About two years ago, Ken Susanto, a Ph.D. student in mechanical engineering at USC started to think about his thesis research project. Guided by his advisor, Bingen "Ben" Yang in the Department of Aerospace and Mechanical Engineering, Susanto chose to work on the application of motion-control devices to biomedical instruments. After an extensive literature review, he centered the thesis work on "smart" forceps that could find use in minimal invasive surgery. Electrical actuation of forceps and other surgical instruments also might bring "telesurgery"-surgery done by remote control-closer to reality, he thought. Under Yang's tutelage, it took Susanto about a year to devise a practical approach for his research and another six months to develop a working prototype. But that effort paid off and earned Susanto and Yang the third annual Design News College Design Engineering Award.
Yang, an expert in the areas of flexible structures, funded the research from a grant he received from the Charles Lee Powell Foundation (LaJolla, CA). He also contributed his expertise in the area of piezoelectric materials. (An applied voltage causes piezoelectric materials to change their shape.) The researchers settled on lead zirconate titanate (PZT), a polycrystalline ceramic, as their actuator. After six months of investigation and lab work, Susanto successfully bonded PZT to specially manufactured forceps made of multiple layers of aluminum and stainless steel. The layered metal sandwich and the adhesives that bind the layers pre-stress the forceps. This pre-stressed structure lets the PZT material deflect the forceps tips much more than it could under normal conditions, without cracking.
Applying a voltage to the 8-mil-thick PZT material on each side of the forceps can open the forceps several millimeters or force them closed. The forceps can grasp with a force of 15g, more than enough for many surgical procedures. An applied potential of about +300V closes the jaws and flattens the forceps, and -300V opens the jaws by forcing each arm into an arch. Adjusting the voltage between high and low limits lets an operator control the opening and the force it exerts. According to Susanto, a linear relationship exists between the forceps' operation and the applied voltage, so the forceps don't undergo any abrupt changes.
Susanto and Yang had to overcome many obstacles on the path to forceps that actually worked. Key was the need to optimize the number of metal layers and search for the proper dimensions for a working prototype. Susanto modeled many types of stainless-steel structures and geometric configurations before he arrived at a combination of metal and piezoceramic that looked promising. He also had to figure out how to work with PZT.
So how did an electric charge get into piezoelectric materials in the first place? In the case of minerals such as quartz or tourmaline, the natural structure of the solid imparts piezoelectric properties. But ceramics produced in a lab require the intervention of scientists. Although the word ceramic has its roots in the Greek word keramos, (potter's clay), we generally consider modern ceramics to cover polycrystalline inorganic non-metal materials. (Although ceramics do include metal such as barium and zirconium, they combine with elements such as oxygen and do not exhibit metallic characteristics, such as high electrical and thermal conductivity.)
Scientists use specific formulations for proven piezoelectric ceramics-not all ceramics exhibit piezo properties. During a high-temperature chemical reaction, the chemicals combine to produce new molecules to form the piezoelectric structure. But the ceramic isn't quite ready for use yet. After grinding, the manufacturer passes the material through several sintering processes that fuse the molecules into a solid ceramic structure for strength. During sintering, dipoles-molecules of the ceramic with a positive and a negative end-orient themselves randomly in the structure. This material exhibits the same electrical and mechanical behavior in all directions and falls into the category of isotropic ceramic.
At this point, the manufacturer can cut, grind, and polish to meet specifications set by the user. After these steps, the manufacturer applied electrodes to two opposite sides of the ceramic and subjects the material to a poling process. As the name implies, poling helps align the dipoles into a more regular order, plus to minus, plus to minus, and so on. The poling takes place at high temperature and with applied electrical fields of several kV/mm of thickness. During poling, the ceramic elongates between the poling electrodes and shrinks, or narrows, along the perpendicular axes. The realignment of the material's dipoles accounts for the change in dimensions.
Users can specify the poled axis, so they'll know how the final piezoelectric element, now considered anisotropic, will operate in their application. The greatest dimensional change occurs along the poled axis, and developers can use this characteristic to actuate devices when they apply a potential across the element. The element also can serve as a sensor that supplies a potential depending on the force exerted on it. Now maybe you can better enjoy those grilled steaks and veggies, knowing where the igniting spark came from.
Thanks to Physik Instrumente GmbH (Palmbach, Germany) and Morgan Electro Ceramics (Bedford, OH) for information about the ceramic fabrication processes.
At present, an operator manipulates the forceps under computer control, but eventually, Susanto and Yang expect doctors to use more sophisticated controls that provide greater flexibility during surgery. The forceps' small size and grasping power make it an excellent candidate for separating blood vessels during surgery, obtaining biopsy samples, and grabbing small gallstones. Susanto and Yang have filed a patent application and hope to commercialize the product.
Asked what inspired him to pursue engineering as a career, Susanto recalls his father's advice to always maintain a technical curiosity. "You can't accept that something like a cell phone just works," said his dad. "You have to think about what it does and why it works." That admonition led Ken to a B.S. degree in mathematics from UCLA, and an M.S. in electrical engineering from USC.
When reflecting on his choice of USC for his doctoral work, Susanto says the school offers a pool of faculty talent and goes out of its way to provide resources and funds for research. He adds that USC actively encourages faculty and students to collaborate on research projects that may develop marketable products. After he finishes his Ph.D, next year, Susanto plans to develop other products, and in a few years he expects to start his own product-design or consulting company.
Contact Jon Titus at [email protected].