Design and Characterization of a Flexible Pressure Sensor Using Starch‐Based Biopolymer Films Doped with Electrographite

1. Introduction

The growing demand for flexible, lightweight, and environmentally sustainable devices has driven the development of piezoresistive sensors based on biocompatible and biodegradable materials. Starch is an abundant biopolymer and a promising candidate for sustainable materials due to its renewability and biodegradability, although its processability and dimensional stability are limited without formulation strategies such as plasticization, reinforcement, or blending [1]. In particular, biopolymers derived from polysaccharides such as starch have emerged as promising candidates for use as active matrices in sensors due to their low cost, abundance, aqueous processing, and ease of chemical modification [2,3,4].

Recent high-impact studies also demonstrate that biopolymer-rich soft conductors can achieve competitive sensing performance when engineered as ionic hydrogels or organohydrogels for wearable applications. For example, a multifunctional poly(acrylic acid)/chitosan hydrogel incorporating boric acid and glucose was reported to combine high stretchability (up to 1694% strain), self-healing self-adhesion, and durable strain sensing over 5000 cycles, with a gauge factor of 5.7 [5]. Enzyme-crosslinked gelatin organohydrogels prepared exclusively from food-grade components have similarly enabled sustainable soft strain and temperature sensing, achieving ≈450% stretchability, a reported gauge factor of 2.86, and long-term electromechanical stability over 5000 cycles [6].

In parallel, polyacrylamide/gelatin ionic hydrogels reinforced with sodium alginate have been shown to reach tensile strength on the order of 110 kPa and elongation up to 1500%, while providing stress/pressure sensing functionality (reported GF = 1.07 and pressure sensitivity = 0.0107) [7]. While these hydrogel platforms emphasize highly deformable strain- or stress-sensing, they highlight the broader momentum toward sustainable soft sensors and motivate the exploration of simpler, film-based biopolymer composites as low-cost sensing layers.

Despite this, their inherently insulating nature poses a challenge for achieving the adequate electrical conductivity required for signal transduction under mechanical stimulation. An effective strategy to overcome this limitation involves incorporating coal-based materials in powder form (e.g., electrographite) as conductive agents, thereby forming internal percolation networks that facilitate charge transport under deformation [8,9,10].

Starch-based films remain highly sensitive to moisture and aging, leading to changes in mechanical response and functional performance over time. Moisture uptake and conditioning at high relative humidity can reduce mechanical integrity in starch-based films and contribute to defects such as microcracking, thereby motivating protective strategies for real applications [11]. Moreover, the mechanical behavior of starch films is affected by plasticizer content, storage humidity, and thickness through their influence on chain mobility and crystallinity development [9]. Aging-related recrystallization can further modify film properties during storage, emphasizing the importance of controlling formulation and post-processing history [12].

In this work, a starch-based sensing layer is combined with conductive carbon fillers to enable a pressure-dependent electrical response. Reinforcement and formulation control are widely used to tailor starch film properties, including via microcrystalline cellulose reinforcements and variations in plasticizer type, which affect stiffness and moisture uptake [13,14]. Cassava starch films, in particular, have been studied as edible films with measurable mechanical and transport properties, underscoring the role of composition and processing in performance [15]. Related studies on starch films from different botanical sources further confirm that mechanical behavior and biodegradability depend on the starch source and formulation [10,16].

Beyond starch, polysaccharide-based films from alternative sources, such as soluble soybean polysaccharides and kefiran, demonstrate that additives and formulation strategies can markedly influence mechanical and barrier performance, illustrating the broader sensitivity of biopolymer films to compositional design [17,18,19]. Starch itself varies with botanical source; for example, plantain starch exhibits distinct physicochemical properties that can influence gelatinization and film behavior [20]. More recent work has explored biopolymeric films and membranes derived from agricultural residues such as banana peel, highlighting both the opportunities and water-sensitivity limitations of such matrices [21,22,23,24]. Hybrid approaches incorporating mineral fillers (e.g., eggshell-derived CaCO3 in cornstarch) provide additional examples of property tuning in biodegradable films [25].

