Red blood cells (RBC) circulate the human body several hundred thousand times in their life span. Therefore, their deformability is really important, especially when they pass through a local capillary whose diameter can be as narrow as 3 μm. While there have been a number of works discussing the deformability in a simulated capillary such as a microchannel, as far as we examined in the literature, no work focusing on the change of shape after reciprocated mechanical stress has been reported so far. One of the reasons is that there have been no appropriate experimental systems to achieve such a test. This paper presents a new concept of RBC fatigue evaluation. The fatigue state is defined by the time of reciprocated mechanical stress when the extensibility and the recoverability characteristics meet each other. Our challenge is how to construct a system capable of achieving stable and accurate control of RBCs in a microchannel. For this purpose, we newly introduced two fundamental components. One is a robotic pump capable of manipulating a cell in the accuracy of ±0.24 μm in an equilibrium state with a maximum response time of 15 ms. The other is an online high speed camera capable of chasing the position of RBCs with a sampling rate of 1 kHz. By utilizing these components, we could achieve continuous observation of the length of a RBC over a 1000 times reciprocated mechanical stress. Through these experiments, we found that the repeat number that results in the fatigue state has a close correlation with extensibility.

This paper proposes a new index for evaluating the stiffness-based deformability of a cell using a microchannel. In conventional approaches, the transit time of a cell through a microchannel is often utilized for the evaluation of cell deformability. However, such time includes both the information of cell stiffness and viscosity. In this paper, we eliminate the effect from cell viscosity, and focus on the cell stiffness only. We find that the velocity of a cell varies when it enters a channel, and eventually reaches to equilibrium where the velocity becomes constant. The constant velocity is defined as the equilibrium velocity of the cell, and it is utilized to define the observability of stiffness-based deformability. The necessary and sufficient numbers of sensing points for evaluating stiffness-based deformability are discussed. Through the dimensional analysis on the microchannel system, three dimensionless parameters determining stiffness-based deformability are derived, and a new index is introduced based on these parameters. The experimental study is conducted on the red blood cells from a healthy subject and a diabetes patient. With the proposed index, we showed that the experimental data can be nicely arranged.

Large deformability of erythrocytes in microvasculature is a prerequisite to realize smooth circulation. We develop a novel tool for the three-step “Catch-Load-Launch” manipulation of a human erythrocyte based on an ultra-high speed position control by a microfluidic “robotic pump”. Quantification of the erythrocyte shape recovery as a function of loading time uncovered the critical time window for the transition between fast and slow recoveries. The comparison with erythrocytes under depletion of adenosine triphosphate revealed that the cytoskeletal remodeling over a whole cell occurs in 3 orders of magnitude longer timescale than the local dissociation-reassociation of a single spectrin node. Finally, we modeled septic conditions by incubating erythrocytes with endotoxin, and found that the exposure to endotoxin results in a significant delay in the characteristic transition time for cytoskeletal remodeling. The high speed manipulation of erythrocytes with a robotic pump technique allows for high throughput mechanical diagnosis of blood-related diseases.

This paper presents experimental investigations of passive mixing in a microfluidic channel with different zigzag angles. Zigzag channel is commonly used for microfluidic mixing because it does not need an additional control unit and can be easily implemented in a lab-on-a-chip system. In this work, microfluidic channels with six different zigzag angles, from $\theta$ = 0° to $\theta$ = 75°, are tested under ten different flow rates corresponding to Reynolds number from 0.309 to 309. Two colored liquids are mixed with the zigzag channels and mixing performance is evaluated based on the color of the pixels on the region of interest from captured images. According to the results, we found that the mixing performance is almost independent of the zigzag angle in the low-speed regime where its Reynolds number is less than 4. The mixing became very much depending on the zigzag angle in the high-speed regime where its Reynolds number is greater than 100. Microfluidic mixing is needed for Lab-on-a-chip applications in both low flow speed, such as medium perfusion for cell culture, and high flow speed, such as high-speed sensing on a point-of-care device. This work is aimed to provide practical information on zigzag mixing for chip design and applications.

An on-chip deformability checker is proposed to improve the velocity–deformation correlation for red blood cell (RBC) evaluation. RBC deformability has been found related to human diseases, and can be evaluated based on RBC velocity through a microfluidic constriction as in conventional approaches. The correlation between transit velocity and amount of deformation provides statistical information of RBC deformability. However, such correlations are usually only moderate, or even weak, in practical evaluations due to limited range of RBC deformation. To solve this issue, we implemented three constrictions of different width in the proposed checker, so that three different deformation regions can be applied to RBCs. By considering cell responses from the three regions as a whole, we practically extend the range of cell deformation in the evaluation, and could resolve the issue about the limited range of RBC deformation. RBCs from five volunteer subjects were tested using the proposed checker. The results show that the correlation between cell deformation and transit velocity is significantly improved by the proposed deformability checker. The absolute values of the correlation coefficients are increased from an average of 0.54 to 0.92. The effects of cell size, shape and orientation to the evaluation are discussed according to the experimental results. The proposed checker is expected to be useful for RBC evaluation in medical practices.

A microfluidic design for evaluating red blood cell deformability with geometrical alignment mechanism is proposed.