High-performance reverse thermoresponsive hydrogel enabled by one-pot PDMS-enriched domain crosslinking

Abstract

Reverse thermoresponsive hydrogels, which exhibit low transparency at ambient temperature and become transparent upon heating, offer distinct advantages in information encryption, thermal display, and emergency signaling. However, integrating such optical responsiveness with mechanical robustness, moisture retention, and interfacial adhesion remains a challenge. Herein, we report a highly stretchable and reverse thermoresponsive hydrogel based on polyacrylamide (PAM) crosslinked by PDMS-enriched microgel-like domains, synthesized via an emulsion-assisted one-pot strategy. During polymerization, hydrophobic PDMS chains form domain aggregates and covalently integrate with PAM at the interface, resulting in a robust and deformable domain network. The hydrogel exhibits excellent mechanical performance (5680% stretchability, 5.8 MJ m⁻³ toughness) and reversibly transitions from opaque to transparent upon heating, due to entropy-driven domain reorganization that reduces interfacial light scattering. This enables rapid thermal decryption and high-contrast visual display without external energy inputs. The hydrogel also shows enhanced water retention, strong adhesion to various substrates, and sodium chloride (NaCl)-enabled strain sensing. This work provides a structurally simple yet multifunctional platform for next-generation optical encryption materials and flexible photonic devices.

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New concepts

Conventional thermoresponsive hydrogels typically transition from transparent to opaque upon heating, which limits their use in rapid optical switching and thermal encryption due to poor visibility and high energy demand. Here, we introduce a reverse thermoresponsive hydrogel that uniquely becomes transparent at elevated temperatures, enabling rapid thermal decryption, visual signaling, and energy-free information display. This hydrogel is synthesized via a one-pot emulsion-assisted strategy, wherein hydrophobic PDMS chains form covalently integrated domains within a PAM matrix. These PDMS-rich domains not only drive the reverse thermoresponsive optical response through entropy-mediated refractive index homogenization, but also reinforce the hydrogel mechanically via progressive domain deformation and chain unfolding. This dual mechanism leads to a rare integration of ultrastretchability (5690%), high toughness (5.8 MJ m⁻³), strong interfacial adhesion, and excellent water retention. Compared to existing thermoresponsive systems, our design eliminates the need for LCST-type phase separation or complex copolymer synthesis, offering a structurally simple yet functionally rich platform. This concept introduces a new paradigm in hydrogel engineering where thermoresponsive optics are integrated with mechanical resilience and practical processability, opening pathways for advanced applications in encryption materials, wearable photonic devices, and emergency-response systems.

1. Introduction

Smart hydrogels that dynamically respond to external stimuli such as temperature,¹ pH,² light,³ or mechanical force⁴ have attracted significant attention for their potential applications in information encryption,⁵ smart displays,⁶ optical modulation,^7,8 and flexible electronics.^9,10 Among these, thermoresponsive hydrogels are especially promising due to their ability to undergo temperature-induced optical transitions, such as changes in transparency or color.^11,12 These properties offer valuable opportunities for anti-counterfeiting, secure information display, and intelligent optical regulation. Currently, widely studied thermoresponsive hydrogels include poly(N-isopropylacrylamide) (PNIPAm)-based systems,^13,14 polyethylene glycol (PEG)-based block¹⁵ or random copolymer hydrogels,^16,17 as well as cellulose derivatives such as hydroxypropyl cellulose (HPC)¹⁸ and hydroxypropyl methylcellulose (HPMC).¹⁹ These materials typically exhibit a conventional optical response in which the hydrogel remains transparent at low temperatures and becomes opaque above its lower critical solution temperature (LCST).^20,21 The opacity is mainly attributed to microphase separation or hydrophobic aggregation, which enhances light scattering due to the dehydration of polymer chains.²² However, this conventional “transparent-to-opaque” thermoresponsive mode faces intrinsic limitations when applied to information encryption or thermally controlled optical displays. Specifically, the encoded information or image becomes visible only at elevated temperatures, which not only requires relatively slow heat diffusion and high energy consumption, but also compromises display clarity due to high-temperature-induced opacity. These drawbacks restrict their practical effectiveness, particularly in scenarios that demand rapid optical switching and high visibility under thermal stimuli.

To address these issues, the development of reverse thermoresponsive hydrogels that appear opaque at room temperature and become transparent upon heating has emerged as an important direction. Such systems offer greater flexibility, responsiveness, and contrast in optical regulation, making them highly suitable for thermal decryption, smart displays, and emergency visual signaling. In addition to the limitations in optical behavior, conventional thermoresponsive hydrogels often suffer from poor mechanical performance.^23–25 They tend to fracture or degrade under large deformation or cyclic loading, which significantly hinders their utility in wearable and flexible optical devices.^26–28 Strategies such as solvent exchange,^29–31 nanofiller incorporation,^32–34 stepwise crosslinking,^35–37 and thermal annealing^38–40 have been proposed to enhance their mechanical robustness. However, these methods often involve complicated synthesis protocols, long processing times, and strict reaction conditions.⁴¹ Furthermore, these hydrogels frequently exhibit low adhesion to common substrates and inadequate water retention, resulting in poor long-term durability and functional reliability.^42,43 Therefore, developing a hydrogel system that simultaneously integrates reverse thermoresponsive optical switching, high mechanical resilience, environmental stability, strong adhesion, and scalable preparation remains a key challenge and a crucial goal for advancing practical applications.

