A low-cost carbon electrode made from a common photolithography material has matched the performance of platinum in a microfluidic system designed to measure hematocrit, the proportion of blood occupied by red blood cells. The development could help make disposable blood-testing devices cheaper, more robust, and easier to manufacture at scale. In a study published in Biomedical Microdevices, researchers from Michigan Technological University report a new three-step pyrolysis process that converts patterned SU-8 photoresist into planar carbon electrodes with enhanced surface roughness, increased thickness, and a larger electrochemically active area than conventional thin metal electrodes.
Hematocrit is a routine but medically important measurement. It helps assess the blood’s oxygen-carrying capacity and can signal conditions including anemia, dehydration, hemorrhage, chronic kidney disease, leukemia, polycythemia, and certain lung disorders. Standard testing usually relies on centrifugation or automated hematology analyzers, which are accurate but require laboratory equipment. The new system instead combines a small polydimethylsiloxane, or PDMS, channel with two electrodes integrated onto a quartz substrate. A blood-derived sample travels through the channel, while an electrical potential is applied across the fluid. The resulting current changes with the concentration of red blood cells, allowing the device to estimate hematocrit.
The researchers built the electrodes using SU-8, a negative photoresist widely used in microfabrication because it can be patterned into precise structures with ultraviolet light. After exposure and development, the polymer is transformed into carbon by heating it to approximately 900 degrees Celsius in an oxygen-free environment. This process, often called carbon-MEMS fabrication, can produce chemically stable carbon structures with a broad electrochemical potential window, low background current, and relatively low overpotentials. Unlike platinum or gold, SU-8 is inexpensive and can be patterned using established semiconductor-style manufacturing techniques. The challenge is that carbonization causes the photoresist to shrink dramatically, while gases, solvents, and internal stresses can cause the emerging electrode to crack, peel away, or combust.
Conventional two-step pyrolysis has generally worked well for tall, high-aspect-ratio carbon posts, where the polymer has exposed sidewalls that allow gases and volatile compounds to escape. The planar electrodes in this study presented a different mechanical problem. Their broad bases adhered closely to the quartz substrate, while their height was small compared with their width. During heating, residual oxygen and solvent could become trapped, and stress accumulated at the polymer-substrate interface. In repeated trials using the conventional approach, the patterned SU-8 partially burned or separated from the substrate instead of converting cleanly into carbon. Cracking and peeling were especially problematic for the T-shaped electrode design, which combined micrometer-scale conducting strips with millimeter-scale circular contact pads.
To solve the problem, the team added an extended vacuum hard-baking stage before the high-temperature conversion. In the first step, patterned SU-8 was heated to 200 degrees Celsius under a continuous vacuum of approximately 10 millitorr for five to six hours, with the duration adjusted for the polymer thickness. This prolonged treatment was designed to remove residual oxygen and solvent while allowing additional cross-linking and adhesion to develop. The samples were then sealed inside evacuated quartz tubes and heated to 200 degrees Celsius for 30 minutes before being ramped to 900 degrees Celsius for one hour. Natural cooling followed the pyrolysis stage. The sealed-tube configuration also contained ash and carbonaceous byproducts, reducing contamination inside the furnace. Afterward, oxygen plasma was used to remove residue from the quartz substrate without substantially damaging the patterned carbon.
The resulting electrodes were not perfectly flat in the conventional sense. As the SU-8 contracted, surface tension and unequal shrinkage generated a concave cross-sectional profile with raised “horns” along the edges of the carbon strips. These features were more than a visual curiosity: they contributed to a rougher, three-dimensional surface that increased the area available for electrical interaction with the fluid. A 25.4-micrometer SU-8 2025 pattern shrank to approximately 5.88 micrometers after pyrolysis, a 76.8 percent reduction in height, while its final width was about 73.3 percent of the original. A thinner SU-8 2 pattern decreased from roughly 4.70 micrometers to 1.07 micrometers. Despite the extreme dimensional changes, the researchers report that the optimized process produced intact, well-defined electrodes with a 100 percent fabrication yield across the characterized batches.
