Advancements in technologies for assessing biomechanics at the cellular level have led to discoveries in mechanotransduction and the investigation of cell mechanics as a biomarker for disease. With the recent development of an integrated optical tweezer with micron resolution particle image velocimetry, the opportunity to apply controlled multiaxial stresses to suspended single cells is available (Nève, N., Lingwood, J. K., Zimmerman, J., Kohles, S. S., and Tretheway, D. C., 2008, "The µPIVOT: An Integrated Particle Image Velocimetry and Optical Tweezers Instrument for Microenvironment Investigations," Meas. Sci. Technol., 19(9), pp. 095403). A stress analysis was applied to experimental and theoretical flow velocity gradients of suspended cell-sized polystyrene microspheres demonstrating the relevant geometry of nonadhered spherical cells, as observed for osteoblasts, chondrocytes, and fibroblasts. Three flow conditions were assessed: a uniform flow field generated by moving the fluid sample with an automated translation stage, a gravity driven flow through a straight microchannel, and a gravity driven flow through a microchannel cross junction. The analysis showed that fluid-induced stresses on suspended cells (hydrodynamic shear, normal, and principal stresses in the range of 0.02-0.04 Pa) are generally at least an order of magnitude lower than adhered single cell studies for uniform and straight microchannel flows (0.5-1.0 Pa). In addition, hydrostatic pressures dominate (1-100 Pa) over hydrodynamic stresses. However, in a cross junction configuration, orders of magnitude larger hydrodynamic stresses are possible without the influence of physical contact and with minimal laser trapping power.
Chondrocytes and osteoblasts experience multiple stresses in vivo. The optimum mechanical conditions for cell health are not fully understood. This paper describes the optical and microfluidic mechanical manipulation of single suspended cells enabled by the μPIVOT, an integrated micron resolution particle image velocimeter (μPIV) and dual optical tweezers instrument (OT). In this study, we examine the viability and trap stiffness of cartilage cells, identify the maximum fluid-induced stresses possible in uniform and extensional flows, and compare the deformation characteristics of bone and muscle cells. These results indicate cell photodamage of chondrocytes is negligible for at least 20 min for laser powers below 30 mW, a dead cell presents less resistance to internal organelle rearrangement and deforms globally more than a viable cell, the maximum fluid-induced shear stresses are limited to ~15 mPa for uniform flows but may exceed 1 Pa for extensional flows, and osteoblasts show no deformation for shear stresses up to 250 mPa while myoblasts are more easily deformed and exhibit a modulated response to increasing stress. This suggests that global and/or local stresses can be applied to single cells without physical contact. Coupled with microfluidic sensors, these manipulations may provide unique methods to explore single cell biomechanics.
A novel instrument to manipulate and characterize the mechanical environment in and around microscale objects in a fluidic environment has been developed by integrating two laser-based techniques: micron-resolution particle image velocimetry (μPIV) and optical tweezers (OT). This instrument, the μPIVOT, enables a new realm of microscale studies, yet still maintains the individual capabilities of each optical technique. This was demonstrated with individual measurements of optical trap stiffness (∼70 pN μm −1 for a 20 μm polystyrene sphere and a linear relationship between trap stiffness and laser power) and fluid velocities within 436 nm of a microchannel wall. The integrated device was validated by comparing computational flow predictions to the measured velocity profile around a trapped particle in either a uniform flow or an imposed, gravity-driven microchannel flow (R 2 = 0.988, RMS error = 13.04 μm s −1 ). Interaction between both techniques is shown to be negligible for 15 μm to 35 μm diameter trapped particles subjected to fluid velocities from 50 μm s −1 to 500 μm s −1 even at the highest laser power (1.45 W). The integrated techniques will provide a unique perspective toward understanding microscale phenomena including single-cell biomechanics, non-Newtonian fluid mechanics and single particle or particle-particle hydrodynamics.
The Invention Bootcamp is a four-week interdisciplinary program where twenty-five high school students underrepresented in STEM (Science, Technology, Engineering and Math) are invited to discover and experience the worlds of engineering, innovation, and entrepreneurship in a college setting. The course creates, deploys and tests in the field a new educational approach to inspire future inventors. In addition to teaching STEM skills in a hands-on and collaborative manner, the course presents high school students with role models in the form of undergraduate mentors, instructors, researchers, and guest speakers in class and during field trips. The course thus helps empower them, helps them gain confidence in the classroom, but also experience a foretaste of being a college student. By the end of the pilot course in Summer 2016, we asked students if they felt they could be engineers or inventors in the future. A strong majority (91%) agreed they could. Several aspects of the bootcamp are unique, and we would like to share the key learnings. They include: 1) The application process, which was based on non-cognitive variables. No grades were required. Applicants needed to deliver a 2-min video showing their motivation and how they would improve their school cafeteria. Students needed to have a curiosity towards STEM fields and the invention process. A recommendation letter was also needed.2) The population targeted, which is underrepresented students in STEM such as minorities, women, and low income students. 3) The hiring and training of eight undergraduate mentors and a mentor coordinator. We had one mentor per group of three high school students. The mentor program created a supportive environment to provide students with the emotional, academic and technical support they needed to be successful in this course. By offering close, near-peer support, we enhanced student learning, classroom effectiveness, and retention of students. The majority of mentors was in the classroom with students for the entire program. They all are engineering students with a strong engineering background, and a good attitude under stress and in groups. 5) The hands-on curriculum, that meshed engineering tools (soldering iron, milling machine, hand tools, laser cutter, 3D printer), visit of guest lecturers (local entrepreneurs and innovators), and work on group projects using a human-center design thinking approach.
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