Uwe Pelz

Smart materials that can sense and react to environmental stimuli are enabling technologies for use in healthcare devices, soft robotics, aerospace and microelectromechanical systems. Among these, paraffin-based phase change material (PCM) actuators have received significant attention due to their high mechanical energy density, affordability and versatility. However, their low thermal conductivity generally limits heat transfer, resulting in slow response times and hindering multifunctional integration. This study presents a dual-paraffin thermoelectric generator-PCM (TEG-PCM) actuator comprising controllable actuation, thermal energy storage and thermoelectric energy harvesting within a leakproof PCM-PDMS matrix. The system uses two paraffins with complementary functions. A low-latent heat paraffin (P1), enhanced with 1 wt.% graphene nanoplatelets (GnP), was filled into a copper cavity and optimized for actuation. A high-latent heat paraffin (P2), embedded in a PDMS matrix, provides thermal stabilization and heat storage. A TEG couples these two functions, enabling Peltier-driven actuation and reset as well as energy harvesting from external temperature gradients via the Seebeck effect. Micro-computed tomography and differential scanning calorimetry revealed that 30 wt.% P2 content offered the optimal balance between latent heat storage and mechanical integrity, with no leakage over multiple thermal cycles. The dual paraffin actuator exhibited maximum displacements and forces of 1400 μ m and 150 mN, with response times of 208 s cm − 3 , representing a fivefold improvement over comparable paraffin-PDMS actuators. Directional infrared heating produced temperature differentials of up to 8 ∘ C and open-circuit voltages of approximately 27 mV, corresponding to an estimated maximum power output of ∼ 228 μ W under matched-load conditions. When employed as an autonomous thermal switch, the actuator uses the P1-GnP expansion to temporarily bridge a thermal gap, thus doubling the thermoelectric output voltage. This multifunctional device couples autonomous and controllable actuation with thermoelectric energy conversion, representing a promising building block towards multifunctional systems for high-temperature microfluidic and thermal management applications.

Engineering systems increasingly demand multifunctional and energy-efficient integration within constrained volume and energy budgets. One promising solution is the monolithic integration of components and functions to minimize occupied volume and simplify control interfaces. Paraffin-based phase change material (PCM) actuators provide high mechanical work density and can be coupled with thermoelectric generators (TEGs) for multifunctional operation. However, their dynamic response is typically constrained by the intrinsically low thermal conductivity of PCM materials. This work introduces a quasi-monolithic fabrication method for a fully integrated TEG-PCM system combining standard four-layer printed circuit board (PCB) technology and CNC milling. By constructing the system as a quasi-monolithic block, thermal interface materials are considerably reduced, thereby diminishing parasitic thermal resistance and promoting faster heat transport from the TEG to the PCM cavity. The system is fabricated using CNC milling with high depth resolution enabled by an electrical sensing-via structure. Experimental validation shows a 76% improvement in displacement rate (15.03 µm/s) at half the input power (1 W) compared to a conventional hybrid-assembled TEG-PCM actuator system consisting of a commercial TEG and an aluminum PCM container. The exploitation of the PCM as a thermal flux modulator for energy harvesting has been preliminarily investigated; considering the measured 5 K temperature difference sustained during a simulated short “day–night” cycle, an estimated open-circuit voltage of ∼13.5 mV is expected to be retrieved under load-match conditions. The actuator is compatible with PCB-based power management and thermal routing, enabling scalable incorporation into compact microsystems and multifunctional MEMS devices.

Thermoelectric Generators (TEGs) are a promising solution for sustainable energy harvesting in Internet of Things (IoT) devices, offering compactness, reliability, and maintenance-free operation. This paper presents the design, optimization, fabrication, and experimental evaluation of double-sided vertical micro thermoelectric generators (µTEGs) based on printed circuit board (PCB) technology. Building on prior work with single-sided µTEGs, the double-sided design leverages both sides of the PCB substrate to enhance thermal gradient utilization, an enhancement in thermolegs density, maximize power density and power conversion efficiency. Dimensional optimization of thermoleg geometries, including diameter and length, was performed through simulations, considering the specific thermoelectric properties of the materials used, to maximize power output. The optimized design was fabricated using a cost-effective CNC milling process, eliminating cleanroom requirements and reducing production time. Experimental results showed that the double-sided µTEG achieved a maximum power output of 1.78 µW at ΔT = 10 °C, representing a 25.4 % improvement over the baseline design and a 54 % enhancement in power-per-area-of-TC and a 380% improvement in thermolegs density compared to previous single-sided µTEGs. The findings demonstrate the potential of double-sided PCB-based µTEGs as efficient, scalable, and cost-effective solutions for energy harvesting in IoT and similar applications.