This work presents a pressure sensor in which the active layer consists of a starch-based biopolymer film doped with electrographite. This layer is confined between two ultrathin aluminum sheets (serving as electrodes) and structurally supported by an acetate substrate. The proposed configuration combines the flexibility of the biopolymer with the electrical conductivity of both the metallic electrodes and the electrographite dopant, aiming to achieve a sensitive and stable device. The presence of electrographite within the starch matrix is expected to form conductive pathways that respond to applied load/pressure variations through changes in electrical resistance (piezoresistive effect). Moreover, the use of ultrathin metallic layers allows for efficient signal collection without compromising the overall flexibility of the system.

This prototype contributes as a proof-of-concept toward flexible pressure sensing using a starch-based biopolymer sensing layer; potential applications (e.g., e-skin or wearables) require additional validation beyond the static loading protocol reported here. The following section outlines the theoretical background supporting the sensor’s physical, chemical, and structural principles.

2. Theoretical Background

2.1. Flexible Pressure Sensors and the Piezoresistive Principle

Piezoresistive sensors convert mechanical stimuli into variations in electrical resistance as the internal structure of the material changes under applied pressure. This effect originates either from alterations in contact between conductive particles (proximity, direct contact) or from changes in the geometry of the conductive pathway (length, cross-sectional area) [13,22]. In polymer–conductor composites, the application of pressure reduces the spacing between conductive particles, increasing the connectivity of the network and thus lowering the overall electrical resistance.

In flexible sensors, optimizing sensitivity (ΔR/ΔP), minimizing hysteresis, and ensuring long-term cyclic stability (load–unload cycles) are crucial performance factors [12]. Additionally, geometric design elements—such as film thickness, electrode arrangement, and overall dimensions—affect the linearity and detection range of the sensor.

2.2. Starch-Based Biopolymers as Functional Matrices

Starch is a natural polysaccharide composed of amylose and amylopectin, capable of forming continuous films through gelatinization and drying [1,20]. Its use in flexible electronic devices has been explored in strain sensors, supercapacitors, and functional membranes [12,17]. Notably, modifying starch to impart electrical functionality has been pursued through the inclusion of conductive nanoparticles into the matrix.

Recent studies show that starch films can incorporate carbon black to enhance conductivity, maintaining acceptable mechanical properties at low filler concentrations [4]. However, excessive concentrations of coal lead to particle aggregation, weak interfacial adhesion, and compromised mechanical strength. Additionally, increased coal content has been linked to higher water vapor permeability and solubility, reducing the environmental stability of the sensor [4].

Another strategy has been the laser-induced or transfer-based carbonization of starch films to create hybrid materials suitable for biodegradable sensing platforms [23]. Fully starch-based hydrogels with electrical transduction capability have also been developed [24].

The adoption of biodegradable biopolymer matrices in sensor development aligns with the goals of green electronics and sustainable design, as reviewed in recent works on compostable flexible sensors [15,16].

For sensing applications, starch-based matrices must be coupled to a conductive phase to enable electrical transduction. Carbon fillers such as graphite, carbon nano-tubes, and related carbonaceous materials are widely used to create percolated conductive networks in polymer composites, supporting piezoresistive sensing under compression. Recent reviews summarize how micro- and nanoscale engineering and conductive network design can improve sensitivity, working range, hysteresis, and response dynamics in flexible piezoresistive pressure sensors [8].

Graphite-containing conductive films provide a relevant benchmark for particulate carbon networks. For example, Yan et al. [26] reported a multiwalled carbon nano-tube/graphite powder conductive film integrated into a flexible sensor, achieving high sensitivity (980 kPa−1 at 0–3 kPa; 370 kPa−1 at 5–10 kPa) and sub-second response times (105 ms rising; 172 ms releasing), with durability over 1200 load/unload cycles.

Complementarily, recent biopolymer-rich hydrogel and organohydrogel platforms have demonstrated sustainable soft sensing with high deformability and long-term cy-cling stability [5,6,7], motivating the positioning of the present starch–electrographite film as a simpler, aqueous-processed, film-based alternative for proof-of-concept pressure sensing.