Herein, we report a mechanically resilient and reverse thermoresponsive hydrogel (PAM@PDMS hydrogel) prepared via a simple emulsion-assisted one-pot polymerization (Scheme 1). In this system, hydrophobic polydimethylsiloxane (PDMS) prepolymers form stable microgel-like domains within a hydrophilic acrylamide (AM) matrix using Tween 80 as the emulsifier. Upon UV-induced polymerization (250 W, 5 min), PDMS chains bearing Si–H groups react with vinyl-terminated PDMS inside the domains to form crosslinked PDMS-rich domains. Simultaneously, additional vinyl-PDMS copolymerizes with AM at the domain–water interface, constructing a domain-crosslinked network that covalently integrates hydrophobic and hydrophilic segments. This hierarchical architecture enables the sequential unfolding of PAM and PDMS chains and progressive deformation of PDMS domains under strain, endowing the hydrogel with exceptional stretchability (5680%), toughness (5.8 MJ m⁻³), and fatigue resistance. At room temperature, refractive index mismatch between PDMS domains and the PAM matrix enhances interfacial light scattering, resulting in an opaque appearance. Upon heating, entropy-driven domain reorganization reduces scattering and induces rapid, reversible transparency. This reverse thermoresponsive behavior enables thermal information decryption, dynamic optical switching, and emergency visual signaling. Additionally, PDMS domains impart excellent water retention and strong adhesion to diverse surfaces, including metal, plastic, rubber, glass, pigskin, hydrogel, elastomer, and fiber. The incorporation of NaCl further provides strain-sensing functionality, allowing real-time monitoring of deformation. In contrast to previously reported systems—such as micelle-crosslinked hydrogels with ultrahigh stretchability but limited optical tunability⁴⁴ or PDMS-containing hydrogels with tunable oxygen permeability but insufficient mechanical resilience and lack of stimuli-responsiveness⁴⁵—our PAM@PDMS hydrogel uniquely integrates optical responsiveness, mechanical robustness, interfacial adhesion, water retention, and sensing functionality into a single material platform. This work presents a structurally simple yet functionally versatile hydrogel design, offering a practical solution to key challenges in intelligent photonic and encryption materials.

Scheme 1 Schematic illustration of the one-pot emulsion-assisted synthesis of PAM@PDMS hydrogel featuring ultrastretchability, reverse thermoresponsiveness, high adhesion, and water retention capacity.

2. Results and discussion

2.1 Molecular design and structural characterization of PAM@PDMS hydrogel

PAM@PDMS hydrogel is fabricated via a simple and efficient one-pot method assisted by emulsion templating, combining UV-initiated free-radical polymerization and thermally induced hydrosilylation. In this system, the aqueous phase contains monomer acrylamide (AM) and crosslinker N,N′-methylenebisacrylamide (MBA), while the oil phase consists of PDMS precursors—component A is a vinyl-terminated polysiloxane prepolymer, and component B contains hydride-functional siloxane and a platinum catalyst (Pt-catalyst), which promotes the hydrosilylation between vinyl and Si–H groups.⁴⁶ Upon emulsification with Tween 80, a stable water-in-oil emulsion is formed. After the addition of the photoinitiator 1173, the emulsion is irradiated under 250 W UV light for 5 minutes. During this process, AM polymerizes via free-radical initiation to form PAM network, while the heat generated by high-power UV irradiation simultaneously induces the crosslinking of PDMS precursors via hydrosilylation within the domains to form PDMS network.

Macroscopic observation reveals that the PAM@PDMS hydrogel exhibits significantly reduced transparency compared to pure PAM hydrogel or pure PDMS elastomer (Fig. 1(a)), indicating the formation of microscale phase-separated structures, likely due to local enrichment of PDMS. To verify this structural feature, Raman spectroscopy is employed. A strong absorption band appears at 851 cm⁻¹ (Fig. 1(b)), corresponding to the symmetric bending vibration of Si–CH₃ groups, a characteristic signature of PDMS. Raman mapping based on this band (Fig. 1(c)) shows a dotted spatial distribution of Si signals, suggesting that PDMS chains aggregate into localized microdomains rather than dispersing throughout the network. These PDMS-rich domains are believed to originate from the dynamic evolution of emulsion droplets during the one-pot polymerization process. In the precursor solution, PDMS components form nanodroplets (∼1718 nm) stabilized by Tween 80 (Fig. S1). As polymerization proceeds, phase separation and viscosity increase allow these droplets to undergo limited fusion or aggregation, ultimately forming immobilized microgel-like domains within the hydrogel matrix. Scanning electron microscopy (SEM) provides further morphological evidence. The PAM@PDMS hydrogel exhibits a dense multiscale porous structure, in sharp contrast to the loosely networked pure PAM hydrogel, as well as the compact and non-porous PDMS elastomer. Numerous uniformly distributed spherical microstructures are observed (Fig. 1(d)). Energy-dispersive X-ray spectroscopy (EDS) analysis confirms their composition: Si signals are concentrated in discontinuous spot-like regions corresponding to PDMS domains, while N signals—representing PAM chains—are homogeneously distributed, indicating that the PAM network constitutes the continuous phase. The above results demonstrate that the PAM@PDMS hydrogel adopts a domain-in-network architecture, where PDMS-rich domains are embedded and immobilized within the PAM matrix.

Fig. 1 Structural characterization of the PAM@PDMS hydrogel. (a) UV-vis transmittance spectra of PAM hydrogel, PDMS elastomer, and PAM@PDMS hydrogel. (b) Raman spectrum of the PAM@PDMS hydrogel. (c) Raman mapping of Si element distribution in the PAM@PDMS hydrogel. (d) SEM images and corresponding EDS elemental mapping of the PAM hydrogel, PDMS elastomer, and PAM@PDMS hydrogel. (e) FTIR spectra of the PAM hydrogel, PDMS elastomer, and PAM@PDMS hydrogel. (f) XPS spectrum of the PAM@PDMS hydrogel before toluene extraction. (g) XPS spectrum of the PAM@PDMS hydrogel after toluene extraction. (h) TOF-SIMS analysis of the PAM@PDMS hydrogel and positive ion mapping of the characteristic fragment C₅H₁₃NOSi⁺.

