3D-printed carrageenan-based nanocomposites for force-sensing applications

3D-Printed Carrageenan-Based Nanocomposites for Force-Sensing Applications Vera M. Macedo, Nelson Pereira, Carmen R. Tubio, Pedro Martins, Carlos M. Costa,* and Senentxu Lanceros-Mendez 1. Introduction A rapid technological evolution is taking place linked to the Internet of Thing (IoT) concept and the digitalization of the society. [1] The IoT concept is based on the interconnection of physical and virtual objects through sensors, actuators, and the internet. [2] The interconnection between electronic devices allows the continuous monitoring, optimization, and control in intelligent, distributed, self-organizing and energyconstrained networks, [2b] using communication technologies such as mobile and wireless networks and identification by radio frequency, global position systems (GPS), and Bluetooth systems, [2a] which create a complete system, acting as a whole. IoT is becoming widespread in areas including security, smart health systems, agriculture, transport, industry, and construction, among others. [2b] In the industrial sector, IoT allows optimization of processes, effective production times, and reduced manufacturing costs. [1] The merits of the IoT concept strongly rely on the improvement in materials and manufacturing techniques, related to the Industry 4.0 paradigm. [1,2] Smart materials implementation has become a cornerstone of the technological evolution due to their active role in improving materials response and integration. [3] Smart materials can be described as a material that senses environmental changes and responds, in a predictable way, to the respective external stimulus. Depending on the smart material group, they react to different stimuli and provide a specific output or vice versa. [2a,4] Active smart materials groups include piezoelectric, piezoresistive, magnetorheological, V. M. Macedo, C. R. Tubio, S. Lanceros-Mendez BCMaterials Basque Center for Materials, Applications and Nanostructures UPV/EHU Science Park 48940 Leioa, Spain N. Pereira, P. Martins, C. M. Costa, S. Lanceros-Mendez Physics Centre of Minho and Porto Universities (CF-UM-UP) University of Minho 4710-053 Braga, Portugal E-mail: cmscosta@fisica.uminho.pt The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adem.202201806. © 2023 The Authors. Advanced Engineering Materials published by WileyVCH GmbH. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. DOI: 10.1002/adem.202201806 N. Pereira Centro ALGORITMI University of Minho Campus de Azurém, 4800-058 Guimarães, Portugal P. Martins, C. M. Costa, S. Lanceros-Mendez Laboratory of Physics for Materials and Emergent Technologies (LapMET) University of Minho 4710-057 Braga, Portugal C. M. Costa Institute of Science and Innovation for Bio-Sustainability (IB-S) University of Minho 4710-053 Braga, Portugal S. Lanceros-Mendez Ikerbasque Basque Foundation for Science 48009 Bilbao, Spain Technological development is leading to an exponential growth in the implementation of sensors and actuators where the concern about environmental problems is also focusing on electronic waste (e-waste), which is composed of hazardous materials, corresponding to a large part of urban waste, and has a strong environmental impact. Therefore, more environmentally friendly electronic components are required, natural polymers being a suitable approach to solve or attenuate those problems. This work reports on a bio-based polymer, carrageenan, embedded with dielectric barium titanate (BTO) nanoparticles to tailor the electrical response. The inclusion of the filler induces slight modifications in the thermal characteristics and on the physicochemical properties of the polymer matrix. On the other hand, the mechanical and dielectric properties improve with the addition of BTO and a high dielectric constant of ε 013 000 is obtained for the composite with 20 wt% BTO content. The increase of the dielectric constant is accompanied by a high AC electrical conductivity, leading to a highε 0–high-loss material. The 20 wt% BTO composite is used to produce a force measuring sensor, due to the highest dielectric response. The functional response of the sensing system shows good stability over cycling. RESEARCH ARTICLE www.aem-journal.com Adv. Eng. Mater. 