2.3. Conductive Polymers and Hybrid Composites

The incorporation of carbon-based materials—such as carbon black, graphite, carbon nanotubes (CNTs), or graphene—into polymer matrices is a well-established method to induce electrical conductivity. In elastomeric or flexible polymer systems, these fillers form percolation networks that enable electron flow when the critical concentration threshold is surpassed [27]. In pressure sensors, some composites function as quantum tunneling composites (QTC), where pressure induces tunneling between closely spaced metallic or conductive particles [3,11].

The literature on flexible sensors offers numerous examples of polymers blended with carbon fillers—such as CNTs, carbon fibers, and graphene dispersed in elastomeric matrices like polydimethylsiloxane (PDMS)—for highly sensitive piezoresistive sensing [9,19]. Comprehensive reviews also highlight advances in polymer-based pressure sensors using microstructural enhancements (pores, domes, channels) to improve performance [13,16].

One recent example involves a high-sensitivity conductive material fabricated using carbon ink blended with CNTs [18]. In general, the balance between filler content and dispersion quality is critical to achieving optimal conductivity without sacrificing mechanical integrity [28,29].

Within this context, the present work explores a starch-based matrix doped with electrographite as a low-cost, aqueous-processed composite film. The emphasis is on demonstrating a measurable piezoresistive response under a simple static loading protocol, while acknowledging that further optimization and validation (e.g., thickness control, response dynamics, and extended cycling) are required to approach wearable-grade benchmarks exemplified by graphite- and hydrogel-based sensors [5,6,7,8,26].

2.4. Sandwich Configuration with Metallic Electrodes

In many piezoresistive sensors, the active layer is placed between two electrodes to measure changes in resistance across its thickness. In this work, ultrathin aluminum foils are used as top and bottom electrodes, with the doped biopolymer layer in between and supported by a transparent acetate substrate. This configuration is commonly found in membrane- or film-type sensors. Ensuring reliable and low-contact-resistance interfaces between the active material and the metallic electrodes is essential for effective operation.

Similar sandwich structures using metallic or graphite electrodes have been reported in e-skin sensors and tactile pressure devices [17,23]. The thinness of the metallic layers helps reduce stiffness and preserve overall flexibility and user comfort.

2.5. Role of Microstructure and Carbon Dispersion

The effectiveness of the sensor strongly depends on the spatial distribution of electrographite powder within the starch matrix. A homogeneous dispersion promotes the formation of uniform conductive pathways, while particle agglomeration creates non-conductive zones, discontinuities, and mechanical stress points [4]. Techniques such as ultrasonication, surfactant addition, or surface functionalization of the carbon particles are used to improve dispersion in polymer matrices.

Additionally, designing the microstructure with pores, microdomes, or textured patterns can enhance sensitivity by focusing local deformation under pressure [16]. Hierarchical structures have been implemented in composite-based pressure sensors to improve linearity and extend the sensing range.

2.6. Stability, Hysteresis, and Cycling Behavior

A key challenge in flexible sensors is ensuring long-term stability under repeated mechanical loading, reducing hysteresis between loading and unloading cycles, and maintaining signal reproducibility. Interfaces between the biopolymer matrix, carbon fillers, and metallic electrodes may degrade over time due to mechanical fatigue or delamination unless proper interfacial adhesion is achieved. Research on conductive polymers emphasizes the importance of interface compatibility and mechanical robustness to mitigate cyclic degradation [27].

Environmental factors, such as humidity, temperature, and aging, can also alter the properties of starch-based biopolymers, thereby affecting conductivity and mechanical stiffness. Protective encapsulation strategies are therefore recommended to preserve performance under real-world conditions.

2.7. State of the Art and Relevant Comparisons

Several recent studies support the viability of the approach proposed in this work:

• Starch-based films incorporating carbon black have been studied for their modified electrical, mechanical, and barrier properties as biocomposites [4].
• Flexible, degradable sensors using laser-induced carbonized starch films have been reported for multi-sensing applications [23].
• Broad reviews of composite-based flexible sensors have outlined structural design strategies, micro- and nanofabrication methods, and filler-optimization techniques [13,15,16].
• A high-sensitivity pressure sensor based on carbon ink and carbon nanotubes in full mixtures has shown that the filler ratio critically affects electrical conductivity without compromising mechanical performance [18].
• Beyond carbon-based composites, recent high-impact studies on hydrogel/organohydrogel biopolymer sensors further illustrate the state of the art in sustainable soft sensing, showing that network engineering and green crosslinking can deliver high stretchability, gauge factors, and long-term cycling stability [5,6,7].

