Microheater: material, design, fabrication, temperature control, and applications-a role in COVID-19

Point-of-Care Diagnostics Using Self-heating Elements from Smart Food Packaging: Moving Towards Instrument-Free Nucleic Acid-Based Detection

Compromising between accuracy and rapidity is an important issue in analytics and diagnostics, often preventing timely and appropriate reactions to disease. This issue is particularly critical for infectious diseases, where reliable and rapid diagnosis is crucial for effective treatment and easier containment, thereby reducing economic and societal impacts. Diagnostic technologies are vital in disease modeling, tracking, treatment decision making, and epidemic containment. At the point-of-care level in modern healthcare, accurate diagnostics, especially those involving genetic-level analysis and nucleic acid amplification techniques, are still needed. However, implementing these techniques in remote or non-laboratory settings poses challenges because of the need for trained personnel and specialized equipment, as all nucleic acid-based diagnostic techniques, such as polymerase chain reaction and isothermal nucleic acid amplification, require temperature cycling or elevated and stabilized temperatures. However, in smart food packaging, there are approved and commercially available methods that use temperature regulation to enable autonomous heat generation without external sources, such as chemical heaters with phase change materials. These approaches could be applied in diagnostics, facilitating point-of-care, electricity-free molecular diagnostics, especially with nucleic acid-based detection methods such as isothermal nucleic acid amplification. In this review, we explore the potential interplay between self-heating elements, isothermal nucleic acid amplification techniques, and phase change materials. This paves the way for the development of truly portable, electricity-free, point-of-care diagnostic tools, particularly advantageous for on-site detection in resource-limited remote settings and for home use.

Efficient enrichment of free target sequences in an integrated microfluidic device for point-of-care detection systems

Nucleic acid biomarker detection has great importance in the diagnosis of disease, the monitoring of disease progression and the classification of patients according to treatment decision making. Nucleic acid biomarkers found in the blood of patients have generated a lot of interest due to the possibility of being detected non-invasively which makes them ideal for monitoring and screening tests and particularly amenable to point-of-care (POC) or self-testing. A major challenge to POC molecular diagnostics is the need to enrich the target to optimise detection. In this work, we describe a microfabricated device for the enrichment of short dsDNA target sequences, which is especially valuable for potential detection methods, as it improves the probability of effectively detecting the target in downstream analyses. The device integrated a heating element and a temperature sensor with a microfluidic chamber to carry out the denaturation of the dsDNA combined with blocking-probes to enrich the target. This procedure was validated by fluorescence resonance energy transfer (FRET) technique, labelling DNA with a fluorophore and a quencher. As proof of concept, a 23-mer long dsDNA sequence corresponding to the L858R mutation of the EGFR gene was used. The qualitative results obtained determined that the most optimal blocking rate was obtained with the incorporation of 11/12-mer blocking-probes at a total concentration of 6 μM. This device is a powerful DNA preparation tool, which is an indispensable initial step for subsequent detection of sequences via nucleic acid hybridisation methods.

Direct Laser Writing: From Materials Synthesis and Conversion to Electronic Device Processing

Direct Laser Writing (DLW) has been increasingly selected as a microfabrication route for efficient, cost-effective, high-resolution material synthesis and conversion. Concurrently, lasers participate in the patterning and assembly of functional geometries in several fields of application, of which electronics stand out. In this review, recent advances and strategies based on DLW for electronics microfabrication are surveyed and outlined, based on laser material growth strategies. First, the main DLW parameters influencing material synthesis and transformation mechanisms are summarized, aimed at selective, tailored writing of conductive and semiconducting materials. Additive and transformative DLW processing mechanisms are discussed, to open space to explore several categories of materials directly synthesized or transformed for electronics microfabrication. These include metallic conductors, metal oxides, transition metal chalcogenides and carbides, laser-induced graphene, and their mixtures. By accessing a wide range of material types, DLW-based electronic applications are explored, including processing components, energy harvesting and storage, sensing, and bioelectronics. The expanded capability of lasers to participate in multiple fabrication steps at different implementation levels, from material engineering to device processing, indicates their future applicability to next-generation electronics, where more accessible, green microfabrication approaches integrate lasers as comprehensive tools.

