Emulsion-engineered polylactide-based polyurethane/MXene films for high-performance flexible and biointegrated wearable sensors

Summary

Designing soft conductive polymeric materials with mechanical resilience and low filler loading remains a challenge in wearable electronics. This work presents a strategy based on sustainable precursors for fabricating flexible, conductive films from polylactide (PLA)-based polyurethane (PU) and delaminated MXene (Ti₃C₂T_x) through emulsion-assisted processing. Using virgin PLA as a controlled model feedstock, the polymer was depolymerized into hydroxyl-terminated oligomers via microwave-assisted alcohol-acidolysis and employed as polyol precursors for PU synthesis. Emulsification of PLA-PU and MXene with poly(vinyl alcohol) (PVA) as a compatibilizer yielded hybrid films with superior MXene dispersion and lower percolation thresholds (≤10 wt %) compared to solution-cast films (≥40 wt %). The 6:4 and 7:3 PLA-PU/MXene/PVA films offered an optimal balance of tensile strength and flexibility. The films exhibited sensitive, reproducible responses to macro- (finger bending) and micro-strains (vocal/laryngeal vibrations). The study advances PLA upcycling into high-performance electronic materials derived from biocompatible precursors for next-generation health monitoring and bio-integrated systems.

Graphical abstract

Highlights

• •Sustainable emulsion design for flexible conductive PLA-PU/MXene films
• •Microwave-assisted PLA upcycling to polyols for green polyurethane
• •Low-filler MXene hybrids show strong, flexible, and conductive films
• •Films detect strain, enabling wearable and bio-integrated sensors

Sensor; Bioelectronics; Materials science

Introduction

Polylactide (PLA), an aliphatic polyester known for its degradability and biocompatibility, is a sustainable alternative to petroleum-based plastics across packaging, agriculture, biomedical, cosmetic, and automotive industries. Although degradable, PLA requires industrial composting conditions that are difficult to achieve widely, raising environmental concerns about single-use products and emphasizing the need for effective recycling. PLA’s degradation susceptibility limits mechanical recycling, whereas chemical recycling offers a promising route to convert post-consumer PLA into high-purity feedstocks for synthesizing value-added products. These depolymerization methods include hydrolysis, alcoholysis, and aminolysis,^1,2,3 with alcoholysis particularly effective by transesterifying ester bonds into hydroxyl-terminated lactate oligomers. The resulting oligomer and monomer mixtures can be separated and reused as feedstocks,^1,4 with product structures tunable via reaction conditions.^5,6 Microwave-assisted alcoholysis accelerates reaction rates and reduces processing times, improving efficiency and sustainability. Alternatively, when applied to virgin resin, this is called the “sizing-down” approach, producing functionalized lactate oligomers with advantages over bottom-up synthesis from a lactic acid monomer or lactide, such as lower cost, shorter time, and precise chemical control.⁷

Polyurethanes (PUs) are versatile polymers synthesized from diols or polyols and diisocyanates or polyisocyanates, forming urethane linkages often with chain extenders or additives.⁸ Alcoholysis of polyesters, especially PLA, yields hydroxyl-terminated oligomers that can serve as sustainable polyols for PU synthesis. Multifunctional molecules such as ethylene glycol, 1,4-butanediol, 2,2-bis(hydroxymethyl)propionic acid (DMPA), citric acid, and glutaric acid facilitate the breakdown of PLA ester bonds, producing tunable OH-capped PLA oligomers.^1,7 These oligomers enable environment-friendly PU production with comparable strength, lower modulus, higher toughness, and excellent elasticity compared to neat PLA. Moreover, the resulting PUs are degradable under hydrolytic, enzymatic, or designed chemical recycling conditions, with properties fine-tuned by varying diisocyanate type and content.

PU’s biocompatibility, elasticity, and physiological stability make it ideal for biomedical applications such as wound dressings, scaffolds, and wearable sensors.^9,10,11,12 In wearable electronics, PU is a matrix for strain and pressure sensors capable of detecting subtle human motions.¹³ To impart electrical conductivity, PU composites incorporate nanofillers such as MXene (e.g., Ti₃C₂T_x), two-dimensional transition metal carbides with high conductivity, hydrophilicity, and large surface area, enhancing polymer composite properties.^{14,15,16,17,18,19} However, uniform MXene dispersion remains challenging, as conventional blending often results in agglomeration and poor mechanical performance of the composites.^20,21 Emulsification addresses these issues by improving MXene dispersion and lowering percolation thresholds, enhancing electrical conductivity at reduced filler loadings.²² To further improve mechanical flexibility and interfacial adhesion within the composite films, a secondary polymer such as poly(vinyl alcohol) (PVA) is introduced, which enhances the matrix flexibility, interfacial compatibility, and film integrity.²³ Despite growing interest in PU/MXene composites, few studies have examined PLA-derived PU matrices, particularly under macro-strain (e.g., finger bending) and micro-strain (e.g., vocal/laryngeal vibrations) conditions. Furthermore, limited data exist on dynamic sensing parameters such as response and recovery times (t_res and t_rec) for these systems, especially in non-elastomeric, non-stretchable films.

In this study, flexible and electrically conductive films based on alcoholized PLA-derived PU and MXene are developed, comparing two fabrication approaches: solution blending and emulsification. This work utilizes explicitly PLA-derived oligomers as a sustainable, high-performance alternative to traditional petroleum-based polyols, aiming to establish a circular economy pathway for PLA waste. To enable systematic evaluation and reproducibility, virgin PLA resin is used as a model feedstock, providing precise control over oligomeric polyol structures while retaining applicability to post-consumer PLA waste for advancing sustainable circularity. Emulsification is explored to reduce MXene percolation thresholds and enhance the electrical conductivity of its cast-film derivatives at lower filler loadings. poly(vinyl alcohol) (PVA) is incorporated as a secondary polymer to enhance mechanical flexibility and cohesion in the emulsified films. A series of PLA-PU/MXene/PVA films with varied compositions is fabricated and thoroughly characterized for their morphological, thermal, mechanical, and electrical properties. The films exhibiting optimal performance are further evaluated for sensing capabilities under both large and subtle strains, including finger bending and vocal or laryngeal vibrations. Through this integrated approach, the work establishes a proof of concept for upcycling PLA into high-performance wearable strain and pressure sensors, demonstrating a viable pathway for converting PLA-based materials into functional devices suitable for applications in soft robotics, human motion tracking, and biomedical monitoring.

Results and discussion

The fabrication of composite films started with the synthesis of the polymer matrix from sustainable precursors. PLA resin was first converted into alcohol-acidolyzed PLA oligomers through a “sizing-down” process via microwave-assisted alcohol-acidolysis, as outlined in Figure 1. These oligomers subsequently served as the polyol precursors for the synthesis of the PLA-based PU (PLA-PU) matrix, following the reaction pathway detailed in Scheme 1. The final composite films were then prepared by integrating this PLA-PU with delaminated MXene nanofillers using both solution casting and emulsion-assisted methods (Figure 2). To understand the properties of the final composite, the fundamental characteristics of the nanofiller were first established.

Figure 1.

Overview of alcohol-acidolysis of PLA resin by DMPA employing a microwave reactor

Scheme 1.

Reaction scheme for the synthesis of the PLA-derived PU (PLA-PU)

Figure 2.

