Particle Manipulation by Optical Forces in Microfluidic Devices

Abstract

Since the pioneering work of Ashkin and coworkers, back in 1970, optical manipulation gained an increasing interest among the scientific community. Indeed, the advantages and the possibilities of this technique are unsubtle, allowing for the manipulation of small particles with a broad spectrum of dimensions (nanometers to micrometers size), with no physical contact and without affecting the sample viability. Thus, optical manipulation rapidly found a large set of applications in different fields, such as cell biology, biophysics, and genetics. Moreover, large benefits followed the combination of optical manipulation and microfluidic channels, adding to optical manipulation the advantages of microfluidics, such as a continuous sample replacement and therefore high throughput and automatic sample processing. In this work, we will discuss the state of the art of these optofluidic devices, where optical manipulation is used in combination with microfluidic devices. We will distinguish on the optical method implemented and three main categories will be presented and explored: (i) a single highly focused beam used to manipulate the sample, (ii) one or more diverging beams imping on the sample, or (iii) evanescent wave based manipulation.

Keywords: microfluidics; optical manipulation; optical stretcher; optical trap; optical tweezers; optofluidics.

Figure 1

(a) Inset: Scheme of λ-DNA molecules linked by H-NS suspended between polystyrene beads held with optical tweezers; Dynamic force spectrum of the H-NS–DNA interaction. The two distinct regions in the Spectrum are fitted to straight lines. (Reprinted with permission from Springer Nature [50]); and, (b) Scheme of a lab-on-a-chip sorting procedure with optical tweezers (Reproduced from [51] with permission of the Royal Society of Chemistry).

Figure 2

(a) Design of the gear microfluidic pump showing bead movement together with the screen shoots of 3 µm silica beads 2 Hz rotation in a 6 µm channel (From [59]. Reprinted with permission from AAAS); (b) Design of a micropump with the twin spiral microrotor, there’s also shown the Scanning electron microscope image of the polymerized microrotor (From [60]. Reproduced with permission of OSA); (c) Dependence of rotational frequency with trap power for Archimedes micropumps with different numbers (N), in the inset there’s a microscope image of a polymerized microscrew (height 5 µm) (From [61]. Reproduced with permission of OSA); (d) The traced path of a 1 µm silica particle being pumped through a 15 µm wide PDMS channel by the two trapped Vaterite beads. The colour of the trace changes as the particle accelerates and then decelerates through the channel due to the pumping effect (Reproduced from [62] with permission of de Royal Society of Chemistry).

Figure 3

Panel (a) shows a scheme of the dual beam optical manipulation approach, with optical fibers used to deliver light to the microfluidic channel. Panels (b) to (e) show different devices for particles trapping and stretching. In panel b the scheme of the Assembled Optical Stretcher is reported (image reproduced with permission from [70]). Panel (c) shows the design proposed by Faigle et al., constituted by two asymmetrically etched substrates [71], published by the Royal Society of Chemistry. Panel (d) shows the disposable version of the Optical Stretcher (OS) proposed by Matteucci et al. (reproduced from [72]). Panel (e) reports the picture and the microscope image of the monolithic device presented by Yang et al. fabricated by FLICE (reproduced from [73] with permission from the Royal Society of Chemistry). Red traced lines indicate the waveguides position, scale bar is 100 µm.

Figure 4

Panel (a) shows the microscope image of the integrated fluorescence activated cell sorter, presented by Bragheri et al. (reproduced from [97] with permission from the Royal Society of Chemistry). The waveguide used for fluorescence excitation (FWG) and the sorting waveguide (SWG) are highlighted by white arrows. The corresponding device validation is reported in panel (b). Panel (c) shows the scheme of an optical cell rotator, which using a non-rotationally symmetric beam is capable to rotate a particle. Panel (d) shows the different phase masks applied to the spatial light modulator and the corresponding supported mode at the fiber output. Changing the mask it is possible to rotate the mode and therefore the trapped specimen (reproduced with permission from [100]).

Figure 5

(a–d): One-dimensional (1-D) photonic crystal for particle trapping (Reprinted with permission from [119]. Copyright 2012 American Chemical Society: scanning electron microscope image of the resonant cavity (a), numeric simulation of the electric field intensity in the center of the cavity highlighted with a red dashed rectangle in the panel (b), schematics of the setup for trapping experiments (c,d). (e) Drawing illustrating how the rotation of electric field polarization results in the excitation of the structure aligned perpendicularly to it, moving the PS sphere towards the position of minimum optical potential (Reprinted with permission from [121]. Copyright 2014 American Chemical Society).