The platform continuously tracks bacterial growth and cellular responses to pharmaceutical treatments through metabolic heat measurements, potentially accelerating antibiotic susceptibility testing and enhancing the understanding of cellular kinetics.
Living cells function as miniature thermodynamic engines, releasing small amounts of heat as they grow, divide, and metabolize nutrients. These thermal signals provide a direct measure of cellular metabolism, reflecting changes in viability and metabolic state. However, measuring these signals has been challenging due to the low heat generated by small cells.
Traditionally, scientists have relied on indirect calorimetry, using proxy measurements such as oxygen consumption to assess cellular activity. While informative, these methods cannot capture immediate thermodynamic changes, limiting real-time monitoring of cellular responses. The development of ultra-sensitive thermal sensors addresses this challenge by enabling direct measurement of metabolic heat.
Innovative Design of the Pico-Calorimeter
To overcome the sensitivity limitations of previous biological sensors, researchers redesigned the architecture of micro-calorimetry by developing a pico-calorimeter with a microfluidic platform mounted on an ultra-thin, micromachined membrane. This device incorporates three microscopic glass capillaries: one containing bacteria suspended in a liquid growth medium and the other two serving as reference channels.
As the bacteria consume nutrients, the metabolic heat they generate produces minute temperature differences between the sample and reference capillaries. These thermal gradients are captured in real time by an integrated thermopile, a sensitive heat-to-electricity converter that transforms localized temperature differences into measurable electrical signals. The sensor achieves a responsivity of approximately 23–100 nV/nW, with a thermal response time of about 7.9 seconds.
To maximize thermal isolation and suppress background noise, the capillary assembly is enclosed in a vacuum chamber that prevents ambient heat dissipation. This design improves sensitivity compared to earlier pico-calorimeter systems that relied on open liquid droplets.
Monitoring of Bacterial Growth and Antibiotic Response
The pico-calorimeter demonstrated exceptional sensitivity during real-time monitoring of Escherichia coli growth. The sensor tracked metabolic heat production from initial populations of only 30–40 bacterial cells, producing thermal growth curves that reflected cellular replication. Continuous calorimetric measurements enabled researchers to estimate oxygen consumption per cell and the contributions of respiration and fermentation to bacterial metabolism.
The study evaluated the platform as a rapid antibiotic susceptibility probe by exposing E. coli cultures to chloramphenicol, rifampicin, and ampicillin, each with distinct mechanisms of action. By comparing thermal profiles across different antibiotic concentrations with untreated controls, the sensor provided a direct, real-time measure of changes in bacterial metabolism.
These measurements captured metabolic heat directly, allowing the device to detect cellular responses and growth alterations before conventional methods could reveal visible changes.
Implications for Sepsis and Pharmaceutical Applications
The exceptional sensitivity of this microfluidic sensor has significant potential for clinical diagnostics, particularly in conditions such as sepsis. Patients with sepsis often have only tens of bacteria per milliliter of blood, necessitating conventional blood culture methods that take days to identify the pathogen and perform antibiotic susceptibility testing.
Operating at the 100 pW scale and employing nanoliter sample volumes, the pico-calorimeter could detect metabolic activity and antibiotic response of low-biomass bacteria within hours.
Beyond infectious disease diagnostics, the platform offers a quantitative, label-free tool for biological research and pharmaceutical development. It enables real-time monitoring of cell viability, proliferation, metabolic activity, and drug susceptibility without the need for fluorescent dyes, making it suitable for drug-response studies.
Future Directions and Commercialization Efforts
This development in pico-calorimetry results from nearly two decades of research, extending the technology from studies of thin-film materials to highly sensitive microfluidic biological sensing. By enabling direct, real-time measurements of cellular heat, the platform establishes a framework for investigating cellular energetics and opens new opportunities for applications ranging from antimicrobial susceptibility testing to the analysis of embryos and tissue biopsies.
To accelerate clinical use, Harvard’s Office of Technology Development has filed multiple patents covering the device and its applications in antimicrobial susceptibility testing. A spin-off company co-founded by researcher Juanjuan Zheng is developing the technology into standardized commercial instruments.
Supported by university accelerator programs, future generations of the platform are expected to expand its role in biomedical diagnostics, making real-time thermal monitoring a practical tool for studying complex biological systems.
Journal References
Zheng, J., et al. (2026). A pico-calorimeter for cellular metabolism and antimicrobial susceptibility testing. PNAS. 123 (25). https://www.pnas.org/doi/10.1073/pnas.2603171123
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