The synthesis of PLA-PU and the fabrication of PLA-PU/MXene films by solution blending and emulsification methods

Microstructures of MXene and delaminated MXene

Microstructures and morphology of MXene and delaminated-MXene were characterized by transmission electron microscopy (TEM), as shown in Figure 3. Following DMSO-assisted delamination, the interplanar spacing increased from ∼1.09 to 1.14 nm, reflecting layer expansion and fragmentation into smaller, dispersed particles. This is attributed to DMSO intercalation between MXene layers. Delamination significantly affects the properties of MXene, including its mechanical, electronic, and chemical behavior. The expanded interlayer spacing enhances surface accessibility and increases the number of reactive sites, improving chemical reactivity.²⁴ Mechanically, it enhances flexibility and fracture toughness by allowing more deformation before failure.²⁵ Electronically, the increased spacing influences charge transfer between layers, thereby affecting conductivity and related properties.²⁶ These structure-property relationships highlight the importance of controlling interlayer spacing for MXene-based applications.

Figure 3.

Structure characterization of MXene and delaminated MXene

TEM images of MXene (A and B) and delaminated MXene (C and D), and XRD patterns of MXene and delaminated MXene (E). Scale bars: 20 nm in (A and C) and 10 nm in (B and D).

XRD patterns of MXene and delaminated MXene

The crystalline and layered structures of MXene and delaminated MXene were analyzed by X-ray diffraction (XRD), as shown in Figure 3E. In its original state, MXene exhibited characteristic (002) peaks at 2θ = 5.98 and 9.52°, corresponding to interplanar spacings of 14.75 and 9.28 Å, respectively, indicative of well-ordered Ti₃C₂T_x stacking. After DMSO-assisted delamination, these peaks shifted to 5.68 and 8.75°, with expanded d-spacings of 15.52 and 9.36 Å. This shift reflects the transition from multilayer to few-layer MXene, which aligns with previous reports,^27,28 confirming effective interlayer expansion. These structural changes observed by XRD are consistent with the TEM findings and confirm the impact of DMSO in promoting delamination. A residual (104) peak of the MAX phase near 39° further indicates incomplete etching, suggesting that some precursor layers remain.²⁹ This emphasizes the need for precise control over the etching and delamination steps to fully achieve the desired MXene structure and optimize its functional properties.

Morphology of PLA-PU and PLA-PU/MXene films

Field emission scanning electron microscopy (FE-SEM) analysis was performed to evaluate the surface and cross-sectional morphology of PLA-PU/MXene films prepared via solution and emulsion casting (Figure 4). Solution-cast PLA-PU films exhibited a smooth, dense surface (Figure 4A) and a compact, homogeneous cross-sectional morphology without noticeable phase separation or voids (Figure 4F), attributed to complete dissolution in the solvent and uniform film formation during evaporation. The polymer matrix remained relatively uniform with MXene incorporation (Figures 4B and 4G), with well-dispersed MXene flakes (yellow arrows) throughout the PLA-PU phase. These interactions are likely governed by hydrogen bonding and dipole-dipole forces between PLA-PU’s polar groups (–OH, –COO, –NH) and MXene’s oxygen-rich surfaces. However, some MXene stacking was observed, which may hinder the formation of a continuous conductive network and limit electrical performance.

Figure 4.

SEM images of PLA-PU-based films prepared via solution and emulsion casting

Surface morphology of (A) PLA-PU film (solution cast), (B) PLA-PU/MXene film (solution cast), (C) PLA-PU film (emulsification), (D) PLA-PU/PVA film (emulsification), and (E) PLA-PU/MXene/PVA film (emulsification). The corresponding cryo-fractured cross-sectional images are shown in (F–J). Yellow arrows indicate MXene distribution in the PLA-PU matrix. Scale bars: 5 μm in (A–E), 10 μm in (F–I), and 100 μm in (J).

In contrast, PLA-PU films prepared via emulsion casting exhibited a granular surface morphology, as shown in Figure 4C. Rather than forming a smooth, compact matrix, the surface consisted of coalesced PLA-PU particles, resulting from dispersed polymer droplets in the aqueous phase that fused during drying. This process created a particle-based network, unlike the homogeneous structure observed in solution-cast films. Cross-sectional images (Figure 4H) confirmed a densely packed arrangement without large voids, indicating film formation through particle coalescence rather than phase separation. However, interfaces between fused particles may act as mechanical weak points, contributing to increased brittleness compared to solution-cast films. To enhance structural integrity, PVA was introduced as a secondary polymeric compatibilizer. Its amphiphilic and hydroxyl-rich structure stabilizes the emulsion and facilitates hydrogen bonding with PLA-PU, promoting interparticle bridging. Figures 4D and 4I showed that this leads to less-visible particle boundaries and a more cohesive matrix with improved flexibility, an advantage for applications such as wearable sensors.

A distinct morphological transformation was observed when MXene was added to the PU/PVA matrix (Figures 4E and 4J). SEM image (Figure 4E) showed a smooth, featureless texture, likely caused by the partial migration of hydrophilic PVA toward the surface during drying, forming a thin, uniform layer. In contrast, the cross-sectional image (Figure 4J) revealed a phase-separated internal structure, with densely packed PLA/PU particles, reduced PVA encapsulation, and interspersed MXene. This suggests that MXene disrupts the polymer network, inducing localized separation between PLA/PU-rich and MXene-rich regions. The high surface energy and hydrophilicity of MXene likely drive its preferential interaction with PLA-PU, resulting in non-uniform dispersion within the film. Although not evident on the surface, this internal phase separation may significantly influence mechanical properties and hinder the formation of continuous conductive pathways, as discussed in later sections.

PLA-PU/MXene/PVA films with varying PVA compositions (1:1, 6:4, 7:3, and 8:2) were examined (Figure 5). At a 1:1 PLA-PU/MXene:PVA ratio, PVA effectively covered the surface and encapsulated PLA-PU particles, forming a smooth, homogeneous matrix with minimal phase separation. At 6:4 (Figures 5A and 5D), partial encapsulation persisted. Still, phase separation became more evident, with increased MXene-rich domains and visible interfaces between polymer particles, indicating weaker chain entanglement and reduced cohesion. At 7:3 (Figures 5B and 5E), phase separation intensified as PLA-PU aggregates became more distinct and MXene clusters emerged, suggesting a reduced PVA bridging layer and further disruption of polymer cohesion. At 8:2 (Figures 5C and 5F), the morphology showed severe phase separation, characterized by discontinuous surfaces, void-like structures, and poor coalescence of PLA-PU particles. The lower PVA content likely diminished polymer interaction, promoting MXene aggregation and the formation of a fragmented conductive network, potentially compromising mechanical and electrical performance.

Figure 5.

SEM images of PLA-PU/MXene/PVA films prepared via emulsification at different compositions

Surface morphology of the films prepared at PLA-PU/MXene: PVA ratio of (A) 6:4, (B) 7:3, and (C) 8:2. The corresponding cryo-fractured cross-sectional images are shown in (D–F). Scale bars: 5 μm in (A–C) and 100 μm in (D–F).