Microheater Chips with Carbon Nanotube Resistors

The specific and excellent properties of the low-dimensional nanomaterials have made them promising building blocks to be integrated into microelectromechanical systems with high performances. Here, we present a new microheater chip for in situ TEM, in which a cross-stacked superaligned carbon nanotube (CNT) film resistor is located on a suspended SiNx membrane via van der Waals (vdW) interactions. The CNT microheater has a fast high-temperature response and low power consumption, thanks to the micro/nanostructure of the CNT materials. Moreover, the membrane bulging amplitude is significantly reduced to only ∼100 nm at 800 °C for the vdW interaction between the CNTs and the SiNx membrane. An in situ observation of the Sn melting process is successfully conducted with the assistance of a customized flexible temperature control system. The uniform wafer-scaled CNT films enable a high level of consistency and cost-effective mass production of such chips. The as-developed in situ chips, as well as the related techniques, hold great promise in nanoscience, materials science, and electrochemistry.

High-Performance, Easy-to-Fabricate, Nanocomposite Heater for Life Sciences and Biomedical Applications

Microheaters are used in several applications, including medical diagnostics, synthesis, environmental monitoring, and actuation. Conventional microheaters rely on thin-film electrodes microfabricated in a clean-room environment. However, low-cost alternatives based on conductive paste electrodes fabricated using printing techniques have started to emerge over the years. Here, we report a surprising effect that leads to significant electrode performance improvement as confirmed by the thorough characterization of bulk, processed, and conditioned samples. Mixing silver ink and PVA results in the solubilization of performance-hindering organic compounds. These compounds evaporate during heating cycles. The new electrodes, which reach a temperature of 80 °C within 5 min using a current of 7.0 A, display an overall 42% and 35% improvement in the mechanical (hardness) and electrical (resistivity) properties compared to pristine silver ink electrodes. To validate our results, we use the composite heater to amplify and detect parasite DNA from Trypanosoma brucei, associated with African sleeping sickness. Our LAMP test compares well with commercially available systems, confirming the excellent performance of our nanocomposite heaters. Since their fabrication relies on well-established techniques, we anticipate they will find use in a range of applications.

Fabrication of a Three-Dimensional Microfluidic System from Poly(methyl methacrylate) (PMMA) Using an Intermiscibility Vacuum Bonding Technique

In this study, the fabrication of microfluidic chips through the bonding of poly (methyl methacrylate) (PMMA) boards featuring designed patterns to create a three-dimensional sandwich structure with embedded microchannels was explored. A key focus was optimization of the interface quality of bonded PMMA pairs by adjusting the solvent, such as such as acetone, alcohol, and their mixture. Annealing was conducted below 50 °C to leverage the advantages of low-temperature bonding. Because of the differences in the chemical reactivity of PMMA toward acetone, alcohol, and their combinations, the resulting defect densities at the bonding interfaces differed significantly under low-temperature annealing conditions. To achieve the optimal sealing integrity, bonding pressures of 30 N, 40 N, and 50 N were evaluated. The interface was analyzed through microstructural examination via optical microscopy and stress measurements were determined using digital photoelasticity, while the bonding strength was assessed through tensile testing.

Development of a Microheater with a Large Heating Area and Low Thermal Stress in the Heating Area

In this paper, a microheater that can absorb thermal stress and has a large heating area is demonstrated by optimizing the structure and process of the microheater. Four symmetrically distributed elongated support beam structures were machined around the microheater via deep silicon etching. This design efficiently mitigates the deformation of the heated region caused by thermal expansion and enhances the structural stability of the microheater. The updated microheater no longer converts the work area into a thin film; instead, it creates a stable heating platform that can uniformly heat a work area measuring 10 × 10 mm2. The microheater is verified to have high temperature uniformity and structural stability in finite element simulation. Finally, thorough investigations of electrical-thermal-structural characterization were conducted. The test findings show that the new microheater can achieve 350 °C with a power consumption of 6 W and a thermal reaction time of 22 s. A scan of its whole plane reveals that the surface of the working area of the new microheater is flat and does not distort in response to variations in temperature, offering good structural stability.

