With joint appointments in the departments of Materials Science and Engineering and Biological Engineering, MIT Associate Professor Krystyn J. Van Vliet brings a materials scientist's understanding of mechanical triggers for biochemical behavior. Although her group studies many non-biological materials that also show this coupling between chemistry and mechanics, biological cells and tissues are especially complex. She studies stem cells from the central nervous system and from bone marrow, as well as tissues from the brain, heart, and liver.
In recent work, Van Vliet's group has shown that three biophysical markers — size, mechanical stiffness, and how much the nucleus inside the cell moves around — can accurately identify stem cells in a mixed group of cells; engineered polymers that can mimic the response of human tissue to high rates of loading; and established that measurements of cell fluidity, a mechanical property that ranges from 0 to 1, can detect cell responses to different chemical triggers such as salinity or physical triggers such as temperature.
Stem cells hold promise for a procedure known as cellular therapy, in which a person's own stem cells can fight disease or repair tissue damage — regrowing bone, for example. Alternatively, the cells can be used as factories to produce key chemicals that are collected to act as the medicine. The challenge, Van Vliet says, is that stem cells naturally occur in small numbers in our bodies. "Roughly 1 in 100,000 of the cells that are in our bone marrow are actually the bone marrow-derived stem cells," she explains. "The rest are other cell types; so it's called a rare cell type."
Although a stem cell line can be grown, or expanded, in the lab from 100 cells to a million cells, a common problem is that over time the cells change properties. "Instead of them all being stem cells, after several generations of growing in the lab, some of them are stem cells, but many of them are not. So if you want to use them for human therapy, how do you sort out the cells that are still stem cells? You want to do that in a way that you don't have to, ideally, touch the cells or put any other proteins or particles on them to enable the separation. So we sought a method of what's called 'label-free sorting,'" Van Vliet says.
Separating stem cells
In a 2014 paper, Van Vliet and colleagues demonstrated that sought-after bone marrow stem cells could be separated from a large group of cells using a spiral-shaped inertial microfluidic device and sorted by function based on a combination of three identifying characteristics: small cell diameter, low cell stiffness and high nuclear-membrane fluctuations. "We measured those three properties as well as several other properties, but only those three properties together, that triplet of properties, distinguished a stem cell from a non-stem cell," Van Vliet explains.
"We don't have to label the cells, we don't have to put any particles on them, or any fluorescent antibodies on them. Just on the basis of these three physical and mechanical properties, we can say, this fraction of cells grown in the lab are still the stem cells, that you can put back in a human," Van Vliet adds. The study used human cells but tested them in mice.
MIT biological engineering graduate student Frances Liu is continuing stem cell research through the Singapore-MIT Alliance for Research and Technology (SMART) BioSystems and Micro-mechanics (BioSyM) group, of which Van Vliet is lead principal investigator. While the Van Vliet group's expertise is in mechanical characterization of cells, SMART colleague and fellow MIT Professor Jongyoon Han provided his expertise in microfluidics. "We put those two approaches together and developed this approach," Van Vliet explains.
"Even though we now can separate the cells into different categories, there's so much we don't how know about them in terms of how are they different, why are they different, what happens when you put them back in the body? What happens if you expose them to different environments in the lab in terms of what chemicals they produce, what functions they take on," Van Vliet says. "All of that work needs to be figured out in order to find material environments where you can expand these cells to large number without accidentally or inadvertently changing their biological properties. And that's part of what Frances is working on."
Based on their findings with the bone marrow stem cells — specifically, cells called mesenchymal stromal cells — clinical trials are underway in Singapore at various hospitals. "We have pending clinical trials to use these subsets of cells that we've isolated on the basis of their biophysical markers for specific repair processes to affect different diseases," Van Vliet says. Research scientist Zhiyong Poon, a member of the team who lives in Singapore, is taking the lead on using the Van Vliet group's engineering approaches to advance the clinical trial goals.
"It's exciting. This project took five years to make sure we fully understood the system and could really engineer the separation of the cells, but it is very gratifying to see that it's already moving toward clinical trials," Van Vliet says. "These are engineers, like myself, learning how to interact with clinicians and help design studies that make this useful, including needing to redesign, for example, the instruments you're using, so they can be used for human trials. It's not a quick progression of the ideas in the lab to the ideas in the clinic, but we've made good progress."
Mechanically stimulating cell functions
Another study led by research scientist Anna Jagielska focused on stem cells from the central nervous system. "The stem cells that Anna studied are also very sensitive to their environment and to their mechanical cues and their chemical cues, but their goal when they differentiate is to produce myelin, this protein that insulates your neurons so that your nervous system works correctly. There we have a disease context, where there are several conditions such as multiple sclerosis in which the stem cells fail to differentiate and take on the correct functions," Van Vliet explains. "We're trying to understand what are the mechanical cues and the chemical cues that retard that differentiation or inhibit that differentiation, and then how could we engineer the environment so that the cells do what we want, which is to properly insulate the axons (nerve fibers). There are many different stem cell types in the body in adults or in neo-natal animals, and so they all have different possible functions."