SEM-energy dispersive X-ray spectroscopy (EDS) elemental mapping of solution-cast PLA-PU/MXene film and emulsion-derived PLA-PU/MXene/PVA films (6:4 and 7:3) was conducted to assess MXene dispersion, as shown in Figure 6. The solution-cast sample (Figure 6A) exhibited a uniform distribution of Ti elements, indicating that the MXene nanosheets were well-dispersed within the PLA-PU matrix without noticeable aggregation. This homogeneity results from the direct dissolution of both MXene and PLA-PU in the solvent, combined with ultrasonication-assisted exfoliation that promotes even dispersion. However, a relatively high MXene content (40 wt %) was still needed to reach the electrical percolation threshold, suggesting that while spatial distribution was uniform, nanosheet interconnection may remain insufficient at lower loadings. In contrast, the emulsion-cast PLA-PU/MXene/PVA films (Figures 6B and 6C) also showed Ti element dispersion but with visibly large MXene domains, especially at higher MXene contents. This indicates localized aggregation, likely driven by increased phase separation, which can influence conductivity and mechanical integrity. These findings, consistent with SEM morphology, suggest that emulsification at higher MXene concentrations promotes structural phase separation and reduces PU encapsulation by PVA, leading to increased aggregation.

Figure 6.

Morphological analysis and elemental mapping of PLA-PU/MXene composite films

SEM images and SEM-EDS elemental mapping of Ti for evaluating MXene dispersion in (A) PLA-PU/MXene film prepared by solution and emulsion-cast PLA-PU/MXene/PVA films prepared at (B) 6:4 and (C) 7:3 ratios (scale bars, 50 μm).

FTIR spectra of PLA-PU and PLA-PU/MXene films

Fourier transform infrared (FTIR) spectra of PLA-PU films fabricated via solution casting and emulsion casting, with and without MXene and PVA, are compared in Figure 7. The C=O stretching band of the PLA sequence in solution-cast PLA-PU film showed a splitting pattern that indicates a semi-crystalline nature, in which the 1,754 cm⁻¹ band is crystalline, while the 1,746 cm⁻¹ mode is associated with the amorphous domains. The sharp bands of δ_as(CH₃), δ_s(CH₃), and δ(C–H) modes at 1,455, 1,360–1,386, and 875 cm⁻¹ also reflect this semi-crystalline nature. Similar characteristics were observed in the solution-cast PLA-PU/MXene film, indicating PLA crystallization upon solvent evaporation. In contrast, the C=O band splitting disappeared, forming a single amorphous band in all emulsion-cast films. All sharp C–H characteristic modes became broadened, indicating crystalline retardation due to droplet formation and chain restriction by the PVA incorporation. The results agree with those observed from differential scanning calorimeter (DSC) and XRD. The bands of ν_as(–CO–O–) and ν_s(–CO–O–) were observed at 1,184 and 1,085 cm⁻¹, whose relative intensity indicates the length of lactate sequences. The weak shoulder bands at 1,715 (amide I) and 1,560 cm⁻¹ (amide II) reflect the urethane functional groups.

Figure 7.

Chemical structure analysis of PLA-PU films

FTIR spectra of solution-cast (A) PLA-PU and (B) PLA-PU/MXene films; (C) emulsion cast PLA-PU/PVA and PLA-PU/MXene/PVA films at different ratios: (D) 8:2 (E) 7:3, (F) 6:4, and (G) 1:1; and (H) neat PVA.

PVA showed characteristic C=O bands of remaining acetate groups at 1,732 and 1,717 cm⁻¹, δ(C–H) at 1,430 and 845 cm⁻¹, and ν(C–O) at 1,245 cm⁻¹. These characteristic modes were observed in conjunction with the PLA-PU characteristics in all emulsion-cast films, ranging from ratios of 8:2 to 6:4, whose relative intensity increased with the PVA content. This indicates that PVA is homogeneously distributed in the film structure, bridging the PLA-PU particles. In contrast, the spectrum of the 1:1 film was dominated by the characteristic modes of PVA. Since FTIR-ATR measures the functional groups up to 200 μm depth from the sample’s surface, this indicates that the excess PVA migrates to the film surface and forms a separate layer. The results agree well with the film structures observed in SEM images. A shift of the amide II (N–H bending and C–N stretching) band at 1,560 cm⁻¹ in the solution-cast PLA-PU to 1,533 cm⁻¹ in PLA-PU/MXene films indicates the formation of a stronger interaction, likely hydrogen bonding between N–H of the urethane bonds and the polar functional groups on the surface of MXene sheets. In addition, the bands further shifted to 1,525 cm⁻¹ for all emulsion-cast films. This indicates hydrogen bonding between PVA’s N–H and –OH, forming a strong adhesion between the PLA-PU particles and PVA.

DSC analysis of PLA-PU and PLA-PU/MXene films

The thermal properties of PLA-PU composites were assessed by differential scanning calorimetry (DSC), as shown in Figure 8 and summarized in Table 1. The solution-cast PLA-PU film showed a glass transition temperature (T_g) at 50.0°C, which decreased to 46.6°C upon adding MXene. This reduction is likely attributed to competing nucleation and confinement effects, where dispersed MXene promotes nucleation but disrupts local chain packing, increasing segmental mobility.³⁰ The film showed two melting peaks at 110.6°C and 143.2°C, suggesting the coexistence of imperfect and more ordered crystalline domains. In contrast, PLA-PU/MXene films exhibited a single melting peak at 129.1°C, reflecting a more uniform, less-phase-separated structure likely due to MXene interfering with regular chain folding.

Figure 8.

Thermal properties and crystalline structure of PLA-PU composite films

(A) DSC first heating scans and (B) XRD patterns of PLA-PU composite films prepared by solution and emulsion casting.

Table 1.

Thermal properties of PLA-PU and PLA-PU/MXene films

Samples	Tg (°C)	Tm (°C)
PLA-PU (solution-cast)	50.0	143.2
PLA-PU/MXene (solution-cast)	46.6	129.1
PLA-PU/PVA (emulsion-cast)	51.1	–
PLA-PU/MXene/PVA (emulsion-cast)	56.0	–

Conversely, the emulsion-cast films displayed an opposite trend in T_gbehavior. The bared PLA-PU/PVA showed a T_g at 51.1°C, which increased to 56°C upon MXene incorporation. This increase is likely due to enhanced microphase organization and stronger interfacial confinement within the PLA-PU/PVA matrix, promoted by emulsification-facilitated MXene dispersion.³¹ Interestingly, the emulsion-cast films exhibited no melting transitions, indicating largely amorphous structures. Instead, broad exothermic peaks near 140°C–143°C were observed, attributed to the slow thermal reorganization of disordered PU segments or PVA-rich microdomains kinetically trapped in an amorphous state.³²

XRD analysis of PLA-PU and PLA-PU/MXene films

XRD patterns of PLA-PU/MXene composites prepared by solution and emulsion casting are compared in Figure 8D. The solution-cast samples exhibited a semi-crystalline morphology with intense, sharp peaks at 2θ = 14.8, 16.7, 19.1, and 22.4°, corresponding to the (010), (200)/(110), and (203) planes of PLA’s α-form crystalline.³³ MXene incorporation caused the weakening of the peaks at 14.8 and 22.4°, indicating partial disruption of PLA crystalline domains due to polymer-MXene interactions. In addition, new MXene signals at 8.8, 38.8, and 45.1° confirmed the presence of restacked or partially ordered MXene layers within the PLA-PU matrix.^34,35 In contrast, the PLA’s crystalline signals disappeared in the emulsion-cast films containing PVA. This reflects disrupted chain packing of lactate sequences from rapid phase inversion and limited mobility during emulsification. Instead, broad but intense peaks at 11.8 and 19.4° indicated semi-crystalline PVA domains.^36,37 The appearance of the MXene characteristic peaks at 6.3, 38.6, and 44.9° in PLA-PU/MXene/PVA films suggested exfoliated or intercalated layers, contrasting with the more stacked structure in the solution-cast films. The 8.8 to 6.8° peak shift implies subtle interlayer spacing or interfacial strain changes.