Fourier-transform infrared spectroscopy (FTIR) is used to analyze molecular interactions within the hydrogel network (Fig. 1(e)). The PAM@PDMS hydrogel exhibits both the C=O stretching vibration of PAM and the Si–O–Si stretching vibration of PDMS. Compared with pure PDMS, the Si–O–Si peak in PAM@PDMS shifts from 1010 to 1016 cm⁻¹, which is attributed to constrained segmental mobility of PDMS chains confined within domains. This spatial restriction increases the bond force constant and results in a blue shift. Meanwhile, the C=O band shifts from 1659 to 1645 cm⁻¹, possibly due to the increased packing density of PAM chains caused by the incorporation of PDMS domains, which enhances N–H⋯O=C hydrogen bonding, thereby lowering the force constant of the C=O bond and causing a red shift. These molecular-level interactions are consistent with the hierarchical morphology observed by SEM.

To further elucidate the interaction between PDMS domains and the PAM network, solvent extraction is performed using toluene. After immersion in toluene for 24 hours, the freeze-dried hydrogels are analyzed by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 1(f) and (g), the Si content decreases only slightly from 24.2% to 19.6%, indicating that the majority of PDMS remains within the hydrogel network. This result suggests that PDMS is predominantly covalently incorporated rather than physically entrapped or loosely embedded. To probe the nature of this covalent integration, a comparative experiment is conducted using a condensation-type PDMS (DOWSIL 3140 RTV Coating), which lacks vinyl functional groups, in place of the addition-type PDMS (Sylgard 184). Under identical polymerization conditions, the condensation-type PDMS exhibits poor miscibility with PAM and forms macroscopic aggregates that are readily removed by toluene extraction (Fig. S2). In contrast, the addition-type PDMS forms a uniform, extractant-resistant hydrogel network. This contrast strongly implies that the vinyl-terminated PDMS in Sylgard 184 participates in radical copolymerization with acrylamide monomers, enabling covalent integration into the PAM matrix.

To verify this hypothesis, time-of-flight secondary ion mass spectrometry (ToF-SIMS) is carried out (Fig. 1(h)). In the positive ion mode, characteristic fragments corresponding to polyacrylamide are observed at m/z = 59 ([CH₃–C(=O)–NH₃]⁺) and 72 (protonated acrylamide), while PDMS-related peaks appear at m/z = 28 (Si⁺) and 73 ([(CH₃)₂SiOH]⁺), which are attributed to –[Si(CH₃)₂O]– units. A distinct signal at m/z ≈ 147 is detected in the PAM@PDMS hydrogel and is not commonly associated with PAM or PDMS fragments reported in previous studies. The characteristic peaks of Tween 80 do not overlap with those observed in the PAM@PDMS hydrogel (Fig. S3). Based on its mass and composition, this fragment is assigned to [HO–HSi–(CH₃)–CH₂–CH₂–CH(CONH₃)]⁺, which is consistent with a covalent linkage between vinyl-functional PDMS and acrylamide formed during radical polymerization.^47,48 Therefore, the presence of this ion, along with PAM- and PDMS-derived fragments and their spatial colocalization in TOF-SIMS mapping, suggests the formation of a covalently integrated polymer network.

Collectively, the PAM@PDMS hydrogel establishes a composite network structure in which PDMS-rich domains are covalently embedded within a continuous PAM matrix. These domains serve not only as elastic reinforcement points but also enhance interchain interactions through spatial confinement. This hierarchical structure provides a physical and chemical foundation for the cooperative deformation of the network under strain, which underpins the hydrogel's remarkable stretchability and toughness. Additionally, the temperature-induced increase in PDMS chain mobility drives entropy-governed relaxation or reorganization of domains, resulting in a reversible transition from opacity to transparency. These structure–property relationships offer a rational basis for the mechanical and thermoresponsive functionalities of the hydrogel, which are discussed in detail in the following sections.

2.2 Ultrastretchability of PAM@PDMS hydrogel and its mechanism

The PAM@PDMS hydrogel exhibits remarkable tensile performance. Compared to the relatively limited fracture strains of pure PAM hydrogel (980%) and pure PDMS elastomer (282%), the PAM@PDMS hydrogel achieves an impressive fracture strain of 5690% (Fig. 2(a)). This significant improvement highlights the advantage of integrating PAM and PDMS into a unified network, where the combination of a hydrophilic polymer matrix and embedded PDMS-rich domains significantly enhances the deformability of the material. A photographic demonstration of this ultrastretchability is shown in Fig. 2(b), where the hydrogel is stretched to several tens of times its original length without rupture, vividly illustrating its exceptional extensibility. To better understand and optimize this performance, the effects of three key formulation parameters—monomer concentration, PDMS content, and emulsifier (Tween 80) dosage—are systematically investigated. Mechanical properties including fracture strain, tensile strength, toughness, and elastic modulus are quantitatively assessed.

Fig. 2 Ultrastretchability of the PAM@PDMS hydrogel. (a) Tensile stress–strain curves of the PAM hydrogel, PDMS elastomer, and PAM@PDMS hydrogel. (b) Photographic demonstration of the ultrastretchability of the PAM@PDMS hydrogel. (c) Tensile stress–strain curves and corresponding mechanical parameters (fracture strain, tensile strength, toughness, and elastic modulus) of the PAM@PDMS hydrogels with varying AM content. (d) Tensile stress–strain curves and corresponding mechanical parameters of the PAM@PDMS hydrogels with varying PDMS content. (e) Tensile stress–strain curves and corresponding mechanical parameters of the PAM@PDMS hydrogels with varying Tween 80 content.