Microheater with copper nanofiber network via electrospinning and electroless deposition

In this report, we present the development of a copper nanofiber network-based microheater, designed for applications in electron microscopes, gas sensing, and cell culture platforms. The seed layer, essential for electroless deposition, was fabricated through the electrospinning of a palladium-contained polyvinylpyrrolidone solution followed by a heat treatment. This process minimized the contact resistance between nanofibers. We successfully fabricated a microheater with evenly distributed temperature by controlling the electrospinning time, heat treatment conditions, and electroless deposition time. We assessed the electrical and thermal characteristics of the microheater by examining the nanofiber density, sheet resistance, and transmittance. The microheater's performance was evaluated by applying current, and we verified its capacity to heat up to a maximum of 350 °C. We further observed the microheater's temperature distribution at varying current levels through an infrared camera. The entire manufacturing procedure takes place under normal pressure, eliminating the need for masking or etching processes. This renders the method easily adaptable to the mass production of microdevices. The method is expected to be applicable to various materials and sizes and is cost-effective compared to commercially produced microheaters developed through microelectromechanical system processes, which demand complex facilities and high cost.

Nano Hotplate Fabrication for Metal Oxide-Based Gas Sensors by Combining Electron Beam and Focused Ion Beam Lithography

Metal oxide semiconductor (MOS) gas sensors are widely used for gas detection. Typically, the hotplate element is the key component in MOS gas sensors which provide a proper and tunable operation temperature. However, the low power efficiency of the standard hotplates greatly limits the portable application of MOS gas sensors. The miniaturization of the hotplate geometry is one of the most effective methods used to reduce its power consumption. In this work, a new method is presented, combining electron beam lithography (EBL) and focused ion beam (FIB) technologies to obtain low power consumption. EBL is used to define the low-resolution section of the electrode, and FIB technology is utilized to pattern the high-resolution part. Different Au++ ion fluences in FIBs are tested in different milling strategies. The resulting devices are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and secondary ion mass spectrometry (SIMS). Furthermore, the electrical resistance of the hotplate is measured at different voltages, and the operational temperature is calculated based on the Pt temperature coefficient of resistance value. In addition, the thermal heater and electrical stability is studied at different temperatures for 110 h. Finally, the implementation of the fabricated hotplate in ZnO gas sensors is investigated using ethanol at 250 °C.

Advances in Bioresorbable Triboelectric Nanogenerators

With the growing demand for next-generation health care, the integration of electronic components into implantable medical devices (IMDs) has become a vital factor in achieving sophisticated healthcare functionalities such as electrophysiological monitoring and electroceuticals worldwide. However, these devices confront technological challenges concerning a noninvasive power supply and biosafe device removal. Addressing these challenges is crucial to ensure continuous operation and patient comfort and minimize the physical and economic burden on the patient and the healthcare system. This Review highlights the promising capabilities of bioresorbable triboelectric nanogenerators (B-TENGs) as temporary self-clearing power sources and self-powered IMDs. First, we present an overview of and progress in bioresorbable triboelectric energy harvesting devices, focusing on their working principles, materials development, and biodegradation mechanisms. Next, we examine the current state of on-demand transient implants and their biomedical applications. Finally, we address the current challenges and future perspectives of B-TENGs, aimed at expanding their technological scope and developing innovative solutions. This Review discusses advancements in materials science, chemistry, and microfabrication that can advance the scope of energy solutions available for IMDs. These innovations can potentially change the current health paradigm, contribute to enhanced longevity, and reshape the healthcare landscape soon.

Fabrication of a Highly NO2-Sensitive Gas Sensor Based on a Defective ZnO Nanofilm and Using Electron Beam Lithography

Hazardous substances produced by anthropic activities threaten human health and the green environment. Gas sensors, especially those based on metal oxides, are widely used to monitor toxic gases with low cost and efficient performance. In this study, electron beam lithography with two-step exposure was used to minimize the geometries of the gas sensor hotplate to a submicron size in order to reduce the power consumption, reaching 100 °C with 0.09 W. The sensing capabilities of the ZnO nanofilm against NO2 were optimized by introducing an enrichment of oxygen vacancies through N2 calcination at 650 °C. The presence of oxygen vacancies was proven using EDX and XPS. It was found that oxygen vacancies did not significantly change the crystallographic structure of ZnO, but they significantly improved the electrical conductivity and sensing behaviors of ZnO film toward 5 ppm of dry air.