To further quantify the degree of structural ordering, the crystallite sizes were estimated using Scherrer’s equation:

$$D=\frac{K\lambda }{\beta \mathit{cos}\theta }$$ (Equation 1)

where D is the mean size of the crystalline domains, K is the dimensionless shape factor, λ is the X-ray wavelength, β is the line broadening at full width at half maximum (FWHM), and θ is Bragg’s angle. The calculated crystallite sizes are summarized in Table 2. Solution-cast films, including PLA-PU and PLA-PU/MXene, showed narrow FWHM and large crystallite sizes (up to 50.2 nm for PLA and >100 nm for MXene), indicating well-ordered domains. In contrast, emulsion-cast films showed broader peaks with smaller crystallites, mostly below 10 nm. The MXene (002) peak at 6.3° corresponded to a d-spacing of 1.4 nm and a domain size of 9.7 nm, supporting finer dispersion and reduced structural coherence. These findings aligned with DSC results, in which no melting transitions were observed in emulsion-cast films, confirming their largely amorphous nature.

Table 2.

Summary of crystalline characteristics of PLA-PU and PLA-PU/MXene films

Samples	2θ (°)	FWHM, β	da (nm)	Db (nm)
PLA-PU (solution-cast)	14.8	0.25	0.6	31.6
	16.7	0.16	0.5	50.2
	19.1	0.59	0.5	13.6
	22.4	0.25	0.4	32.8
PLA-PU/MXene (solution-cast)	8.8	0.14	1.0	57.1
	16.8	0.12	0.5	64.4
	19.2	0.46	0.5	17.5
	38.8	0.09	0.2	96.1
	45.1	0.09	0.2	100.7
PLA-PU/PVA (emulsion-cast)	10.4	4.17	0.8	1.9
	19.3	3.53	0.5	2.3
PLA-PU/MXene/PVA (emulsion-cast)	6.3	0.82	1.4	9.7
	11.8	1.52	0.7	5.3
	19.4	2.83	0.5	2.8
	38.6	0.25	0.2	33.7
	44.9	0.24	0.2	35.8

^ad-spacing.

^bCrystallite size.

Mechanical properties of PLA-PU and PLA-PU/MXene films

The mechanical properties of the film samples were evaluated by tensile testing, as shown in Figure 9A. The stress-strain curves revealed clear differences between the solution- and emulsion-cast films, influenced by their different fabrication methods, MXene content, and PVA incorporation. Solution-cast PLA-PU films showed plastic deformation with an initial yield point and extended strain due to semi-crystalline lactate domains and elastomeric urethane segments.³⁸ Incorporating MXene enhanced tensile strength and modulus but reduced elongation, as the rigid filler reinforced the matrix while disrupting chain entanglements.²¹ A relatively high MXene loading (40 wt %) was necessary to achieve notable reinforcement and ensure sufficient conductivity (discussed later). PLA-PU films exhibited a tensile strength of 0.34 MPa and 21.4% elongation. In contrast, PLA-PU/MXene films achieved 1.32 MPa with decreased elongation to 7.2% (Figure 9B), indicating a transition toward a stiffer, more brittle structure.

Figure 9.

Mechanical properties and fracture morphology of the composite films

(A–C) Stress-strain curves (A), tensile strength (B), and elongation at break (C) of PLA-PU, PLA-PU/MXene, and PLA-PU/MXene/PVA films; data are presented as mean ± SD (n ≥ 3).

Cross-sectional SEM images of PU/MXene-PVA films prepared at (D) 1:1, (E) 6:4, and (F) 7:3 compositions after tensile failures (scale bars, 50 μm).

The emulsion-derived PLA-PU films exhibited different mechanical behavior compared to solution-cast films, owing to their fused granular structure, which distributed stress more effectively but introduced interfacial weaknesses. Incorporating PVA improved interfacial cohesion and ductility, yielding higher tensile strength (6.56 MPa) and elongation at break (8.1%) through enhanced hydrogen bonding and plasticization. PLA-PU/PVA films showed elastic and plastic deformation, with PVA mitigating premature failure. Adding MXene (1:1 PLA-PU/MXene-PVA) altered the failure mechanism, evidenced by strain hardening and moderate tensile strength (4.60 MPa), suggesting localized reinforcement but weakened cohesion from disrupted hydrogen bonding.³⁹ Increasing MXene content (6:4) enhanced tensile strength (5.82 MPa) but reduced elongation (2.6%) due to restricted chain mobility. At 7:3, tensile strength dropped to 4.58 MPa, attributed to MXene aggregation and reduced PVA content, which impaired interfacial adhesion and structural integrity.^40,41 This is supported by the cross-sectional SEM images of the samples after fracture (Figures 9D–9F). PLA-PU/PVA showed continuous morphology with strong stress transfer. In a 1:1 film, partial encapsulation and strain-hardening persisted, while at 6:4, MXene reinforcement became dominant. At 7:3, PVA encapsulation diminished, correlating with a brittle fracture. Compared to solution-cast films, emulsion-derived films exhibited better mechanical properties at lower MXene loadings, due to denser PLA-PU structure, improved MXene dispersion, and enhanced interfacial bonding. The results indicate that emulsion casting is a more effective fabrication method than solution casting for improving the mechanical performance of PLA-PU/MXene films at lower MXene contents.

Focusing on these optimized emulsion-derived films, the results demonstrated high tensile strength and moderate elongation. Although the relatively low elongation at break suggests the material is unsuitable for high-stretch applications (e.g., over joints such as the elbow or knee), it offers an ideal balance of flexibility and mechanical robustness for low-strain, subtle motion detection. Accordingly, as demonstrated in the following sections, the sensor was specifically designed for this regime, including applications such as monitoring finger bending and vocal cord vibrations.

Conductivity of MXene and PLA-PU/MXene films

MXene and PLA-PU/MXene films’ conductivity was measured and compared with those previously reported in the literature, as summarized in Table 3. Neat MXene films prepared by vacuum-assisted filtration (VAF) exhibited an electrical conductivity of 8.78 × 10³ S/m for native MXene and 1.18 × 10⁴ S/m for delaminated MXene. It is noted that the conductivity of the delaminated MXene is lower than the highest values reported in the literature.^42,43,44,45 This difference stems from the synthesis route chosen for this study, which was selected to prioritize practical composite fabrication over maximizing the intrinsic electronic properties of MXene. The process involves an intermediate drying step to convert the wet MXene precipitate into a stable solid, a route chosen to ensure high reproducibility and scalability for composite formulation. This method enables precise mass-based control of the filler content, which is crucial for maintaining consistent material properties. This presents a known trade-off: the drying process causes flake restacking, and the subsequent sonication required for redispersion reduces the final flake size, thereby lowering electrical conductivity.^46,47

Table 3.