As shown in Fig. 2(c), increasing the AM content from 2.3 mol L⁻¹ to 6.9 mol L⁻¹ significantly enhances both tensile strength (up to 0.55 MPa) and fracture strain (above 5000%). This enhancement is attributed to the formation of a denser and more entangled PAM network at higher monomer concentrations, which facilitates efficient load transfer and improves energy dissipation. However, when the AM content reaches 9.2 mol L⁻¹, both elongation and toughness decline, likely due to excessive network density restricting chain mobility and increasing brittleness. Toughness and modulus also peak at 6.9 mol L⁻¹ AM, indicating that this concentration provides an optimal balance between strength and flexibility. Fig. 2(d) illustrates the effect of PDMS content. Increasing the PDMS-to-AM mass ratio from 3 : 15 to 5 : 15 markedly enhances both fracture strain and toughness, reaching 5680% and 5.8 MJ m⁻³, respectively. This enhancement arises from the increased formation of PDMS-rich domains, which act as soft, deformable energy-dissipating domains embedded within the PAM network. However, when the PDMS content is further increased to a PDMS-to-AM ratio of 6 : 15, the mechanical performance deteriorates. This decline is likely due to excessive domain aggregation, which compromises network uniformity and impairs efficient stress distribution. As shown in Fig. 2(e), the concentration of Tween 80 also plays a supporting but essential role. An optimal level (0.24 mol L⁻¹) ensures effective emulsification and uniform domain dispersion. Insufficient Tween 80 results in poor domain formation, while excess surfactant may destabilize the interface, both negatively affecting network integrity. Collectively, these results highlight that tuning the PDMS concentration is the primary determinant of mechanical performance, with 5 : 15 as the optimal PDMS-to-AM ratio, coupled with 6.9 mol L⁻¹ AM and 0.24 mol L⁻¹ Tween 80.

To explore the underlying deformation mechanism, the stress–strain curve of the optimized hydrogel is divided into three regions (Fig. 3(a)): region I (0–100%), region II (100–4000%), and region III (>5000%). As shown in Fig. 3(b), region I exhibits a steep stress increase and high modulus with minimal energy dissipation, corresponding to elastic-dominated deformation. Region II shows a more gradual stress increase with reduced modulus and a significant rise in dissipated energy (approximately 281.8 kJ m⁻³), indicating nonlinear deformation. In region III, the stress increases sharply again, accompanied by high energy dissipation (approximately 281.5 kJ m⁻³), corresponding to energy accumulation and final network rupture. These deformation characteristics are further supported by cyclic tensile loading–unloading tests (Fig. 3(c) and (d)). At a maximum strain of 50%, the loading and unloading curves nearly overlap, and the hysteresis is negligible (around 0.06 kJ m⁻³), confirming reversible elastic behavior. As the strain increases, the hysteresis loops gradually enlarge, indicating progressive energy dissipation and microstructural evolution. To further validate the structural integrity under repeated deformation, 500 consecutive loading–unloading cycles at 50% strain are performed. As shown in Fig. 3(e), no apparent reduction in strain or modulus is observed, confirming excellent structural resilience and fatigue resistance under small deformation. Moreover, the elastic modulus remains nearly constant over a wide strain rate range (10–100 mm min⁻¹) (Fig. 3(f) and (g)), suggesting a strain-rate-independent response, characteristic of elastic materials. This behavior is consistent with that of conventional chemically crosslinked PAM and PDMS networks (Fig. S4), further validating the presence of covalent integration between the two phases.

Fig. 3 Mechanism of ultrastretchability in the PAM@PDMS hydrogel. (a) Tensile stress–strain curve of PAM@PDMS hydrogel divided into three deformation regions. (b) Toughness and modulus of the PAM@PDMS hydrogel corresponding to each region. (c) Cyclic tensile loading–unloading curves of the PAM@PDMS hydrogel under different strain levels. (d) Corresponding dissipated energy of the PAM@PDMS hydrogel at various strain levels. (e) Loading–unloading curves over 100 consecutive cycles at a maximum strain of 50%. (f) Tensile stress–strain curves and (g) corresponding elastic modulus of the PAM@PDMS hydrogel under different tensile rates. (h) CLSM images showing PDMS domain morphology at different strain states. (i) Schematic illustration of the proposed ultrastretchability mechanism of the PAM@PDMS hydrogel.

To further understand the structural evolution during deformation, confocal laser scanning microscopy (CLSM) is employed to monitor the behavior of PDMS-rich domains, which are fluorescently labeled for visualization (Fig. 3(h) and (i)). In region I, domains remain intact and homogeneously dispersed within the hydrogel matrix. As strain enters region II, the domains begin to elongate, partially rupture, and almost completely disrupted, highlighting their critical role in energy dissipation and network buffering. Based on these observations, a sequential deformation mechanism is proposed. In the initial stage, deformation is primarily accommodated by the stretching of flexible PAM chains between domains, while the domains themselves almost remain intact. In the intermediate stage, as PAM chains approach their full extension, stress is gradually transferred to the PDMS domains. Although chemically crosslinked, PDMS chains initially adopt a coiled conformation and begin to uncoil and extend under stress due to their inherent flexibility. Simultaneously, the domains undergo deformation and rupture, enabling energy dissipation and structural rearrangement. In the final stage, both PAM and PDMS chains approach their contour lengths, and localized chain scission occurs, releasing large amounts of energy and marking the hydrogel's ultimate failure. This progressive and synergistic deformation pathway endows the PAM@PDMS hydrogel with exceptional extensibility and mechanical robustness.

2.3 Adhesion and water retention properties of PAM@PDMS hydrogel

In addition to its excellent mechanical properties, the PAM@PDMS hydrogel also demonstrates strong interfacial adhesion across diverse substrates and remarkable water retention capability. As shown in Fig. 4(a), the hydrogel adheres firmly to various commonly used materials, including aluminum (Al), polyvinyl chloride (PVC), polymethyl methacrylate (PMMA), rubber, pigskin, glass, PAM hydrogel, PDMS elastomer, and non-woven (NW) fabric fibers, exhibiting excellent interfacial adaptability and broad application compatibility. To quantitatively evaluate its adhesive properties, lap shear tests are performed. As shown in Fig. 4(b), the shear force–displacement curves display continuous loading behavior on all interfaces without sudden detachment, indicating stable interfacial contact under shear stress. The calculated adhesion strengths (Fig. 4(c)) further confirm the hydrogel's broad-spectrum adhesion, with consistently high values observed on both hydrophilic and hydrophobic substrates. Notably, the hydrogel achieves a maximum adhesion strength of 354.3 kPa on NW fabric. This exceptionally high adhesion is attributed to the fabric's 3D porous structure and surface roughness, which facilitate strong mechanical interlocking and enhanced interfacial anchoring with the hydrogel network. More importantly, the hydrogel retains moderate adhesion to most of these substrates even under aqueous conditions (Fig. S5), highlighting its applicability in moist or submerged environments. This broad-spectrum adhesion behavior mainly arises from the synergistic effects of PAM and PDMS. The PAM chains provide abundant amide groups capable of forming hydrogen bonds or dipole–dipole interactions with polar hydrophilic surfaces. Meanwhile, the PDMS chains, with their low modulus and high flexibility, offer excellent interfacial wettability and conformability, allowing intimate contact with rough or nonpolar surfaces. Furthermore, the domain-distributed PDMS domains function as soft interfacial cushions that mitigate stress concentration during shear deformation, enhancing the overall adhesive strength and contact durability.