2023,25, 2201806 2201806 (1 of 10) © 2023 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH magnetoelectric, shape memory, electrorheological, chromic, self-healing, and pH-sensitive materials, among others. [5] Since these materials possess a great capability to respond upon an external stimulus, they are excellent for sensing applications. Ferroelectric ceramic–polymer composites are examples of smart materials, since combine the best of the ferroelectric ceramic properties (high dielectric and piezoelectric coefficients, low dielectric and mechanical losses and good thermal stability) with the advantages of the polymer matrices (low density, flexibility, excellent integrability and processable by additive manufacturing techniques), [6] which make it highly suitable for a large variety of sensing, electric power and electronics systems. [7] In this area, the production of sensors based on smart materials has typically relied in synthetic polymers, as they provide an efficient way to achieve the desired properties, through their chemical and structural modification. However, electronic equipment is mass produced, leading to a strong increase of the so-called electronic waste (e-waste). If e-waste is not managed properly, it can lead to contamination by toxicity and health risks, compromising the surrounding community. [8] E-waste has a potential to recover valuable materials as iron, aluminum, copper, gold, silver, and rare earth metals; that is why it is globally recognized as a resource for these elements/materials. [9] The recycling of metals from electronic devices may reduce the need for mining virgin materials. Nonetheless, e-waste collection rates and poor recycling or inefficient end processing for electrical or electronic equipment, and consequently these valuable resources are wasted. [9a] Thus, a different strategy must be implemented. In order to achieve more sustainable smart materials and to develop at the same time more sustainable engineering principles, natural polymers can be used as an alternative to synthetic ones. The advantage of natural polymers is their (bio)degradability, nontoxicity, and the possibility, in some cases, to be processed by additive manufacturing with water as a solvent. [10] In electronic technologies, natural polymers can facilitate the recycling process due to degradability. They can also lead to less toxic components which are also biocompatible, particularly relevant for wearable sensors. [10] The two main types of natural polymerbased materials more suitable on electronic applications are polysaccharides (chitosan, agarose, Arabic gum, dextran, hyaluronic acid, alginate, carrageenan) and proteins (albumin, gelatine, soy, and collagen). [11] Carrageenan is a natural polymer that has shown great potential for several applications, including smart packaging, [12] drug delivery, [13] and biomedicine. [14] Carrageenan is a natural sulfated and anionic polysaccharide (carbohydrate) extracted from the multicellular wall of certain species of red algae seaweeds of the Rhodophyceae family such as Chondrus crispus, Gigartina, Euchuema, and Hypnea. Carrageenan exists in large abundance in nature at low cost. [15] Usually, natural polysaccharides are hybrids and contain repetitive disaccharide units of several carrageenan types attached in a single-polymer chain. [16] Depending on the number and position of sulfate groups in the disaccharide repeating unit, three main types of carrageenan can be found: kappa (κ), iota (ι), and lambda (λ), where its sulfate content is 20%, 33%, and 41% (w/w), respectively. [11,13,15b,17] Carrageenan with higher levels of sulfation tends to decrease gel strength and solubility temperature. [15b] This is the reason for selecting iota (ι)-carrageenan for a large variety of applications, since its sulfation is an intermediate value. [18] Carrageenan is a natural polymer that has shown great potential for several applications, including smart packaging, [12] drug delivery, [13] biomedicine, [14] fuel cells, [19] electrochromic devices, [19] and solid polymer electrolytes. [16] As representative examples, a polymer–salt complex based on ι-carrageenan and ammonium bromide (NH 4 Br) has been produced by solutioncasting method, leading to a maximum ionic conductivity of 1.46 10 5 Scm 1 and an electrochemical stability window of 2.1 V. [20] NH 4 NO 3 within ι-carrageenan has been applied to electrochemical devices [19] and solid polymer electrolytes have been developed with ι-carrageenan as polymer host and lithium chloride (LiC) as filler, leading to a high room temperature conductivity of 5.33 10 3 Scm 1 for the 1.0 g ι-carrageenan: 0.3 g LiCl sample. [16] The properties that drew attention in carrageenan to be used as a polymer matrix for additive manufacturing technologies, in addition to its ecofriendly behavior and being soluble in water, were its rheology and strong gel properties after cooling. [11,15b,17,21] Owing the capability to create a water-retaining hydrogel, ι-carrageenan is suitable for