Biodegradable wearable sensors for simultaneous joint / muscle activities and skin temperature monitoring

Abstract

According to an example aspect of the present invention, there is provided a method for preparing a biomass-based and biodegradable sensor electrode, e.g. for use in a wearable sensor, the method comprising the steps of combining a natural wax with a vegetable oil in a weight ratio of 1:1 to 1:10 melting the wax with stirring to obtain a melted mixture, then gradually incorporating carbon particles at a weight ratio of 1-50% to the melted mixture, solidifying the melted mixture incorporating carbon particles on a biomass-based substrate into a solidified electrically conductive paste thereon. According to another example aspect of the present invention, there is provided a pressure sensor (1) obtainable by said method. The pressure sensor 1 comprises a porous xerogel dielectric material layer 4 sandwiched between two substrates 11 and 12, whereof each one comprises or consists of a layer of a substrate material 2 respectively 6 which are coated on one side with a layer of an electrically conductive paste 3 respectively 5. Copper tape 7 is affixed to the external surface of each substrate film 11 and 12 to establish electrical contacts.

Description

[0001] TITLE Biodegradable Wearable Sensors for Simultaneous Joint / Muscle Activities and Skin Temperature Monitoring FIELD OF THE INVENTION The present invention relates generally to health monitoring via the wearable pressure andtemperature sensors composed of bio-based and biodegradable components and materials.More particularly, the present invention relates to the following:- a method for preparing an electrically conductive paste- a method for preparing a biomass-based and biodegradable sensor electrode- a method for preparing a pressure sensor- a method for preparing a temperature sensor- an electrically conductive paste obtainable by the method of any one of claims 1 to 4- a biomass-based and biodegradable sensor electrode obtainable by the method of any oneof claims 5 to 12.- a pressure sensor obtainable by a method of any one of claims 13 to 17- a temperature sensor obtainable by a method of any one of claims 18 or 19- a biomass-based and biodegradable sensor- a pressure sensor- a temperature sensor.BACKGROUND OF THE INVENTION Disclosed is a field of wearable pressure sensors for healthcare monitoring. Wearable pressure sensors offer real-time, continuous monitoring of vital signs, allowing healthcare professionals to assess various physiological parameters. These sensors are non-invasive and convenient, enabling patients to be monitored during daily activities1. Prior art includes wearable pressure sensors based on capacitive, piezoresistive, piezoelectric, and triboelectric mechanisms. Capacitive sensors are a preferred choice due to their simplicity, high precision, low power consumption, and reduced dependence on external factors like temperature and humidity2.1https: / / doi.org / 10.1002 / adma.2015042442https: / / doi.org / 10.1002 / admt.202001023 These sensors typically comprise a flexible dielectric layer sandwiched between two flexible electrodes. When pressure is applied, the thickness of the dielectric layer decreases, and its permittivity increases, resulting in a measurable rise in capacitance. These sensors can detect a wide pressure range, from subtle (1 Pa-1 kPa) to medium (10 kPa-100 kPa), making them valuable for human motion detection, healthcare monitoring, and human-machine interfaces. 3,4 These sensors typically comprise a flexible dielectric layer sandwiched between two flexible electrodes. Common substrate materials include polymers like PDMS5, Ecoflex6, PET7, PU8, PVDF9, and PVA10. Solution-processable nanomaterials like carbon nanotubes11, graphene12, MXenes13, and silver nanowires7are frequently used for the electrodes. For the dielectric layer, PDMS14, Ecoflex15, PVDF16, polyimide17, and PMMA18are common choices. However, to improve sensor performance, modifications to the dielectric materials are sometimes required. Introducing porosity into the dielectric layer is a promising approach to increase sensitivity. Porous materials tend to have lower Young's moduli, leading to a higher effective dielectric constant and enhanced sensitivity5,6. Conventional sensors often rely on non-biodegradable materials and complex manufacturing processes, contributing to electronic waste and environmental pollution19. Biodegradable electronics have emerged as a solution due to growing concerns about waste management and sustainability20. Research efforts have focused on developing biodegradable electrodes and dielectric materials for these sensors21-26. Biodegradable electrode materials explored include3https: / / doi.org / 10.1016 / j.sna.2021.1128384https: / / doi.org / 10.1002 / admi.2022008665Transparent elastic capacitive pressure sensors based on Xplore thermally evaporated magnesium21and zinc22, along with solution-processed copper23and silver nanowires24. Biodegradable substrates investigated encompass films and sheets of PHB:PHV21, PLA22, PVA25, silk26, tree leaf24, and chitosan23. Additionally, research has explored biodegradable dielectric materials such as PGS film21, PLGA:PCL nanofiber mat25, rose petal film24, and chitosan sheets23. While biodegradability is typically assessed in PBS solution, complete biodegradation remains a challenge due to potential undegraded residues, necessitating further research on waste management.On the other hand, body temperature monitoring is crucial for assessing human health,reflecting physiological processes27. Traditional temperature sensing relies on rigid detectors, which are incompatible with curved surfaces like human bodies, causing discomfort and challenges, especially in children. This has led to the emergence of flexible, wearable temperature sensors, offering