In 1970, Arthur Ashkin reported that spatial variations in beams of light could apply forces to microscopic particles. Since that discovery, this effect, now known as optical tweezers has enabled the growth of an active research community developing the theory and application of optical micromanipulation, especially in the context of biology and biomedical engineering where the objects of interest: cells, bacteria, and even DNA are at a size scale suitable for non-invasive probing and manipulation by suitably prepared beams of light. The light that is used is often in the form of tightly focussed laser beams or beams with complex structure generated by spatial light modulators. In 1996 Professor Cronin-Golomb spent a sabbatical at Stanford University in Steve Chu’s lab where he first gained experience with optical tweezers and noted its relationship to photorefractive nonlinear optics, his first area of research.
At Tufts, the lab’s main line of research has been in the use of optical tweezers for non-invasive measurement of the viscoelastic and rheological properties of fluids and biomaterials, most recently in a collaboration with Irene Georgakoudi and Ana Soto for spatially resolved measurements of the stiffness of the extracellular matrix in three dimensional cultures of mammary gland tissue. This is for the purpose of gaining an understanding of the mechanical cues thought to play a substantial role in determining cell differentiation and mammary gland development in the context of cancer research. The approach involves seeding the cell culture with polystyrene or silica microspheres for use as probe particles. The response of these embedded particles to forces applied by optical tweezers is used to measure local mechanical properties such as Young’s modulus.
Our fully equipped optics lab includes an inverted microscope optical tweezers apparatus with a Labview enabled computer interface which provides opportunities for research in automatic particle tracking and measurement using image analysis and closed loop tracking via galvanometer and piezoelectric beam steering. Measurements are made via phase sensitive detection of forward and backscattered light from micron sized probe particles embedded in the matrix. We have also been using one of Irene Georgakoudi’s two-photon scanning microscopes to use the signal from fluorescently tagged probe particles, while simultaneously imaging the collagen matrix via second harmonic generation.
At the same time, we are developing theoretical background for the measurements by using finite element techniques to model the reponse of microspheres to optical forces in materials with known rheological properties.
Tweezing a bead in gel
Click on the image to see an animated model of a 0.25 micron polystryrene sphere forced by a 25mW near infrared laser beam in a soft gel.
Nonlinear Optics in Biomaterials
There is a long history of the use of the photoisomerizable molecule azobenzene to photosensitize polymers for optical recording and optical holography. In 2008 Amanda Murphy, Peter St. John, and David Kaplan: members of the silk group in our department showed that the tyrosine amino acids in silk fibroin could be functionalized via diazonium coupling to form azobenzene side groups. In 2012 we showed that thin films of silk functionalized in this way behaved in the same way as previously studied azo-polymers. Azosilk shows optically induced birefringence and posesses the ability to act as a holographic recording medium, complete with the surface relief gratings commonly associated with holographic recording in other azopolymers. We were also able to show that a silk-elastin like protein (SELP) could be functionalized with retinal via linkage to the amino acid lysine in the SELP backbone. Retinal, like azobenzene, is photoisomerizable and displays many of the same effects as azobenzene, but is more biocompatible: it forms the basis of the visual system. In 2013-2014 Professor Cronin-Golomb spent a sabbatical year at McGill University in the Chemistry Department with Professor Christopher Barrett, an expert in azobenzene chemistry and optics. There, a collaboration with McGill’s Advanced BioImaging Facility resulted in the demonstration of the use of a two-photon microscope to make lithographically patterned modifications to azosilk film gels. These modifications include fluorescence patterning and the production of hollow microfuidic chambers suitable for developing microfluidic devices and surface patterning to guide cell growth.