Flexible thermoelectric generator for wrist wear smart devices

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Cellulose-based encapsulation for all-printed flexible thermoelectric touch detectors

Publication . Figueira, Joana; Peixoto, Mariana; Gaspar, Cristina; Loureiro, Joana; Martins, Rodrigo; Carlos, Emanuel; Pereira, Luís; CENIMAT-i3N - Centro de Investigação de Materiais (Lab. Associado I3N); UNINOVA-Instituto de Desenvolvimento de Novas Tecnologias; DCM - Departamento de Ciência dos Materiais; Springer

Printed and flexible electronics have gained considerable scientific attention in recent years, driving the demand for low-energy production techniques, eco-friendly materials and flexible substrates. However, effective encapsulation is essential to protect these devices in harsh environmental conditions. Thus, sustainable encapsulant materials are critical for advancing flexible electronics. In this work, we studied three encapsulant materials—commercial plastic, polyvinyl alcohol and ethyl cellulose—applied to thermoelectric touch sensors printed on paper and fabric substrates. Ethyl cellulose demonstrated promising properties in terms of flexibility, water resistance and transparency, along with a low carbon footprint. Encapsulated substrates with ethyl cellulose exhibited high contact angles (121° on fabric and 116° on paper), indicating robust water repellency. Thermal stability tests showed minimal mass loss (10%) at 315 °C, confirming its temperature resilience. Furthermore, sensors encapsulated with ethyl cellulose retained their electric performance after water submersion for 1 min and withstood 100 bending cycles, maintaining response times below 1 s and signal output around 100 µV. These findings highlight ethyl cellulose as a viable green encapsulant material compatible with large-scale sustainable electronics manufacturing.

2025-01Artigo científico

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PRINTED ECO-MATERIALS FOR FLEXIBLE THERMOELECTRIC DEVICES

Publication . Figueira, Joana Roumeliotis Sampaio; Loureiro, Joana; Pereira, Luís

The Internet of Things is already a reality, driving the need for scientists to develop and integrate cost-effective, biocompatible, flexible, and lightweight solutions for universal connectivity, including sensing features. Additionally, sustainability emphasizes the importance of using eco-friendly materi-als and choosing energy-efficient production techniques. While often promoted as a green energy source, thermoelectric materials excel in temperature-sensitive applications, being capable of detecting thermal stimuli such as human touch. The combina-tion of sustainable Seebeck coefficient holding materials with printing methods enables scalable and affordable thermal sensors that may be flexible, lightweight and biocompatible, opening the door for wearable applications. This doctoral research focused on the development of printed thermoelectric sensors using graphite-based materials. Sustainability considerations guided the choice of substrates, solvents, and encapsulating materials. The results show that graphite flakes can be used as purchased, yielding pla-nar, vertical, and planar/vertical thermoelectric architectures, capable of containing multiple elements in series, which increases the signal output. The formulated cellulose-based inks can be printed directly onto untreated flexible substrates (paper and fabric). Sensors were also obtained using silicone elas-tomer-based composites, allowing for significant design flexibility and substrate absence. It was possi-ble to achieve touch sensors with fast response times (bellow 1 s) and to have a VON optimization surpassing 4.5 mV (when connecting multiple elements in series, in ~1.5 cm2) and high SNR values (above 300 for EC/GFlakes and up to 170 for PDMS/GFlakes). Regarding the planar sensors made with EC/GFlakes, the printed elements were evaluated and approved for flexible applications, with curvature radii down to 3.5 mm. At its core, this research addresses the urgent need for versatile and sustainable sensing solu-tions in the era of the 'Internet of Things', showcasing advancements in the field through innovative material applications and low-energy consumption production techniques.

2023Tese de doutoramento

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Entidade financiadora

Fundação para a Ciência e a Tecnologia

Número da atribuição

SFRH/BD/121679/2016