biocompatibility and comfort, advancing wearable technologies28. Temperature sensors include resistive temperature detectors (RTDs) and thermistors4. RTDs operate based on resistance changes in conductive materials due to temperature fluctuations, offering low cost and simplicity but with lower sensitivity and slower response. Thermistors, conversely, leverage conductivity changes in response to temperature, providing high accuracy, sensitivity, and rapid response, with both negative and positive temperature coefficient types available29. The Temperature Coefficient of Resistance (TCR), a crucial parameter for all temperature sensors, indicates their sensitivity. A higher TCR implies greater sensitivity to minor temperature changes. TCR is derived from an equation 1, where R0 represents the initial resistance at a reference temperature T0 (oC), and R1 is the resistance at a different absolute temperature T1(oC)30. Photolithography and printing are the predominant technologies in fabricating flexible and wearable sensors. While photolithography yields high-performance, consistent devices, it incurs substantial costs due to cleanroom facilities, extensive material wastage, and high fabrication expenses. Consequently, there's an increasing demand for cost-effective printing methods offering lower fabrication costs and enhanced producibility. Additive printing techniques address these challenges by eliminating the necessity for stencil masks or cleanroom lithography, marking a significant advancement in sensor fabrication technology31. Several methodologies, including stencil printing32, inkjet printing33, and aerosol printing34, have been employed in the development of temperature sensors. These techniques are integral to the fabrication process, enabling precise and efficient creation of sensor components. Various substrates, including polyamide fabric37, polyimide film35, Polyethylene naphthalate (PEN) film32and Polyethylene terephthalate (PET) film36, have been utilized in the fabrication of temperature sensors. In addition, sensing materials such as Mxene345, graphene34, PEDOT:PSS35, silver nanomaterials36and carbon nanotubes were printed on the above- mentioned substrates. These materials were selected for their specific properties that contribute to the functionality and effectiveness of the sensors. The above-mentioned materials used in temperature sensors are often derived from fossil-based or critical raw materials. This reliance poses environmental concerns, especially regarding electronic waste. Therefore, the integration of biodegradable materials into sensor technology is vital for reducing the ecological impact and promoting sustainability in the production and disposal of electronic devices. SUMMARY OF THE INVENTION In response to these issues, in this invention, we have developed a sustainable, environmentally friendly, alternative in the form of a biomass-based and biodegradable wearable capacitive type pressure and resistive type temperature sensor. The sensor components, including the substrate,electrode, and dielectric materials, were fabricated using fully renewable resources such ascellulose, and pectin, which possess the ability to naturally degrade over time. This therebyminimizes the environmental impact associated with electronic waste. Importantly, the preparation of these materials adhered to green chemistry principles, ensuring that no toxic chemicals were utilized during the fabrication process. The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims. According to a first aspect of the present invention, there is provided a method for preparing an electrically conductive paste, e.g. for use in the preparation of a biomass-based and biodegradable sensor electrode, the method comprising the steps of- combining a natural wax with a vegetable oil in a weight ratio of 1:1 to 1:10- melting the wax with stirring to obtain a melted mixture,- then gradually incorporating carbon particles at a weight ratio of 1-50% to the melted mixture.Preferred embodiments of the method for preparing an electrically conductive paste according to the first aspect of the invention comprises at least one of the following preferable features: ^the natural wax is carnauba wax, candelilla wax, shellac wax, rice-bran wax,beeswax, cocoa butter, mango butter, kokum butter, or shea butter. ^the vegetable oil is sunflower oil, olive oil, canola oil, avocado oil, corn oil, almond oilor sesame oil. ^the carbon particles are graphite or activated carbon particles.According to a second aspect of the present invention, there is provided a method for preparinga biomass-based and biodegradable sensor electrode, e.g. for use in a wearable sensor, the method comprising the steps of- combining a natural wax with a vegetable oil in a weight ratio of 1:1 to 1:10- melting the wax with stirring to obtain a melted mixture,- then gradually incorporating carbon particles at a weight ratio of 1-50% to the melted mixture,- solidifying the melted mixture incorporating carbon particles on a biomass-based substrateinto a solidified electrically conductive paste thereon. Preferred embodiments of the method for preparing a biomass-based and biodegradable sensor electrode according to the second aspect of the invention comprises at least one of the following preferable features:a) the natural wax is carnauba wax, candelilla wax, shellac wax, rice-bran wax, beeswax,cocoa butter, mango butter, kokum butter or shea butter. b) the vegetable oil is sunflower oil, olive oil, canola oil, avocado oil, almond oil orsesame oil. c) the carbon particles are graphite or activated carbon particles.d) the biomass-based substrate comprises cellulose, chitosan, whey protein, alginate, orsilk material. e) the substrate comprising a cellulose, chitosan, whey protein, alginate, or silk materialis prepared by: f) preparing a suspension of said cellulose, chitosan, whey