Jagielska was lead author of a paper with colleagues from the University of Cambridge in England and the Dresden University of Technology in Germany. "Since we discovered and showed for the first time that these central nervous system stem cells were mechanosensitive, it's going to be a few years before we translate this understanding to clinical applications," Van Vliet says. "But we're making very rapid progress in growing this interdisciplinary team, and Anna has done a great job with that."
Polymers as tissue simulants
Another area of research the Van Vliet group is pursuing is developing polymer-based materials that mimic human tissue for use as experimental substitutes for tissue from the brain, heart, and liver. MIT biological engineering graduate student Bo Qing is studying brain tissue from mice and rats to establish the necessary properties for synthetic material and then designing and testing synthetic materials. The work is supported by the U.S. Army Research Lab, which developed some of the materials considered, through the Institute for Soldier Nanotechnologies.
"If we're going to develop either computational models or experimental models of how the brain responds to high rates of mechanical loading in an accident or a blast wave or a bullet, you need to develop materials that will not degrade, be stable under lab conditions, but also mechanically recapitulate the impact response of the brain tissue," Van Vliet explains. Qing conducts a special type of indentation tests on the tissues and the engineered polymer gels. "Surprisingly, there are not much data out there on the response of the brain tissue to the kinds of experiments we do, which is called impact indentation. ... Bo is generating some of the first data on the brain tissue for these length scales and deformation rates, and then using the same experiments to understand how engineered materials dissipate impact energy."
Qing is working with polydimethylsiloxane-based (PDMS) gels, which are stretchy and transparent. "It's a polymer for which we can easily control properties such as its stiffness by just varying the crosslinker-to-base ratio that we had when we make these polymers," Qing says. "We can tune the composition of the materials that we build into the final system, in order to tune the energy dissipation properties upon impact. Understanding the tunability of the engineered polymers allows us also to simulate the properties of specific biological tissues."
A May 2013 paper examined indentation impacts on rodent heart and liver tissue and compared cross-linked PDMS gels for their ability to mimic the response of natural tissue. It identified specific gel compositions that closely matched the impact response of heart tissue.
"We could make a very, very complicated material that looked just like brain tissue, had multiple layers to it, white and gray matter, but that's not really the point," Van Vliet explains. "The point is to make as simple a material as you can to mimic the mechanical behavior of interest so that you can then manufacture a lot of it at scale."
Moving from the lab to the market, manufacturing any material at an industrial scale can present several challenges. To address this issue for a specific set of so-called "critical" materials, Van Vliet is collaborating with Elsa A. Olivetti, the Thomas Lord Assistant Professor of Materials Science and Engineering, and the MIT Portugal Program. They are studying how best to recover discarded electronic devices and track their critical materials content. MIT graduate student Patrick Ford of the Engineering Systems Division and Department of Materials Science and Engineering is co-advised by Olivetti and Van Vliet, and is leading that work in Van Vliet's Laboratory for Material Chemomechanics this semester.
"It's a very neat opportunity for a case study on how materials are used in a particular technology we all use, and how those materials might be used or recovered differently in the future," Van Vliet says. "This builds on Elsa's expertise in technical and economic aspects of materials recycling, and my interest in the global supply chains for some of the materials we use in today's most advanced technologies."
"Portugal is a smaller, more homogeneous country than the U.S., and also follows E.U. and national targets for material recovery, so you can look at how they handle recycling of electronic devices with a very high level of specificity," Van Vliet adds. "We're taking apart those electronic devices such as smartphones here at MIT, characterizing the material in them and understanding how they make it through the supply chain in Portugal. A lot of the work we are doing is to develop materials or technologies that eventually you want to see adopted in industry and scaled up. Most of the time when engineers design new materials or new devices, they don't think much about where the materials come from in the world or how they're processed in order to reach their lab, but that becomes a problem when you start to produce any device at scale."
Many renewable energy technologies, such as solar cells or electric and hybrid car batteries, rely on key materials that are not stable in either their supply or their availability to manufacturers. When key supplies become disrupted, those materials become "critical." That puts the burden on engineers to give more consideration to sourcing issues.
"If you need a particular material that has very special properties, which is often the case in most of my lab-scale research and development, can you design the device so you can get that material out afterwards and reuse it, if not for that application, then for other applications?" Van Vliet asks. "It's a challenging problem. It's one that we're very interested in from both a technical point of view as well as a policy point of view."