being used in 3D printing techniques. [22] Nevertheless, despite their interesting properties, there are also limitations in terms of mechanical and electrical properties for some application areas. [23] Thus, in most natural polymers, nanoparticles are being used to develop polymer composites, allowing to introduce and/or tune specific properties of the material by combining their individual properties of polymer and filler. [23b] Barium titanate (BTO, BaTiO 3 ) is characterized by high dielectric response and ionic conductivity characteristics and appears to be a good candidate for next generation of materials with high-ε0–high-loss material, in combination with natural polymers. [24] BTO is a ferroelectric material, with high dielectric constant and spontaneous polarizations and low dielectric losses. [25] Despite its potential interest for tuning polymer matrix electrical response, no studies have been reported regarding carrageenan composites with BTO nanoparticles, in any of carrageenan types. The combination of carrageenan/BTO and their processability by additive manufacturing, more precisely by direct ink writing (DIW), can allow a new generation of electroactive materials with reduced material waste during processing and improved sustainability due to the bio-based origin of the polymer matrix. [26] In particular, DIW allows to develop both simple and complex customizable sensor designs to improve device integration, while being a scalable, low-cost, sustainable, and high-accuracy printing technology. [27] Thus, this work presents a new composite of ι-carrageenan with BTO nanoparticles, processes by 3D printing, in order to develop a dielectric high ε0–high loss material, optimized for touch sensing applications. www.advancedsciencenews.com www.aem-journal.com Adv. Eng. Mater. 2023,25, 2201806 2201806 (2 of 10) © 2023 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH 15272648, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adem.202201806 by Universidade Do Minho, Wiley Online Library on [18/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 2. Experimental Section 2.1. Materials Iota-carrageenan was purchased from Alfa Aesar. Ultrapure water was obtained from a Milipore-Q system. Barium titanate (BTO, BaTiO 3 ), with 100 nm particle size and the surfactant Triton X-100, were purchased from SkySpring Nanomaterials and Sigma-Aldrich, respectively. 2.2. Sample Preparation BTO nanoparticles in different filler contents (neat carrageenan sample, 0, 1, 5, 10, 20 and 40 wt%) were dispersed for 3 h in a mixture of ultrapure water with the surfactant triton X-100 (0.01 wt%), previously homogenized in an ultrasound bath (ATU, model no. ATM40-3LCD, 50 W) for 1 h to promote BTO nanoparticles dispersion. Further, 3 wt% of carrageenan was added into the solution and stirred for another 3 h at 150 rpm. The main steps for the composite of carrageenan/ BTO nanoparticle production are described in Figure 1a. 2.3. Printing Process The produced gel was introduced into the syringe and, in order to remove the bubbles, the syringe was isolated with parafilm in both edges (so that the gel does not leak) and placed into a centrifuge (MRC Lab) for 15 min at 2500 rpm. The syringe with free bubbles gel was placed in the bioprinter (Single Head 3D Bioprinter - 3D Cultures Tissue Scribe, 10 mL nozzle) and, after an optimization procedure, the following parameters were used for the printing process: needle of 0.41 mm, space line of 0.51, to avoid overlay of lines on the same plane, 30 mm s 1 printing speed, and 0.008 extrusion multiplier. Then a 3D form was printed, which resulted into a film, due the curing process and water evaporation, with final thickness of 80 μm. Figure 1b describes the main printing steps. 2.4. Samples Characterization The morphology of the composites was analyzed via scanning electron microscopy (SEM) using a Carl Zeiss EVO-40 equipped with an energy-dispersive spectroscopy (EDS) detector. To evaluate the polymer structure and possible interface bonding between polymer and filler, Fourier-transform infrared spectroscopy (FTIR) was performed in a Jasco FT/IR-4100 system. Measurements were performed at room temperature, in the attenuated total reflection (ATR) mode, between 600 and 4000 cm 1 , using 64 scans at a resolution of 4 cm 1 . Thermal properties of the sample were evaluated by differential scanning calorimetry (DSC) using a Mettler Toledo DSC 822e apparatus equipped with a sample robot under nitrogen flow of 20 mL min 1 . The scans were carried out from 20 to 150 °C at a heating rate of 10 °C min 1 . The samples were weighed and compacted in an aluminum crucible, with a small hole on the top. Figure 1. Schematic representation of the experimental steps for a) composite solution preparation and for the b) printing process. www.advancedsciencenews.com www.aem-journal.com Adv. Eng. Mater. 