protein, alginate, or silkmaterial, the suspension preferably being an aqueous suspension, most preferably an aqueous suspension wherein the water is deionized water g) continuously stirring the suspension, preferably overnight, to ensure homogenousdispersion h) degassing the suspension in a vacuum oven maintained at ambient temperature toeliminate any entrapped bubbles, the bubbles being bubbles of entrapped air i) filtering the degassed suspension with a filtration apparatus having suitable filtermembrane, such as a polyvinylidene fluoride (PVDF) membrane filter having a pore size of 0.45 µm j) subjecting the resulting wet film subsequently to pressing, such as hot press treatment.k) when used in preparation of a pressure sensor electrode, the melted mixtureincorporating carbon particles is cooled, a portion thereof is applied to the substrate, reheated until liquified, pressed with a Teflon sheet for uniform coating, and finally allowed to cool at room temperature, resulting in a solidified electrically conductive paste. l) when used in preparation of a temperature sensor electrode, a portion of the meltedmixture incorporating carbon particles is transferred to a 3-D printer, reheated until liquified, printed on the substrate via melt extrusion, and finally allowed to cool at ambient temperature, resulting in a solidified electrically conductive paste.According to a third aspect of the present invention, there is provided a method for preparing a pressure sensor, wherein a porous xerogel dielectric material is positioned between two sensor electrodes prepared according to the second aspect of the present invention, i.e. the method for preparing a biomass-based and biodegradable sensor electrode, or any of the itspreferred embodiments a) to k).Preferred embodiments of method for preparing a pressure sensor, according to the third aspectof the invention comprises at least one of the following preferable features: ^the porous xerogel dielectric material is prepared from a biopolymer, such ascellulose, chitosan, pectin, chitin, or collagen, preferably pectin. ^the preparation of a xerogel dielectric material from pectin comprises:(1) dissolving a pectin powder in an aqueous saline solution such as NaCl solution,(2) stirring the mixture at a temperature selected from ambient to 100 degrees ofCelsius for a selected period of time, (3) dispensing the solution into a Teflon mold equipped with a copper base plate,(4) resting the mold on a cold bed such as dry ice to induce freeze-casting and gelformation, (5) submerging the resulting gel in cold ethanol,(6) performing a first solvent exchange using a mixture of ethanol and water,(7) conducting a second solvent exchange with acetone,(8) subjecting the samples to vacuum drying at room temperature in a vacuum oven,and (9) obtaining the final xerogel dielectric material.^ the method comprises- precisely trimming a dielectric xerogel cut along the ice growth direction- positioning the trimmed xerogel in a sandwich-like configuration between the twoelectrodes. According to a fourth aspect of the present invention, there is provided a method for preparing a temperature sensor, wherein a biomass-based and biodegradable sensor electrode obtained according to the second aspect of the invention, i.e. the method for preparing a biomass-basedand biodegradable sensor electrode, or any one of its preferred embodiments a) to j) and l) isencapsulated using natural fatty acids. Preferred embodiments of method for preparing a temperature sensor, according to the fourth aspect of the invention comprises at least one of the following preferable features:^ using natural fatty acids comprises using at least one fatty acid, preferably only onefatty acid ^the fatty acids include at least one of Caprylic acid, Capric acid, Lauric acid, Myristicacid, Palmitic acid, Stearic acid, Arachidic acid, Behenic acid, Lignoceric acid, and Cerotic acid. ^the fatty acid is Stearic acid.According to a fifth aspect of the present invention, there is provided an electrically conductive paste obtainable by the first aspect of the invention, i.e. the method for preparing an electrically conductive paste, or any of its preferred embodiments. According to a sixth aspect of the present invention, there is provided a biomass-based and biodegradable sensor electrode obtainable by the second aspect of the invention, i.e. the method for preparing a biomass-based and biodegradable sensor electrode, or any one of its preferred embodiments. According to a seventh aspect of the present invention, there is provided a pressure sensor obtainable by the third aspect of the invention, i.e. by the method for preparing a pressure sensor, or any one of its preferred embodiments. According to an eighth aspect of the present invention, there is provided a temperature sensor obtainable by the fourth aspect of the invention, i.e. the method for preparing a temperature sensor, or any one of its preferred embodiments. According to a ninth aspect of the present invention, there is provided a biomass-based and biodegradable sensor electrode comprising a biobased substrate coated on one side with a solidified electrically conductive paste, the solidified electrically conductive paste comprising a solidified melted mixture of a natural wax with a vegetable oil in a weight ratio of 1:1 to 1:10 incorporating carbon particles at a weight ratio of 1-50% to the solidified melted mixture. Preferred embodiments of the biomass-based and biodegradable sensor electrode, according to the ninth aspect of the invention comprises at least one of the following preferable features: ^the natural wax is carnauba wax, candelilla wax, shellac wax, rice-bran wax, beeswax,cocoa butter, mango butter, kokum butter, or shea butter. ^the vegetable oil is sunflower oil, olive oil, canola oil, avocado oil, corn oil, almondoil or sesame oil. ^carbon particles are graphite or activated carbon particles.