(J-397) Reverse and hybrid liquid displacement for strain driven flow in microfluidic wearables

Introduction::

Microfluidic strain sensors have been used for intraocular pressure measurement [1] and for human movement recognition [2,3] in our group. These sensors dilate, or expand, when subjected to tensile strain. Negative pressure pulls air from the outlet to the channels that causes a pulling of the liquid.

In this study, we explore an innovative approach to microfluidic strain sensors that typically rely on volumetric expansion. Instead, we leverage mechanical metamaterials [4] to form a channel. The bell-shaped conformal membrane structures provide Poisson's ratio beyond 0.5 and a reverse (push) liquid displacement due to the negative volumetric strain generated when subjected to a tensile strain. Additionally, we introduce a hybrid model, featuring both push and pull actions, due to the flattening of the bell-shaped structure, leading to a positive volumetric strain along the rectangular part at high strains.

3D printing and modified thermal bonding methods are used to achieve the bell-shaped structure using several design iterations, which can push up to 0.1 µl liquid every 1% strain with a device dynamic range of 1.5-15% strain.

We designed skin-mounted chips to demonstrate biomedically relevant applications. In one chip, a human movement-driven, powerless bioreaction that facilitate mixing is demonstrated highlighting sequential operation. In another case, reverse liquid displacement is used as a NOT operator in an integrated sensor. This research paves the way for exciting opportunities in various fields such as drug delivery and movement detection, promising enhanced research capabilities and practical implementations for advanced fluid manipulation and sensing technologies.

Materials and Methods::

Device structure and operation principle

Figure 1 depicts the device schematic. Inlets sealed using UV curable adhesive and epoxy after filling with conductive liquid or DI water-based solution combined with 0.1% Tween 20. Tensile strain deforms the channel in standard characterization experiments (Figure 1). Using bell-shaped auxetic structures, Poisson’s ratio of around 0.8 is achieved (Figure 1).

Fabrication methods

Using two fabrication methods, conformal bell-shaped structures are produced. First, 3D-printed two-piece-clicking molds are used. Figure 2a shows polydimethylsiloxane (PDMS) (10:1) poured between molds to make the conformal structure. Patterned PDMS is adhered to sticky, half-cured blank PDMS. Then baked, filled and sealed.

We also introduce a modified thermal bonding method (Figure 2b). This technique is crucial for fabricating conformal devices in wearable technologies, overcoming the limitations of minimum printable sizes in 3D printing that can be applied to soft lithography molds utilized in sensor technologies for higher spatial resolution. The PDMS (10:1) is spin coated on the mold, cured, then peeled off. The patterned PDMS is baked after bonding with a sticky, half-cured (20:1) blank PDMS. The conformal geometry comes from thermal shrinkage between the two PDMS layers.

 

 

Results, Conclusions, and Discussions::

Conformal devices in Figure 3 exhibit negative volumetric strain when induced in the orthogonal direction to the sensor (Figure 1). This reverses the liquid displacement, pushing it toward the outlet. In contrast, non-conformal devices pull liquid due to volume expansion. These microchannels demonstrate a linear response to applied strain.

In microchannels with rectangular bases with bell-shaped structures (Figure 4b), a hybrid response is observed. At lower strain values, liquid displacement is reversed, pushing liquid toward the outlet. As strain increases, the bump height decreases and eventually flattens, leading to positive volumetric strain along the rectangular part of the microchannel. This results in a fluid displacement in the negative direction (pull) after reaching 7-8% strain.

 

Using rectangular base soft photolithography molds (Figure 5-Ⅰb), commonly employed for fabricating wearable sensors due to their micro-level fabrication sizes, we successfully obtained bell-shape with the proposed thermal bonding method. Then we affixed them to the skin using adhesive to demonstrate their potential application. When the person extends their wrist, the liquid is pushed out from the reservoirs, as depicted in Figure 5-Ⅰ. We achieved both reverse and hybrid liquid displacement.

We combined proposed push reservoirs with rectangular non-conformal reservoirs to achieve a highly repeatable movement action (Figure 5-Ⅱ). The liquid is pushed from the bell-shaped reservoirs into the fluidic transport chamber by applying upward strain. After filling the chamber with the pushed liquid, a strain is applied from the right side while maintaining the upward strain, efficiently drawing the liquid into the pull reservoirs. Figure 5-Ⅱ illustrates the drawn liquid and the subsequent color change in the mixing chamber, resulting from the sequential operation.

We also show that a standard pulling and reverse pushing microfluidic network can be integrated to achieve complex functions. Figure 5-Ⅲ illustrates that they can cancel each other’s effect and providing a NOT function.

Overall, by utilizing water as a medium for application experiments, we showed that a biologically relevant environment to evaluate the interaction between a system and the body's tissues can be created which provides exciting opportunities in various fields from drug delivery to movement detection.

 

Acknowledgements (Optional): : This project was supported by the NSF program 19-582, grant 2045087. The device were fabricated in the Center for Nanostructures at Santa Clara University.


Ethics statement : All experiments were performed in accordance with the institutional guidelines and approved by the Institutional Review Board at Santa Clara University with the protocol number 21-06-1630. Informed consent was obtained from the human participant

References (Optional): :

1. Agaoglu, S., Diep, P., Martini, M., Samudhyatha, K. T., Baday, M., & Araci, I. E. (2018). Ultra-sensitive microfluidic wearable strain sensor for intraocular pressure monitoring. Lab on a Chip, 18(22), 3471-3483.


2. Gasvoda, H., Cmager, N., Altay, R., Lee, J. Y., & Araci, I. E. (2023). Integration of capillaric strain sensors toward recognition of human movements. Sensors & Diagnostics, 2(1), 212-224.


3. Rivas Yepes, L., Demir, E., Lee, J. Y., Sun, R., Smuck, M., & Araci, I. E. (2020). Skin mountable capillaric strain sensor with ultrahigh sensitivity and direction specificity. Advanced Materials Technologies, 5(12), 2000631.


4. Farzaneh, A., Pawar, N., Portela, C. M., & Hopkins, J. B. (2022). Sequential metamaterials with alternating Poisson’s ratios. Nature communications, 13(1), 1041.