2023,25, 2201806 2201806 (3 of 10) © 2023 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH 15272648, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adem.202201806 by Universidade Do Minho, Wiley Online Library on [18/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Thermogravimetric analysis (TGA) measurements were performed in a TGA/SDTA 851e Metter Toledo apparatus. The samples were heated from 30 to 600 °C, at a rate of 10 °C min 1 , with a nitrogen flow rate of 50 mL min 1 . The room-temperature mechanical properties of the composites were evaluated under tensile stress at a constant deformation of 50 μms 1 with a load cell of 200 N in a Linkam Scientific Instruments TST 360. Samples were prepared in rectangular shape with dimensions of 20 mm 10 mm 80 μm. The results were provided as the average of five specimens. Dielectric measurements were performed, in samples with the same thickness (80 μm) and electrode area, by obtaining the capacity, C, and the dielectric losses, tan δ, using a Quadtech 1920 inductance–capacitance–resistance (LCR) precision meter, at room temperature, in the 20 Hz–1 MHz frequency range and with an applied voltage of 0.5 V. In each composite, circular gold electrodes of 5 mm diameter were deposited in both sides in a magnetron sputtering SC502 sputter coater. The real part of the dielectric function (ε0), tan δ, and the real part of the electrical conductivity (σ0) were determined through the following equations ε0¼C:d ε0:A(1) tan δ¼ε00 ε0(2) and σ0ðωÞ¼ε0ωε00 ðωÞ(3) where Cis the measured capacitance, ε 0 is the permittivity of free space, Ais the electrode area (m 2 ), dis the thickness of samples (m), and ω¼2πfis the angular frequency. [28] 3. Results and Discussion 3.1. Morphological Analysis The surface morphology of the samples was analyzed using the representative SEM images presented in Figure 2. For neat carrageenan (Figure 2a), a homogeneous flat surface with low roughness is obtained. The addition of the fillers (Figure 2b–d) leads to rougher surface due to both the presence of the fillers and, in particular, of the number of well-distributed agglomerates that increase with increasing filler content, being particularly evidenced for the sample with 40 wt% BTO content. EDS images from Figure 2e–g confirm this fact by the identification of the distribution of barium in the samples (presented in blue) corresponding to the nanoparticles location. 3.2. Physical–Chemical, Thermal, and Mechanical Properties FTIR-ATR spectra for neat carrageenan and BTO/carrageenan composites with different BTO contents are shown in Figure 3a. For neat carrageenan, the typical vibration bands at 805, 845, 905, 930, and 1070 cm 1 are observed, that correspond to the C─O─SO 3 bonds of 3,6-anyhydrogalactose in C2, C4, and C6 of the galactose unity and the C─O bond of 3, 6-anyhydrogalactse, respectively. [20] Further, the absorption bands of galactose groups and S═O bond of sulphate esters were detected at 970–975 and 1240–1260 cm 1 , respectively. The C═O asymmetric stretch/N–H deformation and the OH/NH stretching are identified at 1635 and 3400 cm 1 , respectively. [29] No significant variations are observed in the spectra, independent of the filler content compared to the FTIR spectra of BTO nanoparticles, indicating that there are no new bonds or strong interactions between the filler and the polymer matrix. The DSC measurements (Figure 3b) reveal that the endothermic peak corresponding to the intrinsic water removal and glass transition temperature of the polymer decreases with increasing BTO content, ranging from approximately 100 °C for the pristine polymer [19,30] to 80 °C for the composite with 40 wt%. From TGA analysis (Figure 3c), the first mass loss, related to water evaporation, [31] occurred at temperatures around 25–100 °C. The mass loss decreases with decreasing BTO content, being particularly evident for the 40 wt% BTO content sample. It can be associated to the moisture encapsulated in the polymer matrix. The higher mass loss was reached at 200–250 and 300 °C, which is related to the carrageenan polymeric backbone degradation. [32] At the end of the test (600 °C), the amount of composites mass detected is found to be directly proportional to the filler content. Figure 3d shows the stress–strain curves obtained by mechanical tests in the tensile mode for all samples. It is observed the typical mechanical response of the carrageenan polymer [33] and