Microheater-Integrated Microlens Array for Robust Rapid Fog Removal

In extreme environments, fog formation on a microlens array (MLA) surface results in a device failure. One reliable solution for fog removal is to heat the surface using a microheater. However, due to the surface interference, the combination of these two microdevices remains elusive. In this study, we introduce lift-off and electroless plating into femtosecond laser processing to fabricate a microheater integrated MLA (μH-MLA) on the same substrate with high light transmittance, durability, and fog removal efficiency. Laser-induced micro-nano grooves enable the microheater to be tightly coupled with the MLA and have high heating performance, thus maintaining a stable performance for over 24 h during continuous operation as well as under long time ultrasonic vibration and mechanical friction. With a rapid response time (τ0.5) of 17 s and a high working temperature of 188 °C, the μH-MLA removed fog that covers the entire face within 14 s. Finally, we prove the use of this fabrication method in large areas and curved surface environments. This study provides a flexible, stable, and economical method to integrate micro-optical and microelectrical devices.

A Micro-Hotplate-Based Oven-Controlled System Used to Improve the Frequency Stability of MEMS Resonators

This paper introduces a chip-level oven-controlled system for improving the temperature stability of MEMS resonators wherein we designed the resonator and the micro-hotplate using MEMS technology, then bounding them in a package shell at the chip level. The resonator is transduced by AlN film, and its temperature is monitored by temperature-sensing resistors on both sides. The designed micro-hotplate is placed at the bottom of the resonator chip as a heater and insulated by airgel. The PID pulse width modulation (PWM) circuit controls the heater according to the temperature detection result to provide a constant temperature for the resonator. The proposed oven-controlled MEMS resonator (OCMR) exhibits a frequency drift of 3.5 ppm. Compared with the previously reported similar methods, first, the OCMR structure using airgel combined with a micro-hotplate is proposed for the first time, and the working temperature is extended from 85 °C to 125 °C. Second, our work does not require redesign or additional constraints on the MEMS resonator, so the proposed structure is more general and can be practically applied to other MEMS devices that require temperature control.

Graphene Microheater Chips for In Situ TEM

Low-dimensional materials are bringing significant innovations to in situ TEM characterization. Here a new graphene microheater chip for TEM was developed by stacking graphene on a suspended SiN_x membrane as the Joule heating element. It could be heated up to 800 °C within 26.31 ms with a low power consumption of 0.025 mW/1000 μm². The bulging was only ∼50 nm at 650 °C, which is 2 orders of magnitude smaller than those of conventional MEMS heaters at similar temperatures. The performances benefit from the employment of graphene, since its monolayer structure greatly reduces the heat capacity, and the vdW contact significantly reduces the interfacial interaction. The TEM observation on the Sn melting process verifies its great potential in resolving thermodynamic processes. Moreover, more multifunctional in situ chips could be developed by integrating other stimuli to such chips. This work opens a new frontier for both graphene and in situ characterization techniques.

Recent advances and challenges in temperature monitoring and control in microfluidic devices

Temperature is a critical-yet sometimes overlooked-parameter in microfluidics. Microfluidic devices can experience heating inside their channels during operation due to underlying physicochemical phenomena occurring therein. Such heating, whether required or not, must be monitored to ensure adequate device operation. Therefore, different techniques have been developed to measure and control temperature in microfluidic devices. In this contribution, the operating principles and applications of these techniques are reviewed. Temperature-monitoring instruments revised herein include thermocouples, thermistors, and custom-built temperature sensors. Of these, thermocouples exhibit the widest operating range; thermistors feature the highest accuracy; and custom-built temperature sensors demonstrate the best transduction. On the other hand, temperature control methods can be classified as external- or integrated-methods. Within the external methods, microheaters are shown to be the most adequate when working with biological samples, whereas Peltier elements are most useful in applications that require the development of temperature gradients. In contrast, integrated methods are based on chemical and physical properties, structural arrangements, which are characterized by their low fabrication cost and a wide range of applications. The potential integration of these platforms with the Internet of Things technology is discussed as a potential new trend in the field.

