UCLA

Manipulation of biological micro-objects in vitro is essential for many biomedical applications, such as the study of cells’ or molecular interaction, single-cell analysis, drug delivery, and tissue engineering. There are various physical mechanisms employed to achieve the manipulation; electrokinetic, optical, magnetic, hydrodynamic, and acoustic mechanisms are all conventional examples that have been deeply researched. In comparison, acoustics has been demonstrated to have superior biocompatibility and wide range of operable sizes of target, making it an attractive option to be widely utilized in many applications. On the other hand, acoustics suffers from limited Degree-of-Freedom (DOF) in objects’ manipulation due to the fundamental constraint stemmed from the principle that conventional acoustic devices built upon. Furthermore, acoustic patterning of micro-objects that allows not only real-time, versatile adjustment but also re-configuration to the patterning profile has not yet been realized. This re-configurable feature is particularly crucial in taking the acoustic manipulation technology from the current research phase to the next stage for broader applications.

For this dissertation, two important concepts are investigated and realized to overcome the acoustic limitations described. The first concept explores a new field of acoustofluidics where a novel manipulation platform is developed, making a breakthrough to the conventional approach in objects’ manipulation in achieving highly complex and non-periodic patterning shapes. The second concept involves integration of a photothermal mechanism with the developed acoustic platform which allows the acoustic potential field to be adjusted and re-configured, leading to a programmable platform for highly complex patterning of micro-objects.

In the first concept, a new acoustofluidic field using deep, sub-wavelength approach is exploited. Different from the traditional techniques that rely on using standing waves to generate the acoustic potential wells only spaced periodically at half the wavelength, the new approach can generate the acoustic wells spaced arbitrarily within half the wavelength. To achieve such approach, we developed a “Complaint Membrane Acoustic Patterning” (CMAP) platform utilizing an air-embedded, viscoelastic Polydimethylsiloxane (PDMS) structure to precisely control the wave fronts by providing barriers to incoming waves using acoustic impedance mismatch between air and PDMS, creating a non-uniform energy field and thus the acoustic wells. The embedded air cavities dictate the shape of the wells. Since the cavities can be fabricated into any geometry, complicated profiles of the wells can be realized. As experiments have demonstrated, we succeeded in the patterning of micro-polystyrene beads and HeLa cells into various numeric-letter shapes with resolution one tenth of the wavelength across a large 3 x 3 mm2 area.

In the second concept, a photothermal mechanism is integrated into the CMAP platform to achieve re-configurable patterning of micro-objects. A layer of light absorbing hydrogenated amorphous silicon (a-Si:H) is incorporated into the bottom of CMAP’s PDMS structure, where the cavities are initially fluid-filled. By focusing a laser light onto the a-Si:H layer to evaporate the fluid within the cavities selectively, we can modify where in the PDMS structure would contain fluid or air. Cavities containing air can also be reverted to contain fluid. Since the acoustic impedance of PDMS matches closely to that of water, acoustic waves could pass through easily. This way, we have the versatility in shaping and re-shaping an array of air cavities, permitting the generation of adjustable potential profile of the acoustic wells which leads to field-programmable functionality for highly complex and non-periodic patterning.