

After the researchers design the network architecture on a computer, they feed the design to the RepRap. The printer begins building the walls of a stabilizing mold. Then it then draws filaments across the mold, pulling the sugar at different speeds to achieve the desired thickness of what will become the blood vessels.

After the sugar has hardened, the researchers add liver cells suspended in a gel to the mold. The gel surrounds the filaments, encasing the blood vessel template. After the gel sets it can be removed from the mold with the template still inside. The block of gel is then washed in water, dissolving the remaining sugar inside. The liquid sugar flows out of the vessels it has created without harming the growing cells.

“This new technology, from the cell’s perspective, makes tissue formation a gentle and quick journey,” says Chen.

The researchers have successfully pumped nutrient-rich media, and even blood, through these gels blocks’ vascular systems. They also have experimentally shown that more of the liver cells survive and produce more metabolites in gels that have these networks.

The RepRap makes testing new vascular architectures quick and inexpensive, and the sugar is stable enough to ship the finished networks to labs that don’t have 3D printers of their own. The researchers hope to eventually use this method to make implantable organs for animal studies.

Source: 

University of Pennsylvania