Fig. 4 Adhesion and water retention properties of the PAM@PDMS hydrogel. (a) Photographs demonstrating the adhesion of PAM@PDMS hydrogel to various substrates. (b) Lap shear curves and (c) corresponding adhesion strength of the PAM@PDMS hydrogel on different substrates. (d) Water retention performance of the PAM@PDMS hydrogels with varying PDMS content (25 °C and 65% relative humidity).

Water retention is another critical factor determining the long-term stability of hydrogels in practical applications. As shown in Fig. 4(d), the water content of the PAM@PDMS hydrogel significantly increases with the PDMS content. Particularly, when the PDMS-to-AM mass ratio reaches 5 : 15, the hydrogel retains over 75% of its initial water content after 48 hours under ambient conditions (25 °C and 65% relative humidity), far outperforming pure PAM hydrogels (PDMS-to-AM mass ratio 0 : 15). This enhanced water retention capability can be attributed to the hydrophobic nature of PDMS chains, which effectively suppresses moisture evaporation, as well as the spatial confinement provided by the domain structure, which prolongs water diffusion pathways and slows water loss. Therefore, the introduction of PDMS domains imparts the PAM@PDMS hydrogel with excellent interfacial adhesion and environmental stability.

2.4 Reverse thermoresponsive behavior of PAM@PDMS hydrogel

The PAM@PDMS hydrogel exhibits a unique reverse thermoresponsive optical behavior, displaying low transparency at low temperatures and high transparency at elevated temperatures. In contrast to widely studied conventional thermoresponsive hydrogels, this system demonstrates significant differences and advantages in terms of response direction, underlying mechanism, and functional applications. Traditional thermoresponsive hydrogels, such as PNIPAM-based systems, typically undergo polymer chain dehydration and shrinkage upon heating, which induces microphase separation and reduces transparency. Moreover, their response rate and reversibility are often limited. In contrast, PAM@PDMS hydrogel exhibits a reverse response, with increased transparency upon heating, making it particularly suitable for visual recognition and thermally triggered display under high-temperature conditions. This reverse thermoresponsive behavior likely originates from the temperature-responsive microgel-like domains formed by PDMS chains within the hydrogel network. At ambient temperature, crosslinked PDMS chains self-assemble into densely packed, polydisperse domains, and the pronounced refractive index mismatch between these domains and the hydrophilic PAM matrix enhances interfacial light scattering, resulting in low transparency.^49,50 Upon heating, several factors collectively contribute to increased transparency. First, differential scanning calorimetry (DSC) reveals that the glass transition temperature (T_g) of the PAM@PDMS hydrogel is around 51 °C (Fig. S6), which closely corresponds to the observed cloud point temperature (∼58 °C). This temperature marks the critical point at which the optical transition occurs, suggesting a correlation between thermal softening of the matrix and the onset of optical clarity. Second, CLSM results indicate that the number of PDMS-rich domains decreases upon heating, while their average size remains relatively unchanged (Fig. 5(a)), suggesting partial coalescence or redistribution.^51,52 Third, SAXS analysis shows that domain ordering is enhanced after heating, as evidenced by stronger scattering peaks and decreased low-q intensity (Fig. S7), reflecting reduced structural heterogeneity. Forth, the critical micelle concentration (CMC) of the surfactant Tween 80 decreases with temperature, facilitating more stable and complete domain formation at elevated temperatures. These changes diminish the refractive index gradient between the domains and the surrounding PAM matrix, thereby reducing light scattering and leading to higher overall transparency. This process is partially reversible, and the hydrogel maintains stable optical switching behavior even after three cycles of alternating low and high temperatures (Fig. 5(b)). However, after multiple cycles, the high-temperature transmittance gradually declines (Fig. S8), likely due to irreversible rearrangement or aggregation of PDMS domains under repeated thermal stimulation, weakening refractive index matching and optical contrast.

Fig. 5 Reverse thermoresponsive behavior of the PAM@PDMS hydrogel. (a) CLSM images of the PAM@PDMS hydrogel before and after heating. (b) UV-vis transmittance spectra of the PAM@PDMS hydrogel at 20 °C and 60 °C. (c) Demonstration of information concealment and thermal decryption using the PAM@PDMS hydrogel for text, image, and QR code masking. (d) Visualization of an emergency fire exit sign triggered by temperature-induced transparency switching.

While our hydrogel exhibits an opaque-to-transparent transition upon heating, resembling the macroscopic optical behavior of UCST-type hydrogels, the underlying mechanism and functional advantages are notably different. Unlike conventional UCST systems that often undergo significant volume changes and require complex supramolecular designs,^53–55 our hydrogel achieves optical switching without macroscopic deformation via a simple one-pot emulsion strategy. Moreover, it integrates thermal responsiveness with mechanical robustness, strong adhesion, water retention, and strain sensing—offering a multifunctional platform rarely realized in UCST hydrogels. These distinctive features not only set it apart mechanistically but also extend its relevance to practical applications.