^ the substrate comprises cellulose, chitosan, whey protein, alginate, or silk material.According to a tenth aspect of the present invention, there is provided a pressure sensor, comprising a porous xerogel dielectric material positioned between two biomass-based and biodegradable sensor electrodes according to the ninth aspect of the invention, wherein the porous xerogel dielectric material is prepared from a biopolymer, such as cellulose, pectin, chitin, or collagen. Preferred embodiments of the pressure sensor, according to the tenth aspect of the invention comprises at least that the biopolymer of the porous xerogel dielectric material is pectin. According to an eleventh aspect of the present invention, there is provided a temperature sensor, comprising a biomass-based and biodegradable sensor electrode according to the ninth aspect of the invention, or any one of its preferred embodiments, encapsulated using natural fatty acids. Preferred embodiments of the temperature sensor, according to the eleventh aspect of the invention comprises that the fatty acids include Caprylic acid, Capric acid, Lauric acid, Myristic acid, Palmitic acid, Stearic acid, Arachidic acid, Behenic acid, Lignoceric acid, andCerotic acid. Preferably, using natural fatty acids comprises using at least one fatty acid,preferably only one fatty acid. The fatty acid may be e.g. Stearic acid. BRIEF DESCRIPTION OF THE DRAWINGSFigure 1 shows the schematic demonstration of the pressure sensor.Figure 2 shows a schematic demonstration of a temperature sensor fabrication.Figure 3 presents properties of the pressure sensor (A) Repeated responses of the sensor to 5kPa pressure, (B) real-time responses of the sensor to various pressure, (C) Sensitivity of sensor in different pressure regimes, (D) Response and recovery time of sensor, and (E) Stability of the sensor tested for 10,000 cycles under an applied pressure of 5 kPa.Figure 4 shows the performance of a temperature sensor. DETAILED DESCRIPTION OF EMBODIMENTS The pressure sensor design is detailed in Fig.1, where a porous xerogel is positioned betweentwo biobased substrates, with the carbon paste-coated side oriented towards the xerogel. Thepressure sensor 1 depicted in Fig.1 comprises or consists of a porous xerogel dielectric materiallayer 4 sandwiched between two substrates 11 and 12. Each one of the substrates 11 and 12comprises or consists of a layer of a substrate material 2 respectively 6 which are coated onone side with a layer of an electrically conductive paste 3 respectively 5. Copper tape 7 isaffixed to the external surface of each substrate film 11 and 12 to establish electrical contacts.The assembly is sealed with a sealing material such as medical-grade tape (not shown in Fig1).- Preparation of sensor electrode: Disclosed is a method for preparing an electricallyconductive paste. The method comprises combining a natural wax (carnauba wax, beeswax, or shea butter) with a vegetable oil (rapeseed oil, olive oil, or corn oil) in a weight ratio of 1:1 to 1:10 melting the wax with stirring, then gradually incorporating carbon particles (graphite or activated carbon) at a weight ratio of 1-50% to the melted mixture. The mixtureis cooled, a portion applied to a cellulose substrate, reheated until liquified, pressed with a Teflon sheet for uniform coating, and finally allowed to cool at room temperature, resulting in a solidified electrically conductive paste.- Preparation of substrate: In some embodiments, the invention utilizes substrates comprisingcellulose, chitosan, or silk materials. A suspension of these materials is prepared and continuously stirred overnight to ensure homogenous dispersion. Prior to filtration, the suspension undergoes degassing in a vacuum oven maintained at ambient temperature to eliminate any entrapped bubbles. The degassed suspension is then transferred to and filtered with a filtration apparatus having suitable filter membrane such as a polyvinylidene fluoride(PVDF) membrane filter having a pore size of 0.45 µm. The resulting wet film is subsequently subjected to pressing such as a hot press treatment.- Fabrication of xerogels dielectric material: Disclosed herein are methods for preparingxerogel dielectric materials from various biopolymers, including cellulose, pectin, chitin, and collagen. As an illustrative example, the method for preparing a pectin xerogel comprises: (1) dissolving a pectin powder in an aqueous saline solution such as NaCl solution, (2) stirring the mixture at temperature selected from ambient to 100 degrees ofCelsius. for a selected period of time, (3) dispensing the solution into a Teflon moldequipped with a copper base plate, (4) resting the mold on a cold bed such as dry ice toinduce freeze-casting and gel formation, (5) submerging the resulting gel in cold ethanol, (6) performing a first solvent exchange using a mixture of ethanol and water, (7) conducting a second solvent exchange with acetone, (8) subjecting the samples to vacuum drying at room temperature in a vacuum oven, and (9) obtaining the final xerogel product.- Preparation of the pressure sensors: Disclosed is a method for sensor assembly. The methodcomprises precisely trimming a dielectric xerogel cut along the ice growth direction and a substrate coated with carbon paste to dimensions such as 2x2 cm. The trimmed xerogel isthen carefully positioned in a sandwich-like configuration between the two substrate films, ensuring the carbon paste-coated surfaces directly contact the xerogel. Copper tape is affixed to the external surface of each substrate film to establish electrical contacts. Finally, the assembly is sealed using a sealing material such as medical-grade tape.The temperature sensor design and fabrication process are illustrated in Fig. 2, where carbonpaste sensing elements was deposited on the substrate via melt-printing, followed byencapsulation. The temperature sensor 13 obtained in the fabrication process presented in Fig.2 comprises or consists of a substrate 8 in the form of a layer having one or more carbon pastesensing elements 9 deposited thereon. Each carbon paste sensing element 9 comprises orconsists of solidified electrically conductive paste made from a melted mixture of a natural waxand a vegetable oil in a weight ratio of 1:1 to 1:10 incorporating carbon particles (graphite oractivated carbon) at a weight ratio of 1-50% to the melted mixture.