These references support the rationale behind this study’s configuration: a starch-based biopolymer doped with electrographite powder, sandwiched between aluminum electrodes, offers a novel but technically grounded solution for flexible pressure sensing.

3. Materials and Methods

3.1. Materials and Reagents

• Cassava starch (food grade, La pradera EC)

Sample 1:

Electrographite powder: sieved to <75 µm

Density: 1.3 — 1.95 g/cm³

Thermal conductivity: 25 — 470 W/m•K

Electrical resistivity: $5\times {10}^{-6}-30\times {10}^{-6}\Omega .m$Sample 2:

Graphite powder: sieved to <75 µm

Density: 1.8 — 2.1 g/cm³

Thermal conductivity: 10 — 20 W/m•K

Electrical resistivity: $1\times {10}^{-4}-1\times {10}^{-3}\Omega .m$

• Ultrathin aluminum foils (thickness < 100 µm)
• Transparent acetate sheets (thickness 100 — 200 µm)
• Distilled water, magnetic stirrer, oven, spatula, analytical balance
• Acetic Acid Glacial JT Baker 99.7%
• Glycerin, Fisher Scientific, high purity

3.2. Preparation of the Doped Biopolymer

Five grams of cassava starch were dispersed in 100 mL of hot distilled water (~80 °C) under constant stirring until gelatinization occurred. Electrographite or graphite was then added at 10 wt% relative to dry starch. For clarity, 10 wt% corresponds to 0.5 g filler per 5 g dry starch. The mixture was stirred for 30 min and then deposited by spin coating (spin casting) at 1000 rpm onto acetate-lined supports to form the sensing film (spin-coating duration: 30 s; resulting film thickness: 300 ± 5 µm, (mean ± SD; measured using a micrometer, n = 30)). After deposition, the assembly was covered as described for the device configuration and sealed using a silicone-based sealant to limit moisture ingress during storage. The samples were left to dry at ambient temperature for 24 h prior to electrical testing. Once dried, they were cast (see Figure 1).

3.3. Sensor Assembly

Two aluminum foils matching the dimensions of the biopolymer film were cut and positioned on both sides of the doped film, forming a sandwich structure. This assembly was mounted on an acetate support and secured using dielectric tape to prevent lateral displacement. Electrical connections were established by attaching conductive wires to each aluminum electrode (see Figure 2).

3.4. Electromechanical Characterization

Electrical resistance was measured at rest and under mechanical load using a high-precision digital multimeter (Fluke 179C). For the static calibration curves (Figure 5, Figure 6 and Figure 7), masses ranging from 10 g to 1000 g were applied using calibrated weights over an area of 5.06 cm² (≈0.2–20 kPa) (Figure 3). For the representative hysteresis evaluations (Figure 8 and Figure 9), the load range was extended up to 3200 g (≈62 kPa) to visualize loading–unloading behavior across a broader compressive regime. Unless otherwise stated, plots are presented as representative curves under the reported loading protocol.

3.5. Optical Analysis

Optical microscopy was used to assess the absence of graphite or electrographite dopant clusters within the starch matrix. These observations helped correlate the device’s internal structure with its electrical performance (see Figure 4).

3.6. Repeatability and Stability

The sensor underwent 30 consecutive cycles to evaluate repeatability under the reported protocol. Furthermore, the device’s stability was assessed over a 7-day period and again, for another 7-day period a year later, under controlled laboratory conditions. Sealed storage was used to minimize moisture ingress in the hydrophilic starch-based film and enable a consistent comparison of the response after prolonged storage.

4. Results and Discussion

4.1. Rheological and Mechanical Properties of the Prepared Gels

To address the rheological behavior of the starch-based formulations, the prepared gels were characterized at 25 °C in terms of storage modulus (G′), loss modulus (G″), tan δ, complex viscosity, compressive strength, elastic modulus, and recovery (see Table 1). These parameters are relevant because they describe the viscoelastic response, mechanical integrity, and recovery capability of the starch-based matrix prior to its integration into the pressure-sensing architecture.