Building on these features, the PAM@PDMS hydrogel demonstrates strong potential across a range of high-value applications. Benefiting from its temperature-driven, reversible optical modulation, the PAM@PDMS hydrogel holds great promise in a variety of high-value application scenarios. Without the need for chemical additives, this material enables rapid, reversible, and high-contrast information display and concealment solely via thermal stimulation. For example, it can be used to cover text, image, and QR code, remaining opaque under ambient conditions while revealing the concealed information upon heating (Fig. 5(c)), offering a simple and efficient solution for thermal anti-counterfeiting, identity authentication, and data encryption. In addition, the hydrogel demonstrates remarkable advantages in emergency visual signaling. In fire-related high-temperature environments, the hydrogel rapidly increases in transparency, instantly exposing previously obscured emergency exits or evacuation indicators (Fig. 5(d)), providing a more practically aligned response direction compared to conventional thermoresponsive materials. Importantly, while the hydrogel experiences a moderate decrease in stretchability upon heating (elongation reduced from ∼6000% to ∼1000%), it exhibits an enhancement in mechanical strength (from ∼0.35 MPa to ∼0.45 MPa) (Fig. S9), which remains sufficient to meet the practical requirements of flexible and stretchable deployment. This change likely stems from temperature-induced rearrangement and densification of the internal PDMS-rich domains, leading to restricted chain mobility and higher resistance to deformation. Thus, the PAM@PDMS hydrogel not only features excellent reverse thermoresponsiveness but also integrates ultrahigh stretchability, strong interfacial adhesion, and superior water retention capacity, forming a multifunctional and adaptable soft material system. These comprehensive properties make it a promising candidate for integration into smart building systems, wearable thermal-responsive devices, and military camouflage interfaces, enabling the development of next-generation intelligent materials with thermally triggered optical switching capabilities.

2.5 Sensing performance of PAM@PDMS hydrogel

Building upon its excellent mechanical properties, reverse thermoresponsive behavior, broad-spectrum interfacial adhesion, and strong water retention capability, the PAM@PDMS hydrogel gains ionic conductivity through the incorporation of 3 g NaCl, enabling strain-sensing functionality. This modification offers a new route for dynamic deformation monitoring in applications such as information encryption, smart displays, and flexible optical devices. Fig. 6(a) presents the relative resistance change (ΔR/R₀) of the NaCl-doped PAM@PDMS hydrogel under different tensile strains. The curve can be divided into three linear regions, with corresponding gauge factors (GF) of 1.93 (0–200%), 5.8 (200–400%), and 5.2 (400–1000%), respectively. These results demonstrate a stable and sensitive strain–resistance response across a wide deformation range. A comparative analysis with various previously reported conductive hydrogels (Fig. 6(b)) shows that this system maintains a relatively high GF under large strains, reflecting its superior combination of sensitivity and working range. To evaluate its performance under repeated strain, Fig. 6(c) shows the ΔR/R₀ response under cyclic loading at maximum strains of 100%, 200%, 250%, and 300%. In all cases, the output signals exhibit clear and periodic waveforms, indicating good repeatability and reliable strain recognition. The dynamic response behavior is further assessed in Fig. 6(d) and (e), which reveal that the hydrogel sensor achieves both loading and unloading response times within approximately 1.2 seconds. The symmetry and rapidity of the response process satisfy the temporal resolution requirements for real-time monitoring in practical scenarios.

Fig. 6 Sensing performance of the PAM@PDMS hydrogel. (a) Relative resistance (ΔR/R₀)–strain curves and corresponding gauge factor analysis of the PAM@PDMS hydrogel. (b) Comparison of gauge factors between the PAM@PDMS hydrogel and previously reported conductive hydrogels.^56–62 (c) ΔR/R₀ variations of the PAM@PDMS hydrogel under different strain levels. (d) Response and recovery times during cyclic loading and unloading. (e)–(i) Real-time ΔR/R₀ responses of the PAM@PDMS hydrogel adhered to different body parts: (e) arm, (f) hand, (g) knee, (h) throat, and (i) ankle.

To verify its sensing capabilities under complex, real-world dynamic conditions, the NaCl-doped PAM@PDMS hydrogel is attached to various parts of the human body for motion detection. As shown in Fig. 6(f)–(i), the sensor successfully captures electrical signals corresponding to arm swings, finger shaking, knee flexion during cycling, facial muscle movement during laughing, and leg movement during running. In all cases, the hydrogel delivers consistent and distinguishable signals with patterns that closely match the physical actions, demonstrating its strong ability to sense and respond to diverse motion states in non-ideal and irregular environments. In summary, the NaCl-doped PAM@PDMS hydrogel exhibits high sensitivity, a wide sensing range, excellent repeatability, and fast response times. These characteristics provide reliable support for structural monitoring in smart display terminals, dynamic encryption interfaces, and flexible electronics, highlighting the hydrogel's potential as a multifunctional intelligent sensing material.

3. Conclusion

In summary, we present a structurally simple yet functionally integrated hydrogel system—PAM@PDMS—fabricated via a one-pot micellar copolymerization strategy that covalently embeds PDMS-rich domains within a polyacrylamide matrix. The resulting hydrogel exhibits a rare combination of properties, including ultrahigh stretchability (up to 5680%), exceptional fracture toughness (5.8 MJ m⁻³), and superior fatigue resistance, enabled by sequential energy dissipation through PAM–PDMS chain unfolding and domain deformation. The material uniquely displays reverse thermoresponsive optical behavior, shifting from opaque to transparent upon heating, which enables rapid and reversible information encryption and thermal display applications. Simultaneously, the hydrophobic PDMS domains enhance both moisture retention and broad-spectrum adhesion to diverse substrates. With additional NaCl doping, the hydrogel achieves reliable and sensitive strain sensing with high gauge factors and fast response times. This work offers a versatile design platform for multifunctional smart hydrogels, offering promising potential for next-generation encrypted optical systems, soft electronic devices, and wearable sensing technologies.

