If Pranav Shrotriya’s research goes as planned, the process of detecting an illegal drug such as cocaine could become much easier and less expensive. The 34-year-old Virginia and William Binger Assistant Professor in Mechanical Engineering is working on a miniaturized sensor to do the job.
The premise is relatively simple: You coat the surface of a device with a compound that reacts only to the substance you are trying to detect. When that substance is present, the compound absorbs molecules that rearrange its atoms. This, in turn, changes the stress on the surface of the device, bending or otherwise deforming it.
Learning how structures fail
Working with atoms and molecules, of course, means working at the nanoscale. While researchers know that materials have different properties at these very small sizes, there is still much to be learned about the causes of these unique behaviors. That is where Shrotriya’s interest lies.Shrotriya, who came to the United States from Gwalior, India, first became interested in the mechanics of nanoscale structures while studying polymer composites as a PhD student at the University of Illinois, Urbana-Champaign. After graduating in 2001, he continued to explore mechanics at the micro- and nanoscale as a postdoctoral fellow, first at Princeton and later at Brown University.
In the course of these appointments, he developed models to study how these small structures fail under different loads and began work on computational simulations to understand how molecular rearrangement occurs. This early work convinced Shrotriya that understanding what is happening at the nanoscale could lead to significant advances in engineering, specifically the development of stronger and lighter materials for everything from automobiles to hip and knee implants.
After joining the Iowa State faculty in 2003, Shrotriya soon found himself collaborating with ME Professors Sriram Sundararajan and Abhijit Chandra to study the deterioration of artificial joints and how these might be made more durable. Today, with funding from Aesculap, Inc., one of the world’s largest medical device manufacturers, the researchers are studying how the body’s chemical environment affects the metal surfaces of artificial joints as they rub against one another, causing corrosion in the implant. By predicting surface toughness, they hope to pinpoint materials that can minimize material loss.
The art of detection
A National Science Foundation CAREER Award in 2006 allowed Shrotriya to expand his research. Titled “High-resolution interferometry-based surface stress sensors for chemical and biological species detection,” the project’s goal is to build a working model of a sensor that is able to detect the presence of substances such as drugs, bacteria, or explosives that pose a threat to health, national security, or the environment.
The project encompasses both experimental and computational components. For the experimental part, Shrotriya has developed a mechanical platform for the sensing device that coats a cantilever with an aptamer, a substance that reacts only with the species that needs to be detected.
“Anytime you deposit something on a solid surface and a chemical reaction happens, the surface stress changes,” Shrotriya explains. “We can’t see that effect in large objects because the magnitude of the energy involved is very small. But if you make the structure very small—for example, one micron thick and 100 microns long—and deposit the species on just one side, the surface stress change causes the structure to deform or bend.”
Shrotriya has developed a high-resolution interferometry-based technique that can measure the deformation differences between a reference surface and a sensor surface. The presence of the harmful species will be indicated by the measurements.
Smaller is better
The primary advantage of Shrotriya’s device, he says, is the ability to miniaturize it. “We have a large-scale model in the lab that we are using now, but the goal of the project is to build a MEMS (i.e., microelectricalmechanical systems) device,” he notes. “The reaction itself can occur on a surface as small as one millimeter by one millimeter.
“Making the device very small will make it easy to transport and use on site,” Shrotriya adds. “In addition, it can be mass produced, which means it will be quick and cheap to manufacture.”
The platform can be used for detecting a wide variety of substances, and Shrotriya is collaborating with researchers in biochemistry, biophysics, and molecular biology to design aptamers that will react to a specific target species. “Anytime you have a very specific interaction between one molecule and another,” he says, “you can put it on our platform to see whether a surface stress change occurs.”
A new three-year project recently funded through the National Institute of Justice is giving Shrotriya the opportunity to use his platform for cocaine detection. The advantage of using such small surfaces, he says, is that they are capable of detecting the very small amounts of the drug that may appear only as trace elements in other substances.
“Usually, cocaine molecules are part of a biological matrix or solution in urine or saliva or blood,” Shrotriya explains, “so we are trying to see how these affect our measurements. It is quite a challenging task with all sorts of variables, but if we can show that it works, we will have a sensor that can be carried anywhere and that has very high sensitivity and specificity for cocaine or whatever you want to detect.”
An interdisciplinary bias
While Shrotriya and his students have been busy conducting experiments on the mechanical platform, his primary interest lies not so much in developing technologies for sensing species, but rather in trying to understand the mechanics underlying the reactions. For this aspect of the project, Shrotriya is using computer simulations to explain exactly what is happening to the molecules and atoms that cause the surface stress changes. To find these answers, he is turning to experts in molecular dynamic simulations and quantum chemical calculations.
“We want to know, for example, how sulfur and gold interact with each other. And how does that interaction change when you attach different molecules?” Shrotriya says. At these very small scales, the energies associated with chemical reaction, deformation, and surface stress change are all equal. All of these are interrelated, and to understand what is happening requires learning new things and collaborating across disciplines.”Shrotriya’s interdisciplinary outlook extends to the classroom as well, where he teaches a course in nanomechanics that attracts students from across the college. The course includes discussion about chemical reactions and applications to such things as detection of harmful species. Shrotriya has also developed a module for the machine design class that allows students to explore nanoscale machines.
Shrotriya believes that constantly learning and working across disciplines will provide future opportunities. “The future of the discipline relies on our ability to learn multiple things and apply that information to a mechanical framework,” he says. “By doing so, we will have the tools to attack the challenging problems of the future.”