As shown in Table 1, the sample of starch biopolymer undoped exhibited the lowest storage modulus, complex viscosity, compressive strength, elastic modulus, and recovery, indicating a softer and less mechanically robust gel network. In contrast, the samples of starch composite gels doped with carbon, graphite, and electrographite, respectively, showed markedly higher G′ values, lower tan δ values, and improved compressive and elastic properties, suggesting a more elastic-dominated, mechanically stable structure. The higher recovery values observed for them also indicate better ability to recover after deformation, which is relevant for pressure-sensing applications where repeated compression is expected. These results provide quantitative support for the material behavior of the starch-based system and strengthen the interpretation of the piezoresistive response observed in the sensor.

4.2. Electrical and Piezoresistive Analysis

The experimental results reveal a strong correlation between the sensor’s electrical response and the type of dopant used in the starch-based biopolymer film. Notably, the undoped starch biopolymer did not exhibit a measurable pressure-dependent change in electrical resistance under the reported static loading protocol (see Figure 5), underscoring the need for a conductive phase to enable piezoresistive transduction in this configuration, as previously discussed in similar studies [15,30].

When modified with graphite, the biopolymer-based sensor exhibited slight, inconsistent resistance variations, suggesting that the conductive network is not sufficiently continuous or mechanically robust under compression at the reported formulation and processing conditions (see Figure 6). In particulate-filled composites, dispersion quality, interparticle spacing, and interfacial contact govern whether a stable percolation network forms and how it evolves under load [8].

In contrast, the sensor fabricated with electrographite-doped biopolymer demonstrated a clear piezoresistive response: the electrical resistance decreased exponentially with increasing applied load (see Figure 7), consistent with compression-induced enhancement of conductive contacts within a particulate carbon network [8]. Also, this behavior is characteristic of composite-based piezoresistive sensors, in which conductive particles form progressively closer contact under mechanical load, thereby reducing the overall resistance [10,13].

Figure 5. Electrical resistance vs. applied load (10–1000 g; 0.2–20 kPa over 5.06 cm²) for the neat (undoped) starch biopolymer (representative curve).

Figure 5. Electrical resistance vs. applied load (10–1000 g; 0.2–20 kPa over 5.06 cm²) for the neat (undoped) starch biopolymer (representative curve).

Figure 6. Electrical resistance vs. applied load (10–1000 g; 0.2–20 kPa over 5.06 cm²) for the graphite-doped biopolymer (representative curve).

Figure 6. Electrical resistance vs. applied load (10–1000 g; 0.2–20 kPa over 5.06 cm²) for the graphite-doped biopolymer (representative curve).

Response/recovery times are not reported because time-resolved measurements were not performed in this study.

Since the sensor response does not follow a single linear trend throughout the entire load range, each region was evaluated separately to ensure an accurate determination of the analytical parameters.

Independent linear regressions were performed for both zones, allowing the calculation of the corresponding sensitivities and correlation coefficients. This approach provides a more reliable assessment of the sensor performance within each operating range Figure 8.

A sensitivity of 0.03 Ω kPa⁻¹ was obtained for the sensor in this operating region (see Figure 9). This magnitude is moderate, indicating that the electrographite-doped biopolymer sensor exhibits a measurable and consistent response to applied pressure. The combination of moderate sensitivity and good linearity suggests the material’s potential as a flexible pressure-sensing platform.

Figure 7. Electrical resistance vs. applied load (10–1000 g; 0.2–20 kPa over 5.06 cm²) for the electrographite-doped biopolymer (representative curve).

Figure 7. Electrical resistance vs. applied load (10–1000 g; 0.2–20 kPa over 5.06 cm²) for the electrographite-doped biopolymer (representative curve).

Figure 8. Linear response in the Electrical resistance vs. applied load (10–1000 g; 0.2–20 kPa over 5.06 cm²), low loads in red and higher loads in blue.

Figure 8. Linear response in the Electrical resistance vs. applied load (10–1000 g; 0.2–20 kPa over 5.06 cm²), low loads in red and higher loads in blue.