Around the turn of the century, paraffin as a phase change material demonstrated great potential in the field of microactuators due to its high thermal expansion and high mechanical work capacity. Since then, paraffin actuator research has vastly expanded as new applications in bioengineering, soft robotics and self-tracking systems have emerged. Despite this progress, the most recent comprehensive review was published over a decade ago, leaving a comparatively huge gap in the literature. This review addresses that gap by examining the recent advances and latest developments over the past 10 years, while also including earlier studies not covered in the previous review. In addition to classical microactuators in which the phase change of paraffin triggers the actuation, new concepts for building alternative actuators are presented. These concepts exploit the synergy with other materials and achieve actuation without the use of phase change. We also describe the use of paraffin actuators as switches for smart surface adaptation in terms of stiffness, transparency and adhesion. The scope of this work is to demonstrate the potential of combining diverse properties of paraffin across different applications to develop autonomous multifunctional systems. These systems hold the promise of consolidating functionality into a single device, thereby reducing complexity, energy consumption, and maintenance requirements.

Triboelectric charge separation has received a recent and significant attention with the development of so-called triboelectric nanogenerators (TEN Gs), to harvest electrical energy from mechanical energy invested into the relative motion of triboelectric materials against each other. This research has shown that a large range of experimental conditions - like contact force, contact formation and separation speed, temperature and humidity - does influence the amount of triboelectric charge separation, together with the composition, geometry, bulk and surface morphology and chemical surface properties of the mating surfaces. This wide parameter space is relevant for the design of optimal triboelectric generators, but mostly not addressed in their design and operation today, as measurement equipment for its exploitation is widely missing. As a result, the testing and operation conditions of many different TENGs are not given in literature, not precisely known and not comparable, as are the results from different basic studies on triboelectric materials. To address these issues we have designed, built and initially tested a measurement system for the characterization of triboelectric materials in contact separation-mode under controlled conditions. A model system of Cu vs PTFE (Teflon) was used to demonstrate and repeatably proof the influence of varying measurement conditions on the recorded output voltage from the material pair. These results show the importance of controlled and well-known conditions for the characterization of triboelectric material pairs, offer a possible standardized method for material characterization and give hints on the optimal design of triboelectric generators.

In order to optimize their system design and manufacturing processes, it is crucial to undertake a thorough electrical and thermal characterization of micro thermoelectric generators (µTEGs). To address this need, a highly advanced and fully integrated in-situ measurement system has been developed. The main objectives of this system are to (1) enable the measurement of ZT and thereby of all thermoelectric (TE) properties of thermolegs made from powder-based TE materials and (2) at the same time accurately measure the contact resistance between the TE material and the electrical contacts. The µTEG fabrication concept used in this study is based on copper-cladded printed circuit board (PCB) material as a substrate, using the Cu layers for easy contact formation. In a first step, an innovative measurement concept, based on a distinctive vertical rendition of the well-established transfer length method, has been realized, allowing for the in-situ measurement of contact resistance between the TE material and the copper conductors on the PCB substrate. This enables a comprehensive assessment of the impact exerted by the applied force and temperature during e.g. a hot-pressing step for compacting the powder-based thermolegs during the manufacturing process. In a second step, a comprehensive measurement platform, referred to as the ZT-Card, has been devised to facilitate the evaluation of all relevant TE material properties—Seebeck voltage, electrical conductivity and thermal conductivity (all measured in vertical cross-plane orientation)—inherent to a highly miniaturized thermoleg. Additionally, the ZT-Card also allows for the assessment of contact resistance between the copper contacts and the TE material. Successful testing of this measurement system inspires confidence in the capabilities of the platform and will aid in future µTEG development.

The development of fabrication procedures for micro-thermoelectric generators (µTEGs) based on low-cost fabrication technologies, appropriate for mass production is discussed and demonstrated in this study. Simulations were carried out, two manufacturing processes were tested, and device performance is compared to simulation results for vertical µTEGs. The substrate for this device is a printed circuit board, and the thermoelectric materials are self-developed Bi_0.5Sb_1.5Te₃ (p-type) and Bi₂Te_2.7Se_0.3 (n-type) pastes. A square µTEG (15 mm × 15 mm × 500 µm) with eight thermocouples (TCs) was fabricated. The characterization of the fabricated 8-TC-µTEG was carried out and a power output of 1.23 µW was obtained for the fabricated 8-TC-µTEG. The measurement results of this 8-TC-µTEG closely match simulation results as well. Moreover, a novel vertical electrical contact resistance measurement setup is designed and implemented into the final evaluation of µTEG production to have a more accurate assessment. The simulation study is also applied for the designed electrical contact resistance measurement setup. As a result, the contact resistivity of Bi₂Te₃/Cu was calculated as 5.65 × 10⁻⁴ Ωcm².