4. Experimental section

4.1 Materials

Acrylamide (AM) was obtained from Chengdu Huaxia Reagent Factory (China). The addition-type polydimethylsiloxane (PDMS, Sylgard 184) prepolymer was purchased from Dow Corning (USA), including part A (vinyl-terminated PDMS prepolymer as curing agent) and part B (Si–H structured PDMS prepolymer and a platinum catalyst). The condensation-type PDMS (DOWSIL 3140 RTV Coating) was also sourced from Dow Corning (USA). N,N′-Methylene bisacrylamide (MBA), 2-hydroxy-2-methylpropiophenone (1173), polysorbate 80 (Tween80), and sodium chloride (NaCl) were sourced from Admas-beta (China). Ultrapure water used in this study was produced by a UPW-10NT water purification system (China).

4.2 Preparation of PAM@PDMS hydrogels

The aqueous precursor was prepared by dissolving 6.9 mol L⁻¹ of AM and 0.04 mol L⁻¹ of MBA in ultrapure water. For the oil phase, PDMS components A and B were mixed at a weight ratio of 10 : 1 (with a PDMS-to-AM mass ratio of 3 : 1), followed by the addition of the emulsifier Tween 80. The aqueous and oil phase precursors were then thoroughly mixed, and 0.02 mol L⁻¹ of the photoinitiator 1173 was added until fully dissolved. The resulting emulsion was irradiated under UV light (250 W) for 5 minutes to obtain a series of PAM@PDMS hydrogels. Unless otherwise specified, the standard formulation of PAM@PDMS hydrogel consists of 6.9 mol L⁻¹ AM, PDMS at a mass ratio of 3 : 1 relative to AM, and 0.24 mol L⁻¹ Tween 80. For comparison, a pure PAM hydrogel was prepared by replacing PDMS with an equivalent mass of AM, while keeping all other components and procedures unchanged. Similarly, a pure PDMS elastomer was prepared by replacing 6.9 mol L⁻¹ AM with PDMS, following the same protocol.

4.3 UV-vis spectroscopy tests

The transmittance of the PAM hydrogel, PDMS elastomer, and PAM@PDMS hydrogel at different temperatures was measured using a Lambda950 UV-Vis spectrophotometer (USA) over a wavelength range of 400–800 nm.

4.4 Raman tests

The phase-separated structure of the PAM@PDMS hydrogel was analyzed using a LabRAM HR Evolution Raman spectrometer (Horiba, France) over the range of 600–1000 cm⁻¹. The characteristic peak at ∼851 cm⁻¹, corresponding to Si–CH₃ vibrations in PDMS, was used for Raman mapping to visualize the spatial distribution of PDMS domains within the hydrogel. The sample was pre-treated by freeze-drying.

4.5 SEM tests

The micromorphology and elemental distribution of the PAM hydrogel, PDMS elastomer, and PAM@PDMS hydrogel were characterized using a Carl Zeiss EV0-18 scanning electron microscope (SEM, Germany) coupled with an X-MAX50MM2 energy dispersive spectrometer (EDS, UK). Before observation, the samples were freeze-dried and then fractured under liquid nitrogen to expose a cross-sectional surface. SEM imaging was conducted at an accelerating voltage of 20 kV with a working distance of 15 mm. Elemental distribution across the samples’ surface was mapped using the EDS detector.

4.6 XPS tests

Elemental characterization of the PAM@PDMS hydrogel samples before and after toluene soaking was performed using an ESCA-LAB Xi X-ray photoelectron spectrometer (USA) operated at 40 kV and 40 mA. All samples were freeze-dried before measurement.

4.7 TOF-SIMS tests

A PHI Nano TOF time-of-flight secondary ion mass spectrometer (Japan) was used to characterize the internal chemical bonding structure of the PAM@PDMS hydrogel. The sample was pre-treated by freeze-drying and then bombarded with a Bi³⁺ ion beam at an acceleration voltage of 25 kV and an average pulse current of 0.3 pA. The analysis area was 200 μm × 200 μm. A PHI Nano TOF time-of-flight secondary ion mass spectrometer (Japan) was used to characterize the internal chemical bonding structure of the PAM hydrogel and PDMS.

4.8 High-resolution mass spectrometry (HRMS) test

Mass spectrum of Tween 80 was analyzed using a high-resolution mass spectrometer (Xevo G2-XS QTOF, Waters, USA).

4.9 FTIR tests

The molecular structure of the PAM hydrogel, PDMS elastomer, and PAM@PDMS hydrogel was characterized using a VERTEX 70 Fourier transform infrared spectrometer (USA) over a wavelength range of 1000–4000 cm⁻¹. The analysis was conducted in ATR (attenuated total reflection) mode.

4.10 Mechanical properties tests

The mechanical properties of pure PAM hydrogel, pure PDMS elastomer, and PAM@PDMS hydrogel samples with varying contents of AM, PDMS, and Tween 80 were evaluated using an Instron 3367 universal tensile testing machine (USA). The PAM@PDMS hydrogel samples were tested at 20 °C and 60 °C. The samples were prepared in a dumbbell shape with a gauge length of 20 mm, width of 4 mm, and thickness of 2 mm. Unless otherwise specified, the tensile test was conducted at a rate of 100 mm min⁻¹. To minimize experimental error and ensure data reliability, three parallel samples were tested for each group. The average values and standard deviations were calculated and reported. Statistical analysis was performed using OriginPro (OriginLab) software. Unless otherwise noted, data were expressed as mean ± standard deviation (SD).

Single-cycle tensile tests were performed to determine the fracture strain, tensile strength, toughness, and elastic modulus of the samples. To assess energy dissipation behavior at different strain levels, cyclic tensile loading–unloading tests were conducted under maximum strains of 50%, 500%, 1000%, 1500%, 2000%, and 2500%. In addition, fatigue resistance and elastic deformation behavior were evaluated by subjecting the PAM@PDMS hydrogel to 500 cycles of tensile loading–unloading at a maximum strain of 50% and a strain rate of 100 mm min⁻¹. To investigate the strain rate dependence of elastic modulus, the PAM hydrogel, PDMS elastomer, and PAM@PDMS hydrogel were stretched to failure at different tensile rates (10, 50, and 100 mm min⁻¹). For each tensile rate, the elastic modulus was calculated based on three independent parallel experiments.