that the addition of the BTO content improves the mechanical properties, the BTO nanoparticles act as a mechanical reinforcement, as well as the observed reduced amount of water which increases the stiffness. [34] This behavior is detected for all filler contents, even the smallest ones. The composites of 20 and 40 wt% BTO content seem to have an identical mechanical behavior (Figure 3d). Young’smodulus was determined by the tangent method in the elastic region [35] (inset of Figure 3d), where the samples with 10 wt% BTO content showed higher Young’sModulus,yieldstrength,and ultimate/rupture strength probably due to better dispersion of the BTO nanoparticles. The incorporation of fillers into a polymer matrix typically enhances its mechanical strength (rigidity, hardness, fracture toughness, among others) due the high surface area-to-volume ratio of the fillers, which improves the interaction between both matrix and filler and is directly influenced by the filler’s size, shape and distribution, concentration, and specific interaction with the matrix. [36] In this way, the mechanical properties of net carrageenan are improved (Figure 3b, inset for the Young modulus) for the composites up to 10 wt% BTO content. For filler concentrations above 10 wt% BTO, there is a decrease in mechanical properties, as large filler concentrations lead to filler agglomerates (Figure 2), acting as defects under mechanical deformation. 3.3. Dielectric Behavior Figure 4a shows the real (ε0) part of the dielectric constant for neat carrageenan and BTO/carrageenan composites at room www.advancedsciencenews.com www.aem-journal.com Adv. Eng. Mater. 2023,25, 2201806 2201806 (4 of 10) © 2023 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH 15272648, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adem.202201806 by Universidade Do Minho, Wiley Online Library on [18/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License temperature, as a function of frequency. From Figure 4a, it is observed that ε0decreases with increasing frequency, independent of the BTO content, corresponding to slow dipolar mobility. [28] A high dielectric response is observed for neat carrageenan, (Figure 4a) due to the high hydrogen bonds, polar groups, and water content, contributing both to the dielectric response (Figure 4a) and to conductivity (Figure 4b). It is observed that ε0increases with increasing BTO content due to the increased mobile charge carriers and interfacial charge contributions of the Maxwell–Wagner–Sillars (MWS) effect. [28] This effect is observed in Figure 4c for ε0as a function of BTO content at 10 kHz. Figure 4b shows the AC electrical conductivity, σ0, value as a function of frequency calculated from Equation (3) for all samples, showing that σ’increases with increasing frequency, due to the increased charge carrier mobility in localized states. In addition, it is noticed that the AC conductivity increases with BTO content, as the inclusion of the fillers increases the interfacial and water effects. [37] Figure 4c shows the ε0of the samples at 10 kHz as a function of BTO content at room temperature. A linear behavior of ε0is observed as a function of filler content up to filler content of 20 wt % and a decrease of the dielectric response is observed for the 40 wt% BTO samples, due to the larger filler aggregates, that reduce the water effect. The highest dielectric constant for BTO/carrageenan composites is 13 000 for the sample with 20 wt% BTO content. Thus, it is verified that ε0increases with increasing BTO content due interfacial and space charge polarization, the composite with 20 wt% BTO with the largest dielectric response at 10 kHz. Figure 2. Surface SEM images of neat carrageenan a) and the composites with different BTO contents: b) 1 wt%, c) 10 wt%, and d) 40 wt% of BTO. EDS mapping images for Ba (Barium, blue) for samples with e) 1 wt% of BTO, f) 10 wt% of BTO, and g) 40 wt% of BTO. www.advancedsciencenews.com www.aem-journal.com Adv. Eng. Mater. 