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Fiber-based dual-beam optical trapping platform for stretching lipid vesicles


Dual-beam fiber trapping is a versatile technique for manipulating micron-sized particles, biological assemblies, and living cells. Here, we fabricate fiber-based trapping platforms from plexiglass chips of polycarbonate and cast acrylic. Single-mode fibers from 980 nm fiber-pigtailed laser diodes are arranged in a counter-propagating configuration and situated into alignment. Fiber alignment is verified via coupling efficiency experiments whereby measured efficiency profiles assure proper alignment for trapping.

The initial stages of this project involved fabrication and testing of these trapping platforms. We calibrate the range of forces exerted on trapped polystyrene microspheres in water by suddenly offsetting one of the laser power outputs. The sudden offset causes a trapped microsphere to undergo a small displacement from the trap center. The displaced particle as a function of time follows a characteristic curve of over-damped harmonic motion. We then extrapolate the related spring constants which is proportional to the scattering force acting along the beam axis. Measured spring constants are in the range of 104-145 nN/m which are in agreement with expected values.

Subsequently, we demonstrate for the first time, stable trapping and stretching of low contrast giant lipid vesicles in solution. Trapped vesicles encapsulate poly(ethylene) glycol solution and are trapped with femtonewton range forces. We analyze the lipid membrane response to a range of optical stresses that initiate membrane stretching. At increasingly higher powers, we observe elongation of a vesicle along the beam axis (vesicle major axis) and contraction in the transverse direction (vesicle minor axis). The peak stress exerted along the beam axis is calculated for each incident power. Lipid vesicles stretch in a linear fashion for relative deformations up to 25%, correlating to applied peak stresses ranging from 7-60 mN/m2. Nonlinear deformations are observed at even higher applied optical stress. These experiments demonstrate the capabilities for trapping a variety of low-contrast, soft biomaterials.

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