Here, I highlight aspects of my education, work, and research experience, and share some of my current research interests. For a full summary and complete list of publications, please refer to my CV.
My formal scientific research career spans over a decade,and is summarized in the timeline below. (Click on image for full-size version.)
Educational background and early career
My B.S. and Ph.D. degrees are both in mechanical engineering. My undergraduate curriculum was traditional, and prepared me adequately for an entry-level position as a design engineer in the manufacturing sector. My primary responsibilities were to design custom flexible shafts for diverse applications (e.g., industrial, automotive, aerospace, and medical), develop the manufacturing and inspection procedures for each product I designed, and oversee their quality control. I gained a tremendous amount of knowledge in the areas of manufacturing and quality assurance, and acquired some valuable skills. Although I was promoted to the position of Quality Assurance Manager after two years and appreciated the experience from that brief stint in management, I decided not to stray far from my original field. I stayed with the company as a part-time engineer and pursued a graduate degree in engineering on a full-time basis.
(A flexible shaft can be used to transmit rotatry motion around obstacles. For more, visit www.sswt.com)
Despite specializing in the area of design in graduate school, my coursework consisted of an eclectic mix of topics. Among the most memorable were: Random Vibrations, Methods of Applied Mathematics, Optimal Design, Advanced Thermodynamics Theory, Advanced Heat Transfer, Advanced Mechanics of Materials, Analytical Dynamics, and Polymer Science and Engineering. As a teaching assistant for several years during my graduate studies, I also gained deeper insight into fundamental engineering concepts and first realized my aptitude for teaching.
Graduate research
Biomechanics of the spine
My research career started in the field of spine biomechanics. Working in the Department of Orthopaedics at the New Jersey Medical school, I was part of an multidisciplinary team that studied the injury mechanism of acute thoracolumbar burst fractures (fractures of the vertebrae in the lower back that often result in spinal cord compromise) and the effectiveness of vertebroplasty (injection of bone cement into fractured vertebrae to restore height and augment structural integrity). Working with orthopaedic surgeons, biologists, biochemists, and engineers in the clinical research setting taught me the significance of effective cross-disciplinary communication. Performing experiments on biological specimens — human cadavers, no less (!) — was also an eye-opening experience. This work culminated in a well-received article in Spine journal, in which we correlated mode of loading and facet joint orientation with extent of injury.
(Left: Model of facet orientation and load transmission through the vertebral body and facet joints. Right: X-ray of acute thoracolumbar burst fracture produced experimentally by maximally loading the facet joints.)
Further reading:
Langrana, N.A., Harten, R.D., Lin, D.C., Reiter, M.F., and Lee, C.K. “Acute thoracolumbar burst fractures: a new view of loading mechanisms”, Spine 27:498-508, 2002.
DNA-crosslinked gels
The use of short strands of deoxyrobonucleic acid (DNA), or oligomers, to crosslink polymer chains was first conceived of by Drs. Bernard Yurke and Allen Mills of Bell Laboratories (then a part of Lucent Technologies). During my Ph.D. studies, we initially explored the use of this gel as an artificial replacement for the nucleus pulposus of the intervertebral disks. That simple idea grew into an elaborate research project that resulted in my Ph.D. dissertation, numerous publications, and the development of a new class of responsive or “smart” materials.
In their simplest form, DNA-crosslinked gels consist of end-modified strands of DNA that are incorporated into long polymer chains. If half of all the polymer chains contains oligomers of some sequence, and the other half contains oligomers of the complementary sequence, then the complementary strands will hybridize (or mutually attract and bind together like a zipper into the well-known double helical structure), thereby joining the strands together to form a meshlike network that is is hallmark of a polymer gel.
(Comparison of a covalently crosslinked gel (left) and one crosslinked using DNA (right). In the DNA-crosslinked network, three strands of DNA form the crosslink: two short strands attached to the polymer chains and a third “crosslinking” strand that is complementary to the two short strands.)
In most gels, these crosslinks are chemical (relying on covalent bonds) or physical (relying on entanglements or other mechanisms such as hydrogen bonding). The use of DNA as the crosslinking agent offers a number of advantages and introduces some intriguing possibilities:
• Because DNA strands dehybridize or separate at elevated temperatures, the gels can be melted. As the temperature is lowered, the strands hybridize again. The cycle can be repeated indefinitely.
• The temperature at which a DNA duplex dehybridizes depends on the length and base sequence, the melting temperature of the gels can be engineered by specifying an appropriate base sequence. Since DNA is made from only four bases (Adenine, Cytosine, Guanine, and Thymine), random sequences are simple to generate.