Electrical conductivity of PLA-PU/MXene films, compared with reports in the literature

Polymer matrix	MXene content (%)	Conductivity (S/m)	Fabrication method	Reference
Hemicellulose	77	6.43 × 106	VAF	Chen et al.48
WPU	50	4.80 × 104	Solution casting	Xu et al.49
PEDOT:PSS	75	2.04 × 103	VAF	Liu et al.50
TPU	29	1.60 × 103	LBL	Gao et al.51
TOCNF	40	1.21 × 103	VAF	Zhan et al.52
SA	10	50	VAF	Shahzad et al.42
PAM	75	3.30	Solution casting	Naguib et al.53
CNF	50	2	VAF	Zhou et al.54
PEO	1	2.70 × 10−1	Electrospinning	Mayerberger et al.55
PVA	40	2.26 × 10−3	Solution casting	Tan et al.56
PVDF	10	7.00 × 10−4	Solution casting	Tu et al.57
MXene	−	8.78 ×103	VAF	This work
D-MXene	−	1.18 ×104	VAF
PLA-PU/MXene (solution)	40	7.60 ×10−4	Casting
PLA-PU/MXene (DMF/water) (emulsification)	7	4.52 ×10−5
PLA-PU/MXene (DMF-water) (emulsification)	10	3.58 ×10−4
PLA-PU/MXene (CHCl3-water) (emulsification)	10	9.47 ×10−1
PLA-PU/MXene/PVA 1:1 (emulsification)	6.25	2.93 ×10−3
PLA-PU/MXene/PVA 6:4 (emulsification)	6.98	2.50 ×10−2
PLA-PU/MXene/PVA 7:3 (emulsification)	7.61	4.97 ×10−2
PLA-PU/MXene/PVA 8:2 (emulsification)	8.16	7.50 ×10−3

PVDF, polyvinylidene fluoride; TOCNF, TEMPO-oxidized cellulose nanofiber; SA, sodium alginate; CNF, cellulose nanofiber; PEO, polyethylene oxide; PVA, polyvinyl alcohol; LBL, layer-by-layer.

Bold entries indicate the samples prepared and characterized in this work.

Within the prepared samples, the higher conductivity of delaminated MXene was attributed to its increased d-spacing, as confirmed by TEM and XRD results, which facilitates charge transport by reducing electron congestion and improving interlayer connectivity.^25,26 However, these MXene films were highly brittle, which prevented their use as standalone films. Impregnating the material in a polymer matrix is therefore required.

The electrical conductivity of solution-cast PLA-PU/MXene films with low MXene contents was undetected. The film containing at least 40 wt % MXene content started to exhibit measurable electrical conductivity (7.60 × 10⁻⁴ S/m), indicating a high percolation threshold due to limited MXene dispersion influenced by dimethylformamide’s (DMF’s) poor miscibility with PLA-PU.⁵⁸ In addition, excessive MXene loading causes aggregation, limiting conductivity gains.⁴⁰ In contrast, emulsion-derived films showed significantly lower percolation thresholds, achieving 4.52 × 10⁻⁵ S/m conductivity at 7 wt % MXene content. This is likely attributed to the improved dispersion facilitated by the water-based emulsion and internal emulsification by the carboxylate groups of DMPA. Solvent choice further impacted performance; chloroform/water emulsions yielded higher conductivity (9.47 × 10⁻¹ S/m at 10 wt % MXene) compared to DMF/water (3.58 × 10⁻⁴ S/m) due to ease of solvent removal and reduced phase separation. While PLA-PU/MXene films from chloroform/water were brittle, adding PVA improved flexibility and interfacial adhesion, enhancing MXene dispersion and toughness. At 1:1 PLA-PU/MXene/PVA (6.25 wt % MXene), the conductivity decreased to 2.93 × 10⁻³ S/m, likely because the insulating polymer encapsulation disrupted the MXene network. Higher ratios (6:4 and 7:3) increased the film’s conductivity to 2.50 × 10⁻² and 4.97 × 10⁻² S/m by improving nanosheet contact, while the value dropped to 7.50 × 10⁻³ S/m for an 8:2 ratio due to aggregation and phase separation.

Although the absolute conductivity values of the developed materials are moderated, a comparison to the literature (Figure 10) shows that the emulsification method developed in this work achieves a conductive network at significantly lower MXene contents than many other material systems,^{42,48,49,50,51,52,53,54,55,56,57} emphasizing the effectiveness of emulsification in forming conductive networks, while maintaining the mechanical integrity of the films. This method shows promise for scalable production of flexible, high-performance wearable devices.

Figure 10.

Comparison of electrical conductivity of PLA-PU/MXene films developed in this study with those reported in the literature

Sensor performance evaluation (human motion detection)

The feasibility of applying PLA-PU/MXene/PVA films as flexible wearable strain sensors was evaluated by measuring resistance changes under finger bending and laryngeal vibrations. Films with 6:4 and 7:3 compositions were selected for their balance of flexibility and conductivity. Upon bending a finger to 90° (Figure 11), both films showed increased resistance (ΔR/R₀), indicating strain-induced disruption of the conductive MXene network due to polymer matrix stretching and nanosheet reorientation.^59,60 The response remains stable and reversible, demonstrating good mechanical durability and flexibility under repeated bending cycles. The 6:4 film exhibited a gradual, stable, and reversible response, demonstrating mechanical durability. The 7:3 film showed a greater resistance change due to its denser MXene network, which is more susceptible to strain-induced disconnections.⁴¹ However, higher applied strain and repeated application may increase microcrack formation risk, potentially affecting long-term durability.

Figure 11.

Electromechanical sensing performance of PLA-PU/MXene-PVA sensors and deformation morphology under bending

(A and B) Relative resistance change (ΔR/R₀) of PLA-PU/MXene/PVA films under 90° finger bending: (A) 6:4 and (B) 7:3 compositions.

(C and D) Resistance response under different bending angles (30, 60, and 90°): (C) 6:4 and (D) 7:3.

(E–H) SEM images of films under 30° bending deformation: (E–G) bent-state morphology of the 6:4 composition at increasing magnifications; (H) corresponding unbent structure.

(I–L) (I–K) Bent-state morphology of the 7:3 composition; (L) corresponding unbent structure.

Scale bars: 500 μm in (E and I), 100 μm in (F and J), 25 μm in (G and K), and 50 μm in (H and L).

The resistance response of PLA-PU/MXene/PVA films with 6:4 and 7:3 ratios was assessed at 30, 60, and 90° bending angles to evaluate their sensitivity and strain-signal correlation. Across all angles, the 7:3 film (Figure 11D) showed greater resistance change (ΔR/R₀) than the 6:4 film (Figure 11C), reflecting higher sensitivity due to its denser but more fragile MXene network. This network disruption increased with strain, leading to larger resistance changes. Even at lower angles, the 7:3 film was more responsive to subtle deformations but likely more susceptible to durability issues. Conversely, the 6:4 film exhibited more stable resistance changes, indicating a resilient conductive network that balances sensitivity with mechanical integrity, suggesting better durability under repeated bending.

SEM images illustrated structural changes of the films under 30° bending strain. The 6:4 film’s bent state (Figures 11E–11G) showed stretched but well-encapsulated PLA-PU particles within PVA, maintaining interfacial adhesion and limited phase separation. This morphology supports the gradual resistance increase and short steady-state plateau as conductive pathways elongate without complete disruption. In contrast, the 7:3 film’s bent state (Figures 11I–11K) revealed exposed PLA-PU and MXene domains with localized voids, indicating strain-induced phase separation. These features correspond to the larger resistance increase and prolonged steady-state plateau, reflecting slower recovery of the conductive network compared to the 6:4 film counterpart.

To quantify the sensitivity, the gauge factor (GF) was calculated from the relationship between the relative change in resistance (ΔR/R₀) and the applied strain (ε). For a comprehensive sensitivity analysis, the strain at different bending angles (30, 60, and 90°) was estimated based on the film thickness and bending radius. As shown in Figure 12A, the sensors showed a clear and linear response to increasing strain. From the slope of the linear fit, the 7:3 film was found to have an excellent GF of 86.5, while the 6:4 film had a GF of 29.6. The 7:3 film, in particular, demonstrated a highly linear response (R² > 0.98), which is highly desirable for predictable and reliable sensing.

