Biomimetic Cilia-based Micro-actuators

This work developed novel biomimetic cilia, investigated models to capture critical fluid-structure interactions; and experimentally demonstrated more than an order of reduction in the time needed for batch micro-mixing.

Background: Critical challenges in emerging bio-fluidic devices lie in bio-compatible transport of small sample volumes and bio-reaction enhancement without damaging biomolecules. This research employs a biomimetric cilia actuated by low frequency excitation (~100Hz) in order to manipulate micro-fluids in a bio-compatible manner. In nature, biological cilia are hair-like structures whose rhythmic beating provides motility for cells and micro-organisms, and hence which transports fluids and particles in biological ducts. We are developing high aspect ratio of polydimethylsiloxane (PDMS) structure to mimic biological cilia and their motions. Applications include transport and mixing at the microscale.

Issue 1: Substantial (1/3) Reduction in Resonance Frequency

Background: The challenge in modeling biomimetic cilia actuators is the coupling between the mechanical dynamics of the cilia and the fluid. Such coupling can lead to damping effects due to drag forces, which change the amplitude and resonant-vibrational frequency of the cilia when operated in liquid in contrast to operation in air or vacuum. However, the experimental results show that the drag or damping effects are not sufficient to explain the substantial reduction in the resonant-vibrational frequency when the cilia actuators are operated in liquid as opposed to the natural frequency when the cilia actuators are operated in air.

Our work Ref 2 showed that the use of an added-mass effect can account for this reduction in the natural frequency when cantilever-type devices are operated in a liquid. The added-mass effect accounts for the inertial loading of the liquid when modeling the vibration of cantilever-type devices in a liquid medium.

The additional mass also tends to reduce the effective damping ratio, which can change the behavior from over-damped to underdamped --- leading to a larger amplitude of vibration.

Issue 2: Sloshing-based Cilia Excitation

Background: Our work showed that Cilia can be substantially excited by low frequency oscillations of the chamber containing the cilia and sample. This low-frequency mechanical excitation of cilia is advantageous for mixing samples that are susceptible to damage from high-frequency excitation and magnetic fields. Modeling requires integrating fluid-structure interactions with sloshing dynamics of the chamber.

Our work Ref 4 developed a model for a cilia-based device, which shows that the liquid sloshing and the added-mass effect play substantial roles in generating large-amplitude motion of the cilia in liquid when the chamber containing the cilia is oscillated to mechanically excite the cilia resonance.

Issue 3: Improving Batch Micro-mixing

Background: In general, micromixing can be improved by generating complex flows in the fluid to overcome the mixing-rate limits of laminar flows that are typical at the microscale. Such flow-type mixing can be used when a sufficiently-large amount of sample is available to achieve the flow through the grooved channel. In contrast, if the amount of sample is limited, then batch-type mixing needs to be achieved in small chambers containing the sample. Batch mixing can be enhanced using a variety of actuation techniques such as high- frequency ultrasound excitation and time-varying external magnetic fields. In the current work, cilia are excited by relatively-low-frequency oscillations of the chamber containing the sample when compared to higher-frequency ultrasound excitation. The low-frequency excitation used in this cilia-based method could reduce the damage of fragile samples that are susceptible to damage from high- frequency excitation.

Our experimental results (Ref 4) show that the average mixing time with cilia is more than one order-of-magnitude lower than the average mixing time without cilia. See presentation slides for highlights of micro-mixing results.

Issue 4: Precision Control to Evaluate Asymmetric Excitation

Background: The goal is to evaluate potential improvements in cilia-based mixing with different excitation waveforms of the cilia-chamber. A challenge in such evaluation studies is that, at high frequencies, vibrations in the piezoactuator can distort the achieved motion (excitation waveform) of the cilia-chamber, and thereby, limit the ability to evaluate the effect of a desired excitation waveform on mixing.

An iterative feedforward approach is used in Ref 5 to account for the vibrational dynamics of the piezoactuator, and reduce unwanted vibrations in the achieved excitation waveforms. These controlled, excitation waveforms were then used to show that the choice of the excitation waveform can improve mixing performance with cilia by about 2.6 times. Thus, the article shows that the choice of the excitation waveform can improve mixing performance with cilia, which suggests the need for further efforts in excitation- waveform optimization.