- Preparation of substrate: In some embodiments, the invention utilizes substrates comprisingcellulose, chitosan, or silk materials. A suspension of these materials is prepared and continuously stirred overnight to ensure homogenous dispersion. Prior to filtration, the suspension undergoes degassing in a vacuum oven maintained at ambient temperature to eliminate any entrapped bubbles. The degassed suspension is then transferred to and filtered with a filtration apparatus having suitable filter membrane such as a polyvinylidene fluoride(PVDF) membrane filter having a pore size of 0.45 µm. The resulting wet film is subsequently subjected to pressing such as a hot press treatment.- Preparation of sensor electrode: The method comprises combining a natural wax (carnaubawax, beeswax, or shea butter) with a vegetable oil (rapeseed oil, olive oil, or corn oil) in a weight ratio of 1:1 to 1:10 melting the wax with stirring, then gradually incorporating carbon particles (graphite or activated carbon) at a weight ratio of 1-50% to the melted mixture. Aportion the mixture was transferred to 3-D printer, reheated until liquified, printed on the cellulose substrate via melt extrusion, and finally allowed to cool at ambient temperature,resulting in a solidified electrically conductive paste.- Encapsulation of the sensor: A novel implementation of temperature sensor encapsulationincorporates the use of natural fatty acids. A unique aspect and advantage of the present invention is the use of these fatty acids as encapsulants due to their high resistivity, good thermal conductivity, and overall robust physical properties, providing an effective barrier against environmental hazards while allowing for precise temperature measurement. Specifically, the invention provides for the encapsulation of temperature sensors using natural fatty acids including, but not limited to, Caprylic acid, Capric acid, Lauric acid, Myristic acid, Palmitic acid, Stearic acid, Arachidic acid, Behenic acid, Lignoceric acid, and Cerotic acid. As environmentally friendly and cost-effective materials, these fatty acids contribute to the protective encapsulation system that prevents damage, enhances durability, and maintains the high-efficiency performance of the temperature sensors. EXAMPLESExample 1: Preparation of pressure sensor- Preparation of sensor electrode: Disclosed is a method for preparing an electricallyconductive paste comprising: combining 0.1 g of carnauba wax and 0.3 g of rapeseed oil in a 2 mL glass vial; heating the mixture on a hot stirrer at 100 °C with stirring at 60 rpm until the carnauba wax liquifies; gradually incorporating 0.4 g of carbon particles selected from the group consisting of graphite and activated carbon into the liquified mixture; cooling the mixture to room temperature; applying a 0.2 g portion of the cooled mixture to a cellulose substrate; heating the applied paste to 100 °C until liquified; pressing a Teflon sheet onto the liquified paste to form a uniform coating; and allowing the coated substrate to cool to room temperature, thereby resulting in a solidified electrically conductive paste.- Preparation of substrate: Disclosed herein is a method for cellulose substrate fabrication.The method comprises preparing a 0.5% wt. cellulose nanofiber (CNF) suspension and continuously stirring it overnight for homogenous dispersion. The suspension is then degassed in a vacuum oven at room temperature to remove bubbles. Subsequently, the degassed suspension is transferred to a filtration apparatus equipped with a polyvinylidene fluoride (PVDF) membrane filter having a pore size of 0.45 µm. The resulting wet film is then subjected to a hot press treatment at a constant temperature of 50 °C for a predetermined time of 15 minutes.- Fabrication of xerogels dielectric material: Disclosed herein is a method for preparing apectin xerogel. The method comprises: (1) dissolving pectin powder in a 0.2 M NaCl aqueous solution to achieve a final concentration of 3 wt%, (2) stirring the mixture for 24 hours at room temperature, (3) dispensing the solution into a Teflon mold equipped with a copper base plate, (4) resting the mold on dry ice at -79 °C to induce freeze-casting and gel formation, (5) submerging the resulting gel in cold ethanol at -15 °C for three days, with the solvent mixture being refreshed twice daily, (6) performing a first solvent exchange using a 60 / 40 (v / v) ethanol / water mixture for three days with twice-daily refreshment, (7) conducting a second solvent exchange with acetone for two days with twice-daily replacement of fresh acetone, and (8) subjecting the samples to vacuum drying at room temperature in a vacuum oven, thereby obtaining the final pectin xerogel product.- Preparation of the pressure sensors: Disclosed is a method for sensor assembly. The methodcomprises precisely trimming a 1 mm thick pectin xerogel cut along the ice growth direction and cellulose films coated with carbon paste to 2x2 cm dimensions. The trimmed xerogel is then carefully positioned in a sandwich-like configuration between the two cellulose films, ensuring the carbon paste-coated surfaces directly contact the xerogel. Copper tape is affixed to the external surface of each cellulose film to establish electrical contacts. Finally, the assembly is sealed using medical-grade cellulose tape.