Figure 12.

Electromechanical performance and durability of the PLA-PU/MXene-PVA sensors

(A) Sensitivity, represented by the GF, for the 6:4 and 7:3 compositions (mean ± SD [n = 3]).

(B) Mechanical stability under cyclic bending, showing the 6:4 film enduring 700 cycles, while the 7:3 film fails after 200 cycles, with insets showing optical images of crack formation (scale bars, 2 mm).

(C and D) Dynamic response and recovery times under 90° finger bending for the (C) 6:4 and (D) 7:3 films.

The higher GF of the 7:3 film compared to the 6:4 film is attributed to its denser conductive network. This denser packing of MXene nanosheets is more susceptible to disruption under strain, leading to a greater change in resistance and thus higher sensitivity. This highlights a key trade-off: the 7:3 composition offers superior sensitivity for detecting subtle motions, while the 6:4 composition provides a more mechanically robust network, although with lower sensitivity.

The mechanical durability of the sensors, a critical property for wearable applications, was evaluated by a cyclic bending test, with the results shown in Figure 12B. The 6:4 film demonstrated superior stability, enduring 700 bending cycles before the baseline resistance increased significantly. In contrast, the more sensitive 7:3 film showed a rapid increase in resistance after only 200 cycles, indicating earlier mechanical failure. This difference in durability is directly related to the films’ mechanical properties and microstructure. The lower MXene content in the 6:4 film creates a more resilient and less-brittle conductive network that can better withstand repeated bending fatigue. Conversely, the denser but more fragile network of the 7:3 film is more prone to fracture. This is visually confirmed by the inset optical images in Figure 12B, where yellow arrows indicate the visible cracks present in each film after their respective durability tests are completed. These results highlight a clear trade-off between the high sensitivity of the 7:3 composition and the superior long-term durability of the 6:4 composition.

To benchmark these results, a comparison to other flexible sensors is summarized in Table 4. The high GF of the 7:3 sensor is competitive with many high-performance systems. Furthermore, the percolation threshold of <7 wt % achieved via emulsification is a notable result for a more rigid, PLA-based matrix and a significant improvement over the 40 wt % required for the solution-cast method. These combined results confirm that the emulsification approach effectively creates well-balanced composites suitable for detecting subtle human motions.

Table 4.

A comparison of key performance metrics for the PLA-PU/MXene/PVA sensors developed in this work against various state-of-the-art flexible sensors from the literature

Polymer matrix	Filler	Fabrication method	Percolation threshold (wt %)	Tensile strength (MPa)	Elongation at break (%)	Sensing strain range (%)	GF	Durability (cycles)	Reference
PDMS	Graphene	Emulsion-cast	∼5	N/A	160	0–80	20	1,000	O’Mara et al.61
PDA@NR	MXene	Solution-cast	<6	4.43	230	0–200	51.58	2,500	Gu et al.62
PDMS	CNT/MXene	Solution-cast	0.5	∼0.24	∼109	0–20	0.63	200	Yu et al.63
PVDF	HEG	Solution-cast	<5	N/A	N/A	0–0.35	10	50	Sankar et al.64
TPU	SWCNT	Coagulation precipitation	<0.1	∼15–30	∼300–900	0–18	82	N/A	Novikov et al.65
TPU/PAN	MXene/ILs	Electrospinning	N/A	1.08	140	0–200	1.07	>2,000	Fu et al.66
PLA-PU/PVA 6:4	MXene	Emulsion-cast	<7	5.82	2.6	0–2	29.6	700	This work
PLA-PU/PVA 7:3	MXene	Emulsion-cast	<7	4.58	2.3	0–2	86.5	200

N/A, not applicable; PDMS, polydimethylsiloxane; PDA@NR, polydopamine-coated natural rubber; HEG, hydrogen-exfoliated graphene; SWCT, single-walled carbon nanotubes.

While the sensor demonstrates excellent sensitivity and mechanical durability for its intended applications, a complete characterization for practical use would also require evaluating its performance against environmental variables. Therefore, anti-interference studies investigating the effects of temperature and humidity are critical for future work to validate the sensor’s reliability in real-world conditions fully.

The dynamic sensing performance of PLA-PU/MXene/PVA films was evaluated under 90° finger bending (Figures 12C and 12D). The response time (t_res) and recovery time (t_rec) were calculated from 90% of the maximum resistance change (ΔR/R₀) over multiple cycles.^67,68 The 6:4 film exhibited a slower response time (479 ms) than the 7:3 film (288 ms), reflecting faster conductive network disruption and reformation at higher MXene content. The recovery times were similar (552 vs. 506 ms), indicating that the MXene content affects the response more than recovery. Compared to other PU-based conductive composites reported in the literature, such as polydopamine/carbon black (CB)/carbon nanofiber/TPU,⁶⁹ CB/carbon nanotubes (CNTs)/thermoplastic polyurethane (TPU),⁷⁰ and sensors incorporating CNTs, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), Ag nanowires, or reduced graphene oxide (rGO),^71,72 these PLA-PU/MXene/PVA films exhibit response and recovery times within a similar range (200–500 ms). This indicates that despite their non-stretchable matrix, the PLA-PU/MXene/PVA films’ response and recovery times are competitive.

Although the 7:3 film showed a longer steady-state plateau, reflecting extended conductivity disruption, its faster dynamics is supported by SEM images, revealing stronger phase separation and rapid MXene network realignment after strain release. In contrast, the 6:4 film’s more uniform matrix and particle encapsulation resulted in smoother, slower resistance changes and stable conductive pathways. These morphological differences explain the distinct sensing behaviors: the 7:3 film offers higher sensitivity and quicker response due to phase separation, while the 6:4 film provides a more stable but slower sensing performance.

The sensor performance toward vocal/laryngeal vibrations of PLA-PU/MXene/PVA 6:4 and 7:3 films was evaluated by measuring their resistance changes (ΔR/R₀) in response to the phrases “How are you?,” “Hello,” and a coughing (Figure 13). Laryngeal deformations produced lower ΔR/R₀ values than finger bending, reflecting subtle vibrations and localized tissue stretching. For “Hello” and coughing (Figures 13B and 13C), the 7:3 film exhibited larger resistance changes than the 6:4 film due to its denser MXene network and greater phase heterogeneity. The abrupt, high-energy nature of coughing induced sharp ΔR/R₀ peaks, while the smoother articulation of “Hello” produced a more gradual resistance curve. Interestingly, in response to “How are you?” (Figure 13A), the 6:4 film showed higher ΔR/R₀, likely due to the phrase’s longer phonetic duration and complex strain modes (i.e., stretching, vibration, and muscle movements) interacting differently with the films’ microstructures.^59,73

Figure 13.

Real-time sensing applications for vocal vibrations signal detection

Relative resistance response (ΔR/R₀) of PLA-PU/MXene/PVA films at 6:4 and 7:3 compositions during the application of laryngeal vibration sensing for (A) “How are you?”, (B) “Hello”, and (C) coughing.

The lower ΔR/R₀ in laryngeal tests versus finger bending highlights the smaller strain magnitude of vocalization. Finger bending causes large, directional strain that significantly disrupts MXene connectivity, while laryngeal motion involves fine vibrations and minor skin stretch, causing less network disruption. Although the exact strain mechanism is unclear, the results show that these sensors can detect both macro- and micro-scale deformations, with the 7:3 film offering higher sensitivity and the 6:4 film more stable responses. Future work should optimize sensor placement and consider phonetic variability for improved real-time monitoring. Despite containing MXene, the films’ low loading and non-invasive design minimize toxicity concerns, supporting their potential for safe, wearable electronics.