• The mechanical properties of the gel are affected by the length of the crosslinks since short DNA duplexes act as stiff rods. Hence, mechanical properties can also be engineered.
• “Competitive binding” occurs whenever two strands of DNA both have portions whose sequences are complementary to a third strand. The pair that has a larger number of matching bases wins out. This mechanism can be used to design crosslinks whereby the presence of a competitor (or “removal”) strand displaces one of the strands of the crosslink and disassembles the gel.
In a series of experiments, the results of which have all been published, the unique properties outlined above were all verified. Perhaps most interesting was one in which a gel with long, slack (e.g., single-stranded) crosslinks was stiffened by introducing stiffening or “fuel” strands of DNA that bound with the crosslinks. Via competitive binding, the process was reversed by adding another DNA sequence that removed the stiffening strands from the crosslinks.
(Reversibly stiffening a DNA-crosslinked gel. (a) crosslink has a long, slack single stranded region; (b) fuel strand hybridizes with the crosslink to stiffen it and generate a prestress in the network; (c) removal strand displaces fuel strand via competitive binding; (d) gel is restored to initial state, with waste product formed.)
Further reading (including papers in which we derived the elasticity equations that served as the basis for our measurements of gel mechanical properties):
Lin, D.C., Yurke, B., and Langrana, N.A. “Use of rigid spherical inclusions in Young’s moduli determination: application to DNA-crosslinked gels”, Journal of Biomechanical Engineering 127:571-579, 2005.
Lin, D.C., Yurke, B., and Langrana, N.A. “Inducing reversible stiffness changes in DNA-crosslinked gels”, Journal of Materials Research 20:1456-1464, 2005.
Lin, D.C., Langrana, N.A., and Yurke, B. “Force-displacement relations for spherical inclusions in finite elastic media”, Journal of Applied Physics 97:043510, 2005.
Lin, D.C., Yurke, B., and Langrana, N.A. “Mechanical properties of a reversible, DNA-crosslinked polyacrylamide hydrogel”, Journal of Biomechanical Engineering 126:104-110, 2004.
Postdoctoral research
Following my Ph.D. work, I joined the National Institutes of Health (NIH) in Bethesda, Maryland as a postdoctoral fellow in the Section on Tissue Biophysics and Biomimetics (STBB). During my four years at the NIH (and for another year afterwards as a visiting scientist), I made contributions to several applied and basic sciences, including elasticity, polymers, and surface probe microscopy.
The overarching aim of my work in STBB was to gain a deeper understanding of the relationship between cartilage physical properties (namely mechanical, microstructural, and compositional) and function. Particularly, we wanted to determine, both quantitatively and qualitatively, how these properties change with the onset of disease (e.g., osteoarthritis) or as the tissue develops. Development and refinement of methodologies and analytical models were an essential component of this extensive research project, which is represented in the schematic below. (Click on image for full-size version.)
Atomic force microscopy
The atomic force microscope (AFM) has become ubiquitous in surface characterization, both in the industrial and research settings. Attractive features of the AFM include the elimination of sample size constraints, the ability to concurrently perform multiple functions (e.g., imaging and surface roughness measurements), and the ability to maintain biological samples under physiologic conditions during testing. Its use as a micro- or nano-indenter to measure local mechanical (i.e., elastic) properties is among the many capabilities of this versatile instrument. However, several hinderances (both methodological and analytical) to accuracy and reproducibility exist. Many research efforts have sought to resolve these issues, but accurate analysis of AFM data remains largely ambiguous.
Teaching
I currently teach engineering mechanics (Statics, Dynamics, and Mechanics of Materials) along with C++ programming at the undergraduate level. I enjoy teaching, and both internal and external reviews say that I’m good at it.
Other research interests
Initial foray into entrepreneurship
From 2011 to 2012, while teaching, I also served as Director of Technology Research and Development at a biomedical devices startup. My work concentrated on two technologies: one for noninvasive blood glucose monitoring, and the other for breast cancer detection. These two areas have great market potential, but are fraught with obstacles. I was involved in product development and responsible for grant writing. Our SBIR proposal received very good reviews. This rather brief stint was a great experience, and led to my current endeavors.
Lin and Keller Exercise Innovations
Contacts are everything when it comes to entrepreneurship. I consider myself fortunate to have partnered with an extremely bright physical therapist to start up a company focused on helping the elderly and disabled attain a better quality of life through exercise. We are designing innovative products that meet the core criteria of ease of use, accessibility, and affordability. Currently, we have three products in development, all of which are in the prototype stages.
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David C. Lin, Ph.D. 