Figure 9. Electrical resistance vs. Pressure (kPa) for the electrographite-doped biopolymer in the low-pressure linear range.

Figure 9. Electrical resistance vs. Pressure (kPa) for the electrographite-doped biopolymer in the low-pressure linear range.

The sensitivity obtained at higher pressures was considerably lower than that observed at lower loads, indicating a reduced ability of the sensor to distinguish small pressure variations under these conditions. This behavior suggests that the conductive network within the electrographite-doped biopolymer tends to reach a saturation regime as the applied pressure increases, resulting in a diminished electrical response, see Figure 10. Consequently, the sensor performs best in the low-pressure region and is therefore more suitable for applications with pressures below 10 kPa, where higher sensitivity and greater measurement resolution are achieved.

Moreover, the device exhibited a well-defined hysteresis loop, with a deviation of less than 5% between the loading and unloading curves in the region of maximum difference, indicating low hysteresis in representative curves under the reported protocol (Figure 11). This hysteresis level is comparable in magnitude to values reported in the literature for flexible piezoresistive sensors [16,22], noting that the present results are shown as representative curves.

Films were deposited by spin coating (1000 rpm is reported for the representative fabrication used to generate the curves shown). The results presented in Figure 6, Figure 7, Figure 8 and Figure 9 are representative curves under the stated protocol. The film thickness was $300\pm 5 \mu m$(mean ± SD; micrometer, n = 30).

To mitigate moisture ingress into the hydrophilic starch-based sensing layer, the assembled device was sealed as described in the Methods. The sensor remained operational after one year of sealed storage, supporting maintained qualitative functionality under moisture-protected conditions (Figure 12). Figure 11 and Figure 12 are presented as representative curves under the stated protocol.

These findings are relevant given the biodegradable nature of the starch-based sensing layer (matrix), the use of abundant, low-cost conductive fillers, and recyclable aluminum electrodes. Full-device biodegradability is not assessed in this study because the prototype includes non-starch components (e.g., electrodes, substrate, and sealing/adhesive materials). This aligns with the proposed device’s recent advances in sustainable electronics and supports the development of eco-friendly alternatives to conventional polymer-based sensors (see Figure 13) [4,12,15].

Related biopolymer film and membrane literature emphasizes that performance improvements often require composites, blends, or protective strategies to mitigate moisture-driven drift and aging [2,3,31,32].

Taken together, these results demonstrate that a biodegradable starch-based biopolymer film doped with electrographite and integrated in a flexible sandwich structure with aluminum electrodes yields a stable, reproducible, and sensitive pressure sensor. The proposed configuration presents a viable, sustainable, and low-cost approach to future development of flexible sensing systems.

The observed results highlight the importance of material selection and formulation control in the development of piezoresistive sensors. The poor response of the pure starch film confirms the need to incorporate a conductive filler to enable signal transduction. While graphite, commonly used in conductive composites, provided only weak and unstable signals, the use of electrographite resulted in a markedly superior response, validating its compatibility with biopolymer matrices.

The nonlinear decrease in electrical resistance under increasing load suggests the formation of load-sensitive percolation pathways within the electrographite-filled starch matrix, consistent with the general behavior of particulate-carbon piezoresistive composites [8]. Additionally, the low hysteresis (<5%) observed in representative curves indicates elastic recovery of the assembled layers under the reported protocol; however, replicate statistics and extended fatigue testing are required to quantify device-to-device variability and long-term durability.

These results support maintained qualitative functionality under the reported protocol, including a one-year sealed-storage check; however, extended durability testing (e.g., ≥1000 cycles) and environmental robustness evaluation are required for validated wearable-grade performance.

When placed in the broader landscape of biopolymer-based soft sensors, the present device should be interpreted as an initial, film-based piezoresistive pressure-sensing concept rather than a fully validated wearable sensor. Table 2 benchmarks this proof-of-concept against representative graphite-containing piezoresistive pressure sensors and recent biopolymer-rich soft sensors [5,6,7,26]. In contrast to elastomeric architectures that report time-resolved response and extended cycling (e.g., [26]) and to hydrogel/organohydrogel platforms that sustain large deformations over thousands of cycles [5,6], our study focuses on a water-processed starch–electrographite composite film operating under static compressive loading (0.2–20 kPa for calibration, with representative hysteresis curves shown up to ≈62 kPa) and reports representative resistance–pressure curves with low hysteresis under the stated protocol; time-resolved response, long-term cycling durability, and systematic environmental drift metrics remain to be quantified in future work.