4.11 DSC tests

Tg of the PAM hydrogel, PDMS elastomer, and PAM@PDMS hydrogel were determined using a Differential Scanning Calorimeter (DSC Q100, TA Instruments, USA). Samples were heated under a nitrogen atmosphere at a constant rate, and the T_g was identified from the midpoint of the baseline shift in the DSC curve.

4.12 Cloud point temperature test

The cloud point temperature of the PAM@PDMS hydrogel was evaluated using a temperature-controlled hot stage with a heating rate of 1 °C min⁻¹. The transition temperature was visually determined as the point at which the hydrogel changed from opaque to transparent during heating.

4.13 SAXS tests

To investigate the internal structural changes induced by temperature, SAXS measurements were performed on the PAM@PDMS hydrogel before and after heating using a Xeuss 2.0 SAXS/WAXS system (Xenocs, France).

4.14 Reserve temperature—sensitive tests

The PAM@PDMS hydrogel samples were fabricated into circular sheets with a diameter of 3 cm. At room temperature (20 °C), the hydrogel was placed over printed elements including text, QR code, graphical pattern, and an “EXIT” emergency sign to evaluate its ability to conceal visual information. The samples were then heated to 60 °C for 3 minutes to trigger a reversible transparency transition and uncover the concealed content.

4.15 LSCM tests

The structural evolution of PDMS domains within the PAM@PDMS hydrogel during tensile deformation and at different temperatures was investigated using a Nikon A1R MP+ two-photon laser scanning confocal microscope (Japan). In situ LSCM imaging under various strain levels was performed with a custom-built micro-tensile device integrated into the microscope system. To visualize the PDMS-rich domains, a hydrophobic fluorescent dye (1,4-bis(2-cyanostyryl)benzene) was employed, which selectively binds to the domains via hydrophobic interactions.

4.16 Adhesion performance tests

Adhesion performance was evaluated using lap shear testing on the Instron 3367 tensile testing machine to characterize the adhesion strength of the PAM@PDMS hydrogel to various surfaces, including aluminum, PMMA, PVC, rubber, glass, pigskin, PAM hydrogel, PDMS elastomer, and NW fiber. The samples were prepared in rectangular form with dimensions of 25 mm in length, 20 mm in width, and 2 mm in thickness. For each substrate, three parallel tests were performed to obtain average values and standard deviations.

4.17 Adhesion performance tests in aqueous environments

The adhesion performance of the PAM@PDMS hydrogel under aqueous conditions was evaluated using lap shear tests conducted on an Instron 3367 universal testing machine. The hydrogel's adhesion strength was assessed against a range of substrates, including Al, PMMA, PVC, rubber, glass, pigskin, PAM hydrogel, PDMS elastomer, and NW fiber. To simulate a fully hydrated environment, all bonding interfaces were completely submerged in deionized water during testing. Rectangular hydrogel samples (25 mm × 20 mm × 2 mm) were prepared for each test. For each substrate, three replicate measurements were conducted to determine the average adhesion strength and standard deviation.

4.18 Water retention tests

The water retention performance of PAM@PDMS hydrogel samples with varying PDMS content was evaluated using a gravimetric method. Test samples were prepared in rectangular sheet form with dimensions of 4 cm × 2 cm and a thickness of 2 mm. The samples were exposed to ambient conditions (approximately 25 °C and 65% relative humidity), and their weight was recorded at 12-hour intervals. The water retention ratio was calculated using the equation:

Water retention (%) = (W_t/W₀) × 100,

where W₀ is the initial weight and W_t is the weight at a given time t. For each sample condition, three parallel measurements were conducted to obtain average values and standard deviations.

4.19 Sensing performance tests

To endow electrical conductivity, 3 g of NaCl was added to the PAM@PDMS hydrogel. The sensing performance of the NaCl-doped PAM@PDMS hydrogel was evaluated using a Keithley 2450 Source Meter (USA) integrated with an Instron 3367 universal tensile testing machine. Strip-shaped samples were prepared with a length of L = 20 mm and a cross-sectional area of 4 mm × 1 mm. Both ends of the sample were affixed with 0.1 mm thick double-sided conductive copper foil, to which electrodes were connected for resistance monitoring. During tensile deformation, the real-time electrical resistance was recorded to calculate the relative resistance change (ΔR/R₀), gauge factor (GF), and response time. Additionally, the hydrogel's sensitivity to various human motions was assessed by tracking resistance changes under different physiological activities.

Author contributions

Qianqian Liang: conceptualization, data curation, formal analysis, investigation, validation, writing – original draft, writing – review & editing, Wanting Yuan: data curation, investigation, writing – original draft, Yi He: data curation, formal analysis, Ziqi Wang: data curation, formal analysis, Jinrong Wu: conceptualization, supervision, writing – review & editing, Lijuan Zhao: funding acquisition, project administration, supervision, Yi Wang: conceptualization, funding acquisition, project administration, resources, supervision, writing – review & editing.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data that support the findings of this study are available in the SI of this article.

Additional figures supporting the main text, including droplet size distribution, comparative images of different PDMS systems, mass spectrum of Tween 80, mechanical tests under varied conditions, thermal analysis, SAXS characterizations, and UV-vis transmittance spectra at different temperatures. See DOI: https://doi.org/10.1039/d5mh01141h

Acknowledgements

This work was supported by National Natural Science Foundation of China (52203013), National Key Research and Development Program of China (2022YFF0904000), Sichuan Science and Technology Program (2024NSFSC1030, 2023YFS0015), Key Laboratory of the Evaluation and Monitoring of Southwest Land Resources (Ministry of Education) (TDSYS202414), and China Scholarship Council Program (202408510215).

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