Cytokines play a vital role in immune system signaling, making their detection crucial for continuous health monitoring. Among the various cytokines, tumor necrosis factor-alpha (TNF-α) stands out as a key regulator of the immune response. Notably, TNF-α can be detected in sweat at concentrations as low as pg/mL, with levels strongly correlated with those in blood. Despite its importance, sensitive, wearable, and continuous monitoring of TNF-α in sweat remains limited. To address this gap, this study presents a flexible electrochemical sensor integrated into a microfluidic system for the sensitive and selective detection of TNF-α under continuous sweat flow. First, we present the fabrication of two distinct, miniaturized designs of flexible screen-printed carbon three-electrode platforms, which are subsequently biofunctionalized with gold nanoparticles (AuNPs) coated with TNF-α-specific thiolated aptamers. Next, we compare the two geometrically distinct AuNP-aptamer-functionalized sensors, utilizing experimental and novel simulation-based characterization techniques. Finally, the sensors are integrated into a custom-built microfluidic system enabling the detection of TNF-α ranging from 0.2 to 1000 pg/mL under constant artificial sweat flow conditions, exhibiting high selectivity with negligible responses to non-specific analytes. These findings highlight the feasibility of integrating wearable cytokine sensors for detecting TNF-α under continuous sweat flow conditions, achieving clinically relevant sensitivity within the pg/mL range.

This study presents a novel in-situ Young’s modulus measurement technique of polydimethylsiloxane (PDMS) microfluidic chips that are designed to measure fluid viscosities. In such chips, viscosity can be accurately determined once the Young’s modulus of PDMS is also known. Yet, the Young’s modulus values of elastomers are highly dependent on their chemical composition and curing temperature. The presented approach involves integrating a piezoelectric sheet underneath the chip, oscillating the chip through sweeping the drive frequency, and monitoring the blur and speckle contrast of the pillar via an external camera. A local maximum in the blur and speckle contrast is achieved at the fundamental resonance of the pillars, from which its Young’s modulus is predicted. The experimental results validate the relationship between the Young’s modulus of PDMS and base polymer:curing agent ratio, curing time, and temperature, demonstrating that it could be measured optically. Six different PDMS microfluidic chips prepared by curing elastomer at different temperatures and using different base polymer to curing agent ratios, 5:1, 10:1, and 20:1, were utilized. The Young’s modulus of the prepared sample microfluidic chips varied within the range of 0.72 to 3.64 MPa. The successful measurement of the Young’s modulus of PDMS using the proposed techniques demonstrates its potential in determining the mechanical properties of fluidic chips in-situ, with a low-cost and simple setup.

Compared to the established monolayer approach of two-dimensional cell cultures, three-dimensional (3D) cultures more closely resemble in vivo models; that is, the cells interact and form clusters mimicking their organization in native tissue. Therefore, the cellular microenvironment of these 3D cultures proves to be more clinically relevant. In this study, we present a novel easy-to-fabricate microfluidic shallow trench induced 3D cell culturing and imaging (STICI3D) platform, suitable for rapid fabrication as well as mass manufacturing. Our design consists of a shallow trench, within which various hydrogels can be formed in situ via capillary action, between and fully in contact with two side channels that allow cell seeding and media replenishment, as well as forming concentration gradients of various molecules. Compared to a micropillar-based burst valve design, which requires sophisticated microfabrication facilities, our capillary-based STICI3D can be fabricated using molds prepared with simple adhesive tapes and razors alone. The simple design supports the easy applicability of mass-production methods such as hot embossing and injection molding as well. To optimize the STICI3D design, we investigated the effect of individual design parameters such as corner radii, trench height, and surface wettability under various inlet pressures on the confinement of a hydrogel solution within the shallow trench using Computational Fluid Dynamics simulations supported with experimental validation. We identified ideal design values that improved the robustness of hydrogel confinement and reduced the effect of end-user dependent factors such as hydrogel solution loading pressure. Finally, we demonstrated cultures of human mesenchymal stem cells and human umbilical cord endothelial cells in the STICI3D to show that it supports 3D cell cultures and enables precise control of cellular microenvironment and real-time microscopic imaging. The easy-to-fabricate and highly adaptable nature of the STICI3D platform makes it suitable for researchers interested in fabricating custom polydimethylsiloxane devices as well as those who are in need of ready-to-use plastic platforms. As such, STICI3Ds can be used in imaging cell–cell interactions, angiogenesis, semiquantitative analysis of drug response in cells, and measurement of transport through cell sheet barriers.

During the transition from research to market, the fabrication of microfluidic devices in thermoplastic substrates is inevitable. For short production runs of several hundred products, hot embossing is the typical method before moving on to a typically more expensive injection molding process for higher production volumes. In this work, we investigated the effect of mold material used during hot embossing on feature fidelity for microfabrication in cyclic olefin polymer (COP) substrate. Specifically, we designed a simple flow-focusing microfluidic device and fabricated three different molds using silicon wafer by deep reactive ion etching (DRIE), aluminum filled high temperature epoxy by soft lithography and aluminum by CNC milling. We performed hot embossing experiments with 2mm thick COP substrates and these three different molds using automatic bench top Carver hot press. Finally, we characterized the hot embossed substrates by optical and scanning electron microscopy. Fabrication results demonstrate that the mold material plays a big role in feature fidelity. Among the mold materials used, silicon substrate performed the worst based on defects after demolding. Epoxy and aluminum molds were similar in terms of microfabricated feature defects in the substrate which could be mostly attributed to their coefficient of thermal expansion (CTE). A mold material with a CTE closer to the thermoplastic will result in much better feature fidelity.