[0002] Example 2: Preparation of temperature sensor- Preparation of substrate: Typically, cellulose, chitosan and silk materials were utilized assubstrates. As an example, procedure for cellulose fabrication is mentioned. A suspension of cellulose nanofiber (CNF) was prepared with a concentration of 0.5%, and it was subjected to continuous stirring overnight to assure homogeneous dispersion. To eliminate any bubbles formed within the suspension, a vacuum oven maintained at ambient temperature was utilized prior to the filtration process. Thereafter, the degassed suspension was transferred to the filtration apparatus, using a polyvinylidene fluoride (PVDF) membrane filter with a pore size of 0.45 µm. The resultant wet film was subsequently exposed to a hot press (Carver Inc., USA) treatment, conducted at a steady temperature of 50 °C for a fixed interval of 15 minutes.- Preparation of sensor electrode: An electrically conductive paste was prepared using thefollowing methodology. Initially, a 0.1 g of carnauba wax and 0.3 g of rapeseed oil was sequentially put into a 2 mL glass vial. This vial was then placed on a hot stirrer maintained at 100 °C and mixed until the carnauba wax had completely liquified. Subsequently, 0.4 g of carbon particles (graphite or activated carbon) were gradually incorporated into the liquid mixture while gently stirring at a rate of 60 rpm. Following the cooling of the mixture to ambient temperature (RT), a required quantity of paste was taken from the vial and placed on a printer with melt writing capability. This was accompanied by a heating phase at 100 °C until the paste was thoroughly liquefied and printed via melt extrusion on the substrate.- Encapsulation of the sensor: The temperature sensor was prepared and ensured to be cleanfrom dust or impurities. A suitable quantity of stearic acid was placed into a heat-resistant container and was melted on a hot plate. The controlled temperature setting was adjusted to approximately 70-75°C, slightly above the melting point of stearic acid. Once fully melted, the temperature sensor was carefully submerged into the molten stearic acid using heat- resistant tweezers, ensuring complete immersion. After this, the coated temperature sensor was gently removed and placed in a predetermined heat-resistant molding case, which was designed to form the proper shape and size around the sensor as the stearic acid solidified. The encapsulated sensor was allowed to cool slowly at room temperature, after which it was placed in a refrigerator or cool environment to expedite the solidification process. Once the stearic acid had completely solidified, forming a protective shell around the sensor, the encapsulated sensor was removed from the molding case.