These findings are relevant given the biodegradable nature of the starch-based sensing layer (matrix) and the use of abundant, low-cost conductive fillers and recyclable aluminum electrodes. Full-device biodegradability is not assessed in this study because the prototype includes non-starch components (e.g., electrodes, substrate, and sealing/adhesive materials).

Another critical outcome is the demonstrated long-term stability of the device. Through encapsulation and control of film-deposition parameters (such as spin-coating at 1000 rpm), the device maintained functionality for 1 year and across 30 reproducible fabrications. These results support maintained qualitative functionality under the reported protocol; however, extended durability testing (e.g., ≥1000 cycles) and time-resolved response characterization are proposed as immediate future work for validated wearable-grade benchmarking [16,17,22].

Importantly, this work supports the potential of using biodegradable and recyclable components in sensor design. The starch matrix is fully biodegradable, the electrographite filler is abundant and low-cost, and aluminum is recyclable and widely available. These features align the sensor with the principles of green electronics and life-cycle sustainability [5,12,15].

Future studies could complement electromechanical testing with standardized water-absorption and tensile measurements (ASTM D570 and ASTM D882) and thermal analysis (e.g., TGA) to characterize moisture sensitivity and thermal stability in the composite films [29,33]. If optical properties are retained as a scope item, UV–Vis/absorbance measurements can be reported using established UV–Vis principles and recent examples of UV-resistant biopolymer films [28,34].

Furthermore, the process simplicity and scalability, no need for toxic solvents, high temperatures, or advanced lithography, position this sensor as a promising platform for low-cost, eco-conscious pressure sensing technologies in biomedical, agricultural, and environmental monitoring applications.

5. Conclusions

A flexible pressure-sensor prototype was fabricated using a starch-based biopolymer sensing layer doped with electrographite and integrated in an aluminum/biopolymer/aluminum sandwich configuration. The results presented are representative curves under the reported protocol.

• The electrographite-doped film exhibits a measurable piezoresistive response under 0.2–20 kPa (10–1000 g over 5.06 cm²), whereas the neat starch film remains insulating under the reported protocol; graphite-doped films show a weaker and less consistent response in representative curves.
• Representative curves show a pronounced piezoresistive effect for the electrographite-doped film, with resistance decreasing nonlinearly as pressure increases within the tested range.
• Representative load–unload curves show low hysteresis (<5%) under the stated conditions, and repeatability was evaluated over 30 consecutive cycles under the reported protocol.
• The device remained operational after one year of sealed storage, supporting maintained qualitative functionality under moisture-protected conditions.
• Biodegradability claims are restricted to the starch-based sensing layer; full-device biodegradability is not assessed.
• The developed electrographite-doped biopolymer sensor demonstrated a stable and reproducible piezoresistive response with two differentiated linear regions. The higher sensitivity observed below 10 kPa confirms the effectiveness of the conductive network in detecting small mechanical stimuli, whereas the lower sensitivity at higher pressures is attributed to the gradual saturation of conductive contacts within the composite structure. Therefore, the sensor is particularly promising for low-pressure sensing applications, where accurate detection and enhanced resolution are required. These findings demonstrate that electrographite incorporation into the biopolymer matrix provides an environmentally friendly and cost-effective strategy for the development of flexible pressure sensors with potential applications.

Limitations and Future Work: This study is a proof-of-concept based on static mass loading and representative curves. Response/recovery time, and extended fatigue testing are not reported, principally because the sensor uses the starch in a sandwich rather than alone, and the sensor assumes its properties. However, the rheological properties are proposed to be measured, along with detailed recovery times, in the immediate future work to improve the prototypes and validate wearable-grade performance.

Future work will focus on refining the electrode geometry, exploring encapsulation alternatives, and integrating the sensor into wireless platforms for wearable or distributed monitoring systems. The results of this work suggest that the current configuration can be further optimized and adapted for a wide range of sustainable sensing applications.