3D printing-enabled uniform temperature distributions in microfluidic devices

Many microfluidic processes rely heavily on precise temperature control. Though internally-contained heaters have been developed using traditional fabrication methods, they are limited in their ability to isothermally heat a precisely defined volume. Advances in 3D printing have led to high resolution printers capable of using bio-compatible materials and achieving geometry resolutions near 20 μm. 3D printing's ability to create arbitrary 3D structures with an arbitrary 3D orientation as opposed to traditional microfluidic fabrication methods enables new three-dimensional heater geometries to be created. As examples, we demonstrate three new 3D heater geometries: a non-planar serpentine channel, a tapered helical channel, and a diamond channel. These new geometries are shown through finite element simulation to isothermally heat microfluidic channels of cross section 200 μm × 200 μm with a 0.1 °C temperature difference along up to 91% of a 10 mm length, compared to designs from the literature that are only able to have that same temperature distance over several μms. Finally, a set of design rules to create isothermal regions in 3D based on the desired temperature, heater pitch, heater gradient, and radial space around a target volume are detailed.

Design and Thermal Analysis of Flexible Microheaters

With the development of flexible electronics, flexible microheaters have been applied in many areas. Low power consumption and fast response microheaters have attracted much attention. In this work, systematic thermal and mechanical analyses were conducted for a kind of flexible microheater with two different wire structures. The microheater consisted of polyethylene terephthalate (PET) substrate and copper electric wire with graphene thin film as the middle layer. The steady-state average temperature and heating efficiency for the two structures were compared and it was shown that the S-shaped wire structure was better for voltage-controlled microheater other than circular-shaped structure. In addition, the maximum thermal stress for both structures was from the boundary of microheaters, which indicated that not only the wire structure but also the shape of micro heaters should be considered to reduce the damage caused by thermal stress. The influence resulting from the thickness of graphene thin film also has been discussed. In all, the heating efficiency for flexible microheaters can be up to 135 °C/W. With the proposed PID voltage control system, the response time for the designed microheater was less than 10 s. Moreover, a feasible fabrication process flow for these proposed structures combing thermal analysis results in this work can provide some clues for flexible microheaters design and fabrication in other application areas.

Microfluidics Temperature Compensating and Monitoring Based on Liquid Metal Heat Transfer

Microfluidic devices offer excellent heat transfer, enabling the biochemical reactions to be more efficient. However, the precision of temperature sensing and control of microfluids is limited by the size effect. Here in this work, the relationship between the microfluids and the glass substrate of a typical microfluidic device is investigated. With an intelligent structure design and liquid metal, we demonstrated that a millimeter-scale industrial temperature sensor could be utilized for temperature sensing of micro-scale fluids. We proposed a heat transfer model based on this design, where the local correlations between the macro-scale temperature sensor and the micro-scale fluids were investigated. As a demonstration, a set of temperature-sensitive nucleic acid amplification tests were taken to show the precision of temperature control for micro-scale reagents. Comparations of theoretical and experimental data further verify the effectiveness of our heat transfer model. With the presented compensation approach, the slight fluorescent intensity changes caused by isothermal amplification polymerase chain reaction (PCR) temperature could be distinguished. For instance, the probability distribution plots of fluorescent intensity are significant from each other, even if the amplification temperature has a difference of 1 °C. Thus, this method may serve as a universal approach for micro–macro interface sensing and is helpful beyond microfluidic applications.

Research Progress on Coating of Sensitive Materials for Micro-Hotplate Gas Sensor

Micro-hotplate gas sensors are widely used in air quality monitoring, identification of hazardous chemicals, human health monitoring, and other fields due to their advantages of small size, low power consumption, excellent consistency, and fast response speed. The micro-hotplate gas sensor comprises a micro-hotplate and a gas-sensitive material layer. The micro-hotplate is responsible for providing temperature conditions for the sensor to work. The gas-sensitive material layer is responsible for the redox reaction with the gas molecules to be measured, causing the resistance value to change. The gas-sensitive material film with high stability, fantastic adhesion, and amazing uniformity is prepared on the surface of the micro-hotplate to realize the reliable assembly of the gas-sensitive material and the micro-hotplate, which can improve the response speed, response value, and selectivity. This paper first introduces the classification and structural characteristics of micro-hotplates. Then the assembly process and characteristics of various gas-sensing materials and micro-hotplates are summarized. Finally, the assembly method of the gas-sensing material and the micro-hotplate prospects.