Claims

CLAIMS:

1. A method for preparing an electrically conductive paste, e.g. for use in the preparation of abiomass-based and biodegradable sensor electrode, the method comprising the steps of- combining a natural wax with a vegetable oil in a weight ratio of 1:1 to 1:10- melting the wax with stirring to obtain a melted mixture,- then gradually incorporating carbon particles at a weight ratio of 1-50% to the meltedmixture.

2. A method according to claim 1, wherein the natural wax is carnauba wax, candelilla wax,shellac wax, rice-bran wax, beeswax, cocoa butter, mango butter, kokum butter, or shea butter.

3. A method according to claim 1 or 2, wherein the vegetable oil is sunflower oil, olive oil,canola oil, avocado oil, corn oil, almond oil or sesame oil.

4. A method according to any one of claims 1 to 3, wherein the carbon particles are graphiteor activated carbon particles.

5. A method for preparing a biomass-based and biodegradable sensor electrode, e.g. for usein a wearable sensor, the method comprising the steps of -combining a natural wax with a vegetable oil in a weight ratio of 1:1 to 1:10- melting the wax with stirring to obtain a melted mixture,- then gradually incorporating carbon particles at a weight ratio of 1-50% to the meltedmixture, -solidifying the melted mixture incorporating carbon particles on a biomass-basedsubstrate into a solidified electrically conductive paste thereon.

6. A method according to claim 5, wherein the natural wax is carnauba wax, candelilla wax,shellac wax, rice-bran wax, beeswax, or cocoa butter, mango butter, kokum butter or shea butter.

7. A method according to claim 5 or 6, wherein the vegetable oil is sunflower oil, olive oil,canola oil, avocado oil, almond oil or sesame oil.

8. A method according to any one of claims 5 to 7, wherein the carbon particles are graphiteor activated carbon particles.

9. A method according to any one of claims 5 to 8, wherein the biomass-based substratecomprises a cellulose, chitosan, whey protein, alginate, or silk material.

10. A method according to claim 9, wherein the substrate comprising acellulose, chitosan, whey protein, alginate, or silk material is prepared by:- preparing a suspension of said cellulose, chitosan, whey protein, alginate, or silkmaterial -continuously stirring the suspension, preferably overnight, to ensure homogenousdispersion -degassing the suspension in a vacuum oven maintained at ambient temperature toeliminate any entrapped bubbles -filtering the degassed suspension with a filtration apparatus having suitable filtermembrane, such as a polyvinylidene fluoride (PVDF) membrane filter having a pore size of 0.45 µm -subjecting the resulting wet film subsequently to pressing, such as hot press treatment.

11. A method according to any one of claims 5 to 10, for use in preparation of a pressure sensorelectrode, wherein the melted mixture incorporating carbon particles is cooled, a portionthereof is applied to the substrate, reheated until liquified, pressed with a Teflon sheet for uniform coating, and finally allowed to cool at room temperature, resulting in a solidified electrically conductive paste.