The carbon electrodes were then integrated into a PDMS microfluidic device containing an 8-millimeter-long channel measuring 180 micrometers wide and 70 micrometers high. Each electrode consisted of a narrow strip connected to a 6-millimeter circular contact pad, allowing copper wires to be attached with silver epoxy. Red blood cells obtained from a volunteer donor were separated, washed, and resuspended in phosphate-buffered saline at concentrations ranging from 10 to 60 percent by volume. The researchers applied 100 volts of direct current for 30 seconds and recorded the current response at 16.13 measurements per second. The short measurement period helped limit errors caused by red blood cells settling under gravity inside the channel and reservoirs.
The electrical signal increased linearly as the red blood cell concentration increased in the phosphate-buffered saline. This behavior differs from measurements made in whole blood or blood plasma, where red blood cells generally act as insulating particles and increasing their concentration can lower the current. In the researchers’ low-conductivity buffer, however, electrical contributions from the cell membranes and surface glycoproteins produced a different overall response. The thicker SU-8 2025-derived carbon electrodes generated approximately 40.65 microamperes with a 50 percent red blood cell suspension, compared with 31.59 microamperes from the thinner SU-8 2 electrodes. The thicker design had a 13 percent larger active surface area in the relevant fluidic regions, helping explain its stronger signal.
For hematocrit determination, the SU-8 2025 system achieved 3.67 percent precision and 3.97 percent accuracy, while the thinner SU-8 2 version reached 5.37 percent precision and 4.21 percent accuracy. The stronger-performing carbon device therefore approached the reported performance of earlier systems based on carbon-coated platinum electrodes, which achieved approximately 2.8 percent precision and 2.6 percent accuracy. The comparison is significant because the new electrode requires no noble-metal coating and can be fabricated through a simpler sequence of photolithography, vacuum baking, pyrolysis, and plasma cleaning. The authors also report that a device integrating carbon electrodes cost approximately 15 U.S. dollars to fabricate in their facility, compared with about 38 dollars for an equivalent platinum-electrode device.
Microscopic surface analysis helps explain why the carbon electrodes performed so well. Atomic force microscopy showed a root-mean-square roughness of approximately 14.2 nanometers for the pyrolyzed carbon, compared with 5.29 nanometers for platinum. The carbon surface was therefore roughly 2.7 times rougher, with more pits, grains, and three-dimensional features. Its measured three-dimensional surface area in the scanned region was about 14.14 square micrometers, compared with 12.82 square micrometers for platinum. When the electrode geometry and thickness in the fluid reservoirs were included, the researchers estimated that the carbon design provided an active surface area of approximately 307,346 square micrometers, about 25 percent greater than the 245,787 square micrometers estimated for platinum. A larger electroactive area reduces the current density required to produce a given total current and can lower the charge-transfer overpotential described by the Butler–Volmer equation. The rough and textured surface may also improve local mass transport by disturbing fluid movement near the electrode.
The study does not present the device as an immediate replacement for clinical hematology analyzers, and several limitations remain. The experiments used red blood cells from a single A-positive donor suspended in prepared phosphate-buffered saline rather than a broad set of untreated patient blood samples. The system also requires a relatively high applied voltage and depends on controlling sedimentation, electrode fouling, electrolysis, and fluidic reproducibility. Even so, the work demonstrates that low-aspect-ratio carbon electrodes, once considered difficult to fabricate reliably, can be made using a practical modification of a well-known process. Because the carbon precursor is inexpensive, abundant, and compatible with precise patterning, the approach could be adapted to other electrochemical sensors, cell-analysis platforms, and disposable microfluidic diagnostic devices. By turning a photoresist into a rugged, high-area electrode, the researchers have created a route toward simpler blood-testing technologies that use less platinum while preserving much of its analytical performance.
Subject of Research: Low-cost pyrolyzed carbon electrodes integrated into a microfluidic hematocrit detection system
Article Title: Fabrication of photoresist SU-8 pyrolyzed carbon electrodes and their integration in a microfluidic hematocrit detection system
Article References: Lee, H. Y., Rogers, J. A., Kendrick, C., Habibi, S., et al. “Fabrication of photoresist SU-8 pyrolyzed carbon electrodes and their integration in a microfluidic hematocrit detection system.” Biomedical Microdevices, volume 28, article 57 (2026).
Image Credits: AI Generated
DOI: 10.1007/s10544-026-00838-6
Keywords: Carbon electrodes, microfluidic devices, hematocrit detection, pyrolysis fabrication, biosensing, SU-8, electrochemical sensors, point-of-care diagnostics