A scalable and efficient process for fabricating flexible, high-performance conductive films of PLA-based PU by incorporating delaminated MXene through emulsion-assisted assembly has been successfully developed. By taking advantage of the intrinsic merits of emulsification, specifically improved phase control, superior dispersion of fillers, and interface engineering, PLA-PU/MXene/PVA films exhibit excellent electrical conductivity and mechanical compliance at MXene loadings lower than 10 wt %. This is superior to a solution-cast process, which requires up to 40 wt % MXene for optimum performance. In-depth structural and spectral characterization identifies that emulsification and PVA incorporation contribute to MXene distribution, reduce aggregation, and stabilize the polymer network. The fabricated films show tunable phase-separated architecture and morphology, which are responsible for their improved electromechanical response. The films with PLA-PU/MXene/PVA ratios of 6:4 or 7:3 exhibit the optimum trade-off between conductivity and stretchability, ensuring reproducible, stable responses under large-strain (e.g., bending of a human finger) or low-strain (e.g., vibration of a human larynx) deformations. The results elucidate the synergistic effect of processing routes and polymer-filler in determining the composite performance. This study establishes a viable and promising pathway for the future upcycling of PLA waste into multifunctional materials by demonstrating the successful fabrication of high-performance sensors from a model virgin PLA feedstock. The reported methodology and materials, which are based on a sustainable and biodegradable precursor, hold wide-ranging promise in next-generation wearable electronics, soft robotics, and bio-integrated sensors, in which competing flexibility, conductivity, and scalability requirements must be addressed.

Limitations of the study

This work serves as a foundational proof of concept; however, several aspects must be addressed to bridge the gap toward real-world deployment. The upcycling pathway was validated using a model virgin PLA feedstock; applying this process to actual, varied post-consumer PLA waste is a necessary next step. Similarly, the claims of sustainability and biocompatibility are based on the properties of the precursor materials. Definitive validation would require a full analysis of the final composite’s degradation behavior and direct biocompatibility testing. Furthermore, a direct comparison with composites based on conventional PU/MXene or neat PLA/MXene would be valuable to isolate the unique advantages of the PLA-PU backbone quantitatively. Additionally, in-depth studies using advanced characterization techniques, such as surface analysis and simulations, would be valuable to elucidate the precise microscopic interaction mechanisms that govern performance. Finally, a thorough evaluation of industrialization challenges, such as cost-effectiveness and batch-to-batch consistency, will be crucial for translating this laboratory-scale method to scalable production.

Resource availability

Lead contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Prof. Pakorn Opaprakasit (pakorn@siit.tu.ac.th).

Materials availability

The unique PLA-PU polymer generated during this study is available from the lead contact upon reasonable request.

Data and code availability

• •All data reported in this article will be shared by the lead contact on request.
• •This article does not report original code.
• •Any additional information required to reanalyze the data reported in this article is available from the lead contact on request.

Acknowledgments

The authors acknowledge financial support from the Thailand Science Research and Innovation Fundamental Fund and the Center of Excellence in Functional Advanced Materials Engineering (CoE FAME), Thammasat University. O.D.P. is grateful for the scholarship support from the JAIST-SIIT-NSTDA collaborative program.

Author contributions

O.D.P., investigation, visualization, and writing – original draft. A.P., S.H.H., M.O., P.S., C.K., and K.M., methodology, visualization, writing – review & editing. P.O., conceptualization, methodology, visualization, supervision, and writing – review & editing.

Declaration of interests

The authors declare no competing interests.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
Polylactide (PLA 4043D)	NatureWorks	4043D
Lithium fluoride (LiF)	Acros Organics	Cat#218270025
2,2-bis(hydroxymethyl)propionic acid (DMPA)	Acros Organics	Cat#147975000
1,4-butanediol (BDO)	Acros Organics	Cat#L03491.0B
Isophorone diisocyanate (IPDI)	Acros Organics	Cat#427602500
Tin(II) 2-ethyl hexanoate (Sn(Oct)2)	Sigma-Aldrich	Cat#S3252
Poly(vinyl alcohol) (PVA)	Sigma-Aldrich	Cat#P8136
Chloroform	Carlo Erba Chemicals	Cat#528326
Dimethylformamide (DMF)	Carlo Erba Chemicals	Cat#444923
Dimethyl sulfoxide (DMSO)	Carlo Erba Chemicals	Cat#508001
Triethylamine (TEA)	Carlo Erba Chemicals	Cat#LA12646AP
Hydrochloric acid (HCl)	Carlo Erba Chemicals	Cat#302626
Ti3AlC2 (MAX phase)	Luoyang Tongrun Nano Technology Co., Ltd.	Cat#TR-Ti3AlC2
Software and algorithms
OriginPro 2022	OriginLab	www.originlab.com
OMNIC Software	Thermo Fisher	www.thermofisher.com
Other
Microwave Reactor	CEM Matthews, NC, USA	Discover SP series
FTIR Spectrometer	Thermo Scientific	Nicolet iS5
FE-SEM	Hitachi, Tokyo, Japan	SU-8030
TEM	JEOL, Tokyo, Japan	JEM-2100Plus
X-ray Diffractometer	Bruker, Germany	D8 Advance
DSC	Mettler Toledo, Switzerland	DSC 3+
Tensile Machine	Tinius Olsen	H5KT
Digital Multimeter	Keithley	DMM6500
Probe Sonicator	Branson	Sonifier SFX550

Experimental model and study participant details

As this study serves as a preliminary assessment of sensor performance, all experiments involving strain sensing under large-strain bending of a finger and low-strain vibration arising from laryngeal deformation were conducted on a single participant, the author of this manuscript, Oceu Dwi Putri (Female, Asian, 32 years old). The experiment was performed with the permission of Sirindhorn International Institute of Technology (SIIT), Thammasat University, and oversight by Prof. Pakorn Opaprkasit, the corresponding author. No other human subjects were involved in the experiments. This, therefore, leads to the limitation of the study that the influence of sex, gender, or both on the results, cannot be evaluated.

Method details

Materials

Polylactide (PLA 4043D, ${\overline{M}}_w$ = 1.2–1.5 × 10⁵) was purchased from NatureWorks. Lithium fluoride (LiF), 2,2-bis(hydroxymethyl)propionic acid (DMPA), 1,4-butanediol (BDO), and isophorone diisocyanate (IPDI) were obtained from Acros Organics. Tin(II) 2-ethyl hexanoate (Sn(Oct)₂) and poly(vinyl alcohol) (PVA, ${\overline{M}}_w$ = 30,000−70,000) were purchased from Sigma Aldrich. Chloroform, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone, triethylamine (TEA), and hydrochloric acid (HCl) were purchased from Carlo Erba Chemicals. Ti₃AlC₂ MAX phase (99.5%) was purchased from Luoyang Tongrun Nano Technology. All materials were used without further purification.