2023,25, 2201806 2201806 (5 of 10) © 2023 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH 15272648, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adem.202201806 by Universidade Do Minho, Wiley Online Library on [18/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 3.4. Force Sensor Development and Functional Response Figure 5 shows the schematic representation of the sensor (a), its working principle (b), and the actual image (c). A force sensor has been developed with the BTO/carrageenan composite with 20 wt% of BTO considering its high dielectric value. The sample was placed between two conductive PET/ITO films with a PET separator. The geometry of the developed sensor allows the measurement of a compression force based on a 200 μm air gap, created by the separator and the BTO/carrageenan composite with 20 wt% of BTO. The gap reduces its size with the pressure, which changes the distance between the two electrodes, changing the electrical response of the sensor, measured as a capacity signal. The results of Figure 6a show the capacity value of the sample at a frequency of 1 kHz while applying a force from 0 to 10 N (0–55 kPa) at 1 N s 1 . Figure 6a,b shows the normalized capacity variation under cyclic applied force for 80 and 70 (detail) cycles, respectively. Figure 6c shows the sensitivity of the sample. It is shown that the force sensor presents good repeatability for over 80 cycles. Figure 6c shows that the sample presents good linearity until the 30 kPa range, with a sensitivity of 0.0667 kPa 1 (the sensitivity is defined as S=(ΔC/C 0 )/p, 1000 1500 2000 2500 3000 3500 4000 BTO (a) 40wt.%BTO 1wt.%BTO Transmittance / a. u. Wavenumber / cm -1 C-O-C O=S=O -O-SO C-O C-H O-H 0wt.%BTO 5wt.%BTO 10wt.%BTO 20wt.%BTO 20 40 60 80 100 120 140 endo (b) Heat Flow / W.g -1 Temperature / ºC 80 ºC 89 ºC 100 ºC 99 ºC 102 ºC 40wt.% BTO 20 wt.% BTO 10 wt.% BTO 5 wt.% BTO 1 wt.% BTO 0wt.% BTO 105 ºC 100 200 300 400 500 600 20 30 40 50 60 70 80 90 100 (c) Weight / % Temperature / ºC 0 wt.% BTO 1 wt.% BTO 5 wt.% BTO 10 wt.% BTO 20 wt.% BTO 40 wt.% BTO 0 4 8 1216202428 0 10 20 30 40 50 60 (d) Stress / MPa Strain / % 0 wt.% BTO 1 wt.% BTO 5 wt.% BTO 10 wt.% BTO 20 wt.% BTO 40 wt.% BTO Figure 3. a) FTIR-ATR spectra, b) DSC scans, c) TGA thermograms, and d) stress–strain mechanical curves (insert: Young’s modulus as a function of BTO content in the composites) for neat carrageenan and BTO/carrageenan composites containing different BTO contents. 10 2 10 3 10 4 10 5 10 6 10 2 10 3 10 4 10 5 (a) / Hz 10 2 10 3 10 4 10 5 10 6 / Hz 0wt.% BTO 1wt.% BTO 5wt.% BTO 10wt.% BTO 20wt.% BTO 40wt.% BTO 1E-3 0,01 0,1 (b) 0wt.% BTO 1wt.% BTO 5wt.% BTO 10wt.% BTO 20wt.% BTO 40wt.% BTO ' / S.m -1 010203040 4000 6000 8000 10000 12000 14000 (c) wt.% of BTO 10kHz Figure 4. a) ε0and b) σ0for neat carrageenan and BTO/carrageenan nanocomposites. c) Variation of the dielectric constant as a function of BTO content at 10 kHz. www.advancedsciencenews.com www.aem-journal.com Adv. Eng. Mater. 2023,25, 2201806 2201806 (6 of 10) © 2023 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH 15272648, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adem.202201806 by Universidade Do Minho, Wiley Online Library on [18/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License where Cand C 0 denote the capacity with and without applied pressure, and pdesignates the applied pressure), the signal starting to saturate for higher compression forces. Once characterized, the developed force sensor was connected to an electronic circuit based on a microcontroller Arduino Uno (Figure 7a,b). The circuit sends a continuous signal at 1 kHz to one of the electrodes of the sensor and reads the value on the other electrode that is connected to the microcontroller analog–to-digital converter (ADC). A digital Butterworth low-pass filter with a cut frequency of 10 Hz was implemented in the microcontroller firmware, in order to filter the high-frequency signal, while leaving the signal from the pressure event. Figure 7c shows the data received by the microcontroller ADC that was sent to a graphical user interface (GUI) created in QT Creator, where the data can be visualized in real time (see supplementary video). The results show good correlation with the previous tests using the LCR meter. The sensor can detect a finger pressure and quantify an increase or decrease in pressure. In Figure 5. a) Schematic diagram of the sensor, b) sensor working principle, and c) picture of the fabricated force/touch sensor. -10 0 10 20 30 40 50 60 Pressure / kPa 0 500 1000 1500 2000 2500 3000 3500 4000 4500 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 Pressure C/C 0 Time / s C/C0 -10 0 10 20 30 40 50 60 Pressure / kPa 500 550 600 650 700 750 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 (b) (a) Pressure C/C 0 Time / s C/C 0 0 102030405060 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 (c) C/C0 Pressure / kPa Mean+ SD Fit Figure 6. a) BTO/carrageenan composite with 20 wt% of BTO under 80 cycles of compression with varying pressure from 0 to 55 kPa. b) Magnification area of the cycles (rectangle in a)) and c) sensitivity of the sample. www.advancedsciencenews.com www.aem-journal.com Adv. Eng. Mater. 