Microrheology of Biopolymers at Non-thermal Regimes - Tissue Engineering: Computer Modeling, Biofabrication and Cell Behavior - page 107

Fig. 6 Twin optical tweezer system. Dual optical trap system using 1064 nm and 830 nm lasers.
AOD , acousto-optical device; M , mirrors; M3 , steering mirror; PBS , polarizing beam splitters;
BFP O , backfocal plane objective; DM , dichroic mirror; TL , telescope lens; f , focal length; QPD ,
quadrant photodiode; L ,lens; BFP C , backfocal plane condenser; DIC , differential interference
contrast microscopy; S , sample; CCD , charge-coupled device camera
for the intended unfolding applications (Fig. 6 ). The design for the twin optical trap
system is based on Moffit et al. (2006), which utilized a rotating mirror with the po-
larizing beam splitters to separate one laser beam for steering and recombining the
steered beam and the other beam before the microscope [ 37 ]. However, to approach
steering at frequencies above 100 Hz, our novel design inserts an acousto-optical
device to steer one of the beams, while the other beam is steered using the rotating
mirror, which is capable of linear mechanical steering only within 100 Hz (Fig. 6 ).
Thus, both beams are steered. The limitation however in the use of the same wave-
length for trapping and monitoring is the cross-talk at detection (using quadrant
photodiodes, QPDs) [ 24 , 38 ].
The techniques derived in this paper (various microrheology modes and exper-
imental schemes) provide robust tools for characterizing mechanical properties of
soft samples that would lead to insights in fundamental biological functions, which
are at the forefront for understanding health. However, the applications are further
easily extended to soft matter in general, which are not necessarily of biological ori-
gin. Furthermore, since the approach is performed at very small volumes (micro-liter
range), the benefits are directed specifically where large bulk samples are not avail-
able. Results coming from the study of mechanics at microscopic scales surrounding
living cells, down to the investigations of unfolding in protein units provide insights
on the workings of living matter and therefore opens the avenue for finding novel
non-linear and non-equilibrium physical phenomena.
Acknowledgements The research work reported in this paper was partially funded by the Uni-
versity of San Carlos (USC, Cebu City, Philippines) Research Office and the Department of Physics
(USC).