Alcohol-acidolysis of PLA resin by DMPA

The “sizing down” of PLA resin to lactate oligomers was conducted by alcohol-acidolysis using DMPA in a microwave reactor (Discover SP series, CEM Matthews, NC, USA), as outlined in Figure 1. PLA pellets and DMPA were placed in a 35 mL vessel tube with a magnetic stir bar. The PLA: DMPA molar ratio was fixed at 6:1. The reactions were carried out at 180°C, with a reaction time of 10 min, under self-pressurized conditions (<100 psi). The resulting alcohol-acidolyzed PLA product (aac-PLA) is a mixture of oligolactates with hydroxyl and carboxylic acid terminals at different sizes and structures. This can be directly used as a polyol starting material for synthesizing PU. The mixture was purified by dissolving it in hot acetone and precipitating it in an excess of water/ethanol mixture. The products were then subjected to vacuum filtration and dried in an oven at 60°C overnight.

Synthesis of MXene

MXene (Ti₃C₂T_x) was synthesized by etching the MAX phase (Ti₃AlC₂) using lithium fluoride (LiF) in hydrochloric acid (HCl). In a Teflon vessel, 1.9 g of LiF was added to 10 mL of 9 M HCl with continuous stirring for 10 min at room temperature. Then, 1 g of Ti₃AlC₂ powder was slowly added to the etchant solution under slow stirring. The reaction was allowed to proceed at 50°C for 24 h to extract the Al layers. The obtained suspension was then cooled down, washed using deionized water, and centrifuged (5000 rpm, 5 min/cycle) several times until the final pH value was about 5–6. The precipitate was then collected and dried in a vacuum oven at 60°C for 24 h. The resulting delaminated MXene was dispersed in DMSO (1:12, w/v) and stirred for 24 h at room temperature, followed by centrifuging at 5000 rpm for 5 min.

Synthesis of PLA-based polyurethane (PLA-PU)

The synthesis of PLA-based polyurethane (PLA-PU) is summarized in Figure 2. The aac-PLA product (5 g) and 1 wt. % of Sn(Oct)₂ catalyst were dissolved in chloroform or DMF in a three-necked round-bottom flask. Subsequently, IPDI (1.1 mL) was introduced into the flask under mechanical stirring and heated to 90°C for 3 h under a reflux condenser to obtain PLA-based prepolymers. BDO (0.28 mL) was then added and allowed to react for 1 h to extend the prepolymer chains. Triethylamine (TEA) was added dropwise to the reaction vessel once it was cooled to 30°C and reacted for an additional 30 min. The resulting PLA-PU was then stored at room temperature to fabricate PLA-PU and PLA-PU/MXene films. The chemical reaction is detailed in Scheme 1.

Fabrication of PLA-PU/MXene films by solution casting

PLA-PU/MXene films were prepared by directly blending MXene powder into a PLA-PU solution using DMF solvent (40%, w/w). The mixture was subjected to ultrasonication for 1 h, followed by stirring to ensure good dispersion, as depicted in Figure 2. The resulting mixture was then cast onto a silicone mold and dried at 60°C to facilitate solvent evaporation and the formation of PLA-PU/MXene cast films.

Fabrication of PLA-PU/MXene films by emulsification and casting

PLA-PU/MXene films were also alternatively prepared through emulsification and casting to facilitate MXene distribution, as also described in Figure 2. An aqueous MXene dispersion was first prepared at 7 and 10% (w/w) and were homogenized by probe sonication at an amplitude of 50% for 2 min. Meanwhile, PLA-PU solutions in DMF or chloroform, which were the products of the synthesis described in the previous section, were separately prepared. The aqueous MXene dispersion was mixed with the oil-phase (PLA-PU solution) to form an emulsion with a desired mixing ratio to achieve a final PLA-PU solid content of 20%. The emulsification process was facilitated by probe sonication at an amplitude of 50% for 2 min. Subsequently, the emulsions from the chloroform-water system were continuously stirred at 800 rpm at room temperature overnight to allow solvent evaporation. The emulsions from both DMF/water and chloroform/water systems were then cast on a silicone mold and dried at room temperature for 48 h to yield PLA-PU/MXene films.

To prepare PLA-PU/MXene/PVA films, a 10% (w/v) aqueous PVA solution was prepared by dissolving PVA powder in deionized water under constant stirring at 90°C. The resulting PVA solution was mixed with pre-prepared PLA-PU or PLA-PU/MXene emulsions at specific weight ratios (1:1, 6:4, 7:3, and 8:2 PLA-PU/MXene: PVA). The mixtures were further homogenized using gentle stirring at 300 rpm for 1 h to ensure uniform emulsions. The final emulsions were cast onto a silicone mold and dried at room temperature for 48 h to obtain flexible PLA-PU/MXene/PVA films.

Characterization of PLA-PU/MXene films

The chemical structures and interactions of the resulting PLA-PU/MXene films prepared using different techniques were examined by Fourier transform infrared (FTIR) spectroscopy. The spectra were acquired using a Thermo Scientific Nicolet iS5 spectrometer in attenuated total reflectance (ATR) mode, coadding 32 scans at a resolution of 4 cm⁻¹. The morphological characteristics of MXene and PLA-PU/MXene films were observed using field emission-scanning electron microscopy (FE-SEM Hitachi SU-8030, Tokyo, Japan) and transmission electron microscopy (TEM JEOL JEM-2100Plus, Tokyo, Japan) with an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns of the materials were analyzed on an X-ray diffractometer (D8 Advance, Bruker, Germany) using CuKα radiation (1.5406 Å) at 40 kV and 40 mA. Thermal properties of PLA-PU/MXene films were evaluated using a differential scanning calorimeter (DSC 3+, Mettler Toledo, Switzerland) under a nitrogen atmosphere (a flow rate of 10 mL/min). The samples were examined at a heating and cooling rate of 10°C/min from −20°C to 200°C. The mechanical properties of the film samples were assessed on a Tinius Olsen H5KT tensile machine. The films were cut to 1 × 5 cm² dimensions and subjected to tensile testing at a set crosshead speed of 20 mm/min. The electrical resistance measurements of PLA-PU/MXene films were conducted using a Keithley DMM6500 6 1/2-digit bench digital multimeter, whose electrical conductivity was calculated using the following equations:

$$\sigma =\frac{1}{\rho }$$ (Equation 2)

$$\sigma =\frac{1}{R}\times \frac{\mathcal{l}}{w.t}$$ (Equation 3)

where σ is the electrical conductivity (S/cm), ρ is the resistivity (Ω/cm), R is the resistance (Ω), $\mathcal{l}$ is the distance between the two probes (cm), w is the width of the films (cm), and t is the thickness of the films (cm). The sensor performance was further characterized by evaluating its sensitivity, mechanical durability, and response to human motion. Sensitivity, defined by the Gauge Factor (GF), was determined by measuring the change in resistance as the film was bent to various angles (30°, 60°, and 90°), with the applied strain (ε) calculated from the film thickness and bending radius. Mechanical durability was assessed via a manual cyclic bending test, where the sensor was repeatedly bent around a 1 cm diameter rod for up to 1,000 cycles, with the baseline resistance monitored at set intervals. For vocal cord vibration sensing, the optimal sensor film was attached to a volunteer’s throat, and the change in resistance was recorded as they spoke specific phrases.

Quantification and statistical analysis

All data processing and graphing were performed using OriginPro 2022. The Gauge Factor (GF) was determined from the slope of a linear regression fit to the relative resistance change versus applied strain data. For mechanical and electromechanical performance data, all dispersion and precision measures (mean, standard deviation) and the exact value of n (representing the number of independent samples, n ≥ 3) are reported directly in the corresponding figure legends. Durability data are presented as the percentage change in baseline resistance over the number of cycles. No statistical tests for significance (e.g., t-tests or ANOVA) were performed.

Published: January 7, 2026