2023,25, 2201806 2201806 (7 of 10) © 2023 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH 15272648, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adem.202201806 by Universidade Do Minho, Wiley Online Library on [18/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License order to isolate a touch event from the hand (proximity of the finger to the electrodes), a polylactic acid (PLA) rod was also used to test the pressure detection. The demonstrated simple-circuit implementation shows the versatility of the developed sensor for touch/pressure detection applications. It is the first time that a force sensor based on carrageenan with BTO nanoparticles has been reported and its behavior is comparable to sensors reported in the literature based on composites produced by synthetic polymers such as, carbon nanotube/polydimethylsiloxane (PDMS), [38] MXene/ polyvinylidene fluoride, [39] and BTO with PDMS. [40] The dielectric response of the BTO/carrageenan nanocomposites as a force sensor was demonstrated in a functional device, opening new avenues for the development of environmentally friendly composite materials for electronic applications. 4. Conclusion Carrageenan-based composites with BTO nanofillers have been developed by direct ink writing for force-sensing applications. Further, the influence of BTO nanofillers content, up to 40 wt%, on the composite physical–chemical properties has been also addressed. The samples showed a compact microstructure, the BTO filler being homogeneously distributed into the matrix and observing an increase in filler aggregation for the samples with larger nanoparticle content. The thermal properties of the samples depend on BTO filler content, T g values, and endothermic peak intensity, decreasing with increasing filler content. Mechanical and dielectric properties were also affected by the BTO content. Mechanically, the best sample was the one with 10 wt% of BTO, acting as better mechanical reinforcement. Dielectric properties of the composites were generally improved upon BTO addition. In particular, for the 20 wt% BTO content sample, the dielectric constant was 13 000, showing also an AC conductivity of 0.011 S m 1 at 10 kHz, being therefore a high-ε0–high-loss material. With respect to sensor application, a good repeatability in the sensor electrical response has been obtained with the 20 wt% BTO sample for over 80 cycling tests for force variation between 0 and 10 N. A good linearity has been obtained for forces up to 5 N range (around 86.7 pF N 1 ), starting to saturate at higher compression forces. In summary, the developed BTO/carrageenan composites exhibit a suitable sensing response, thus allowing to develop force sensors for sustainable electronics. 0102030405060708090100110120 200 300 400 500 600 (c) Value / a.u. Time / s (a) (b) Figure 7. a) Photography of the electronic circuit and capacitive sensor, b) schematic of the electronic circuit, and c) data received from the electronic circuit when a pressure event occurs. www.advancedsciencenews.com www.aem-journal.com Adv. Eng. Mater. 2023,25, 2201806 2201806 (8 of 10) © 2023 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH 15272648, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adem.202201806 by Universidade Do Minho, Wiley Online Library on [18/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Acknowledgements The authors thank the Fundação para a Ciência e Tecnologia (FCT) for financial support under the framework of Strategic Funding grants UIDB/04650/2020, UID/FIS/04650/2020, UID/EEA/04436/2020, and UID/QUI/0686/2020 and under projects POCI-01-0145-FEDER-028157 and PTDC/FIS-MAC/28157/2017 funded by national funds through FCT and by the ERDF through the COMPETE2020—Programa Operacional Competitividade e Internacionalização (POCI). The authors also thank the FCT for financial support under grant SFRH/BD/131729/2017 (N.P.) and contracts under the Stimulus of Scientific Employment, CEECIND/03975/2017 (P.M.) and 2020.04028.CEECIND (C.M.C.). Financial support from the Basque Government Industry Department under the ELKARTEK program is acknowledged. The authors thank technical and human support provided by SGIker (UPV/EHU/ERDF, EU). Conflict of Interest The authors declare no conflict of interest. Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request. Keywords carrageenan, polymer–matrix composites, sensors, smart materials Received: December 13, 2022 Revised: February 22, 2023 Published online: March 18, 2023 [1] R. Ashima, A. Haleem, M. Javaid, S. Rab, Adv. Ind. Eng. Polym. Res. 2021,5, 137. [2] a) J. Oliveira, V. Correia, H. Castro, P. Martins, S. Lanceros-Mendez, Addit. Manuf. 2018,21, 269; b) S. Yousefi, H. Karimipour, F. 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