12. A method according to anyone of claims 5 to 10, for use in preparation of a temperaturesensor electrode, wherein a portion of the melted mixture incorporating carbon particles istransferred to a 3-D printer, reheated until liquified, printed on the substrate via melt extrusion, and finally allowed to cool at ambient temperature, resulting in a solidified electrically conductive paste.

13. A method for preparing a pressure sensor, wherein a porous xerogel dielectric material ispositioned between two sensor electrodes prepared according to the method of any one ofthe claims 5 to 11, so that the carbon paste-coated sides of the electrodes are orientedtowards the xerogel dielectric material.

14. A method according to claim 13, wherein the porous xerogel dielectric material is preparedfrom a biopolymer, such as cellulose, chitosan, pectin, chitin, or collagen.

15. A method according to claim 14, wherein the biopolymer comprises pectin.

16. A method according to claim 15, wherein the preparation of a xerogel dielectric materialfrom pectin comprises: (1) dissolving a pectin powder in an aqueous saline solution such as NaCl solution, (2) stirring the mixture at a temperature selected from ambient to 100 degrees of Celsius for a selected period of time,(3) dispensing the solution into a Teflon mold equipped with a copper base plate, (4) resting the mold on a cold bed such as dry ice to induce freeze-casting and gel formation, (5) submerging the resulting gel in cold ethanol, (6) performing a first solvent exchange using a mixture of ethanol and water, (7) conducting a second solvent exchange with acetone, (8) subjecting the samples to vacuum drying at room temperature in a vacuum oven, and (9) obtaining the final xerogel dielectric material.

17. A method according to any one of claims 13 to 16, comprising- precisely trimming a dielectric xerogel cut along the ice growth direction- positioning the trimmed xerogel in a sandwich-like configuration between the twoelectrodes.

18. A method for preparing a temperature sensor, wherein a biomass-based and biodegradablesensor electrode obtained according to a method according to any one of claims 5 to 10 and12 is encapsulated using natural fatty acids.

19. A method according to claim 18, wherein the fatty acids include at least one of Caprylicacid, Capric acid, Lauric acid, Myristic acid, Palmitic acid, Stearic acid, Arachidic acid, Behenic acid, Lignoceric acid, and Cerotic acid.

20. An electrically conductive paste obtainable by the method of any one of claims 1 to 4.

21. A biomass-based and biodegradable sensor electrode obtainable by the method of any oneof claims 5 to 12.

22. A pressure sensor obtainable by a method of any one of claims 13 to 17.

23. A temperature sensor obtainable by a method of any one of claims 18 or 19.

24. A biomass-based and biodegradable sensor electrode comprising a biobased substrate (8,11, 12) coated on one side with a solidified electrically conductive paste, the solidified electrically conductive paste (3, 5) comprising a solidified melted mixture of a natural wax with a vegetable oil in a weight ratio of 1:1 to 1:10 incorporating carbon particles at a weight ratio of 1-50% to the solidified melted mixture.

25. A biomass-based and biodegradable sensor electrode according to claim 24, wherein thenatural wax is carnauba wax, candelilla wax, shellac wax, rice-bran wax, beeswax, cocoa butter, mango butter, kokum butter, or shea butter.

26. A biomass-based and biodegradable sensor electrode according to claim 24 or 25, whereinthe vegetable oil is sunflower oil, olive oil, canola oil, avocado oil, corn oil, almond oil or sesame oil.

27. A biomass-based and biodegradable sensor electrode according to any one of claims 24 to26, wherein carbon particles are graphite or activated carbon particles.

28. A biomass-based and biodegradable sensor electrode according to any one of claims 24 to27, wherein the substrate (8, 11, 12) comprises cellulose, chitosan, whey protein, alginate, or silk material.

29. A pressure sensor (1), comprising a porous xerogel dielectric material (4) positionedbetween two biomass-based and biodegradable sensor electrodes according to any one of claims 24 to 28, wherein the porous xerogel dielectric material is prepared from abiopolymer, such as cellulose, pectin, chitin, or collagen.

30. A pressure sensor according to claim 29, wherein the biopolymer of the porous xerogeldielectric material is pectin.

31. A temperature sensor (13), comprising a biomass-based and biodegradable sensor electrodeaccording to any one of claims 24 to 28 encapsulated using natural fatty acids.

32. A temperature sensor according to claim 31, wherein the fatty acids include Caprylic acid,Capric acid, Lauric acid, Myristic acid, Palmitic acid, Stearic acid, Arachidic acid, Behenic acid, Lignoceric acid, and Cerotic acid.

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