One-step 3D printing of flexible poly(acrylamide-co-acrylic acid) hydrogels for enhanced mechanical and electrical performance in wearable strain sensors | Scientific Reports

Apr 08, 2025

Scientific Reports volume 15, Article number: 11900 (2025) Cite this article

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This study explored the synthesis and 3D printing of an electrolytic hydrogel based on polyacrylamide and acrylic acid copolymer (poly(AM-co-AA)), using lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) as a photoinitiator, along with N,N′-Methylene bisacrylamide (MBA) and sodium alginate (SA) for crosslinking. The hydrogel matrix, incorporated with electrolyte fillers, including sodium chloride (NaCl), calcium chloride dihydrate (CaCl2·2H2O), and aluminum trichloride hexahydrate (AlCl3·6H2O), was fabricated via a one-step approach and printed with an LCD-3D printer, yielding a porous structure with remarkable water absorption capacity and tailored mechanical properties. Scanning electron microscopy (SEM) analysis of the NaCl electrolyte poly(AM-co-AA) hydrogel revealed a highly porous surface structure, contributing to a remarkable water absorption capacity exceeding 800%. The mechanical and electrical properties of this 3D-printed hydrogel were found to be intermediate between those of MBA crosslinked poly(AM-co-AA) and MBA crosslinked poly(AM-co-AA) with SA. This hydrogel exhibited efficient conductivity and flexibility, making it well-suited for potential use in strain sensors and wearable devices, enabling real-time monitoring of human activities, such as finger bending.

Additive manufacturing processes, commonly known as 3D printing (3DP), enable the fabrication of lighter, more complex, and substantial parts. These processes utilize computer-aided designs to directly deposit material layer by layer, creating a three-dimensional shape1,2. Vat photopolymerization (VP) is a 3D printing process that utilizes a photopolymer resin cured layer by layer using UV light or a laser. This technique enables the precise formation of 3D objects with highly detailed structures3. VP is classified into several techniques, including Stereolithography (SLA), Digital Light Processing (DLP), Two-Photon Polymerization (TPP), Continuous Liquid Interface Production (CLIP), and Liquid Crystal Display (LCD)4,5. The LCD technique is the simplest and most affordable VP method, using only a liquid crystal display to irradiate the resin layer with UV light, with the display itself acting as a mask generator. Each pixel on the screen is a small cell containing molecules in a liquid-crystal state. To form a mask, each pixel can be set to either a transparent (emitting) or opaque (non-emitting) state by changing the orientation of the molecules. The resin in an LCD printer is in direct contact with the display, allowing the printed pixels to align with the shape of the beam profile. Moreover, there is only one moving mechanical part in the LCD printer—the motorized z-positioning stage—eliminating the need to focus the laser beam precisely on the resin’s surface. Despite these clear advantages, LCD technology has yet to be widely adopted for material printing due to the challenge of lower pixel density compared to the digital micromirror devices used in DLP5,6.

Hydrogels are widely used in fields such as electronics, strain sensing, drug delivery, adsorption, and more7,8,9,10. Wearable strain sensors, in particular, undergo repeated tensile and bending deformations during use, demanding that hydrogel-based strain sensors exhibit high mechanical strength and flexibility11,12. Recently, 3D printing technologies have enable the precise fabrication of hydrogels, leveraging their biocompatibility, flexibility, and responsiveness to stimuli for applications in biomedical devices, soft robotics, and flexible electronics13,14. Particularly, LCD-3D printing technique allows for the rapid and efficient fabrication of hydrogel in complex three-dimensional shapes15,16,17. Compared to traditional casting methods, LCD-3D printing offers a more efficient and versatile approach for hydrogel preparation. However, conventional hydrogels often suffer from limitations, such as limited stretchability and low resilience, which restrict their use in wearable devices. To advance flexible wearable sensors, it is essential to develop conductive hydrogels that exhibit both high stretchability and strong resilience. Conductive hydrogels, widely used in applications such as tensile sensors and artificial soft tissues, require robust mechanical properties to withstand substantial loads and resist fractures. Therefore, improving the mechanical strength of hydrogel-based wearable strain sensor hold significant practical value18. A widely used approach for strengthening hydrogels involves constructing double-network and dual ionic crosslink network structures, which significantly improve their mechanical properties7,19. 3D printing technology has significantly expanded the potential for fabricating hydrogel sensors with complex geometries and unique features that are challenging or impossible to achieve using traditional manufacturing techniques. 3D printing enables the creation of sensors with enhanced sensitivity, faster response times, and greater detection accuracy. Notably, 3D-printed hydrogel sensors are gaining widespread attention due to their ability to integrate customizable geometries with enhanced functional properties. Additionally, 3D printing facilitates the integration of multiple materials or functional components, further enhancing their versatility. These attributes could make 3D-printed hydrogel sensors particularly well-suited for applications in wearable technology and environmental monitoring20,21.

Polyacrylamide, recognized as a green material, shows strong potential as a basis for developing gel polymer electrolyte materials. When combined with acrylic acid, it forms a copolymer, polyacrylamide and acrylic acid (poly(AM-co-AA)). In this formulation, N, N′-methylene bisacrylamide (MBA) and sodium alginate (SA) serve as chemical and physical crosslinkers, respectively. The photoinitiator lithium phenyl-2,4,6 trimethylbenzoylphosphinate (LAP) facilitates the curing process when printed with an LCD-3D printer.

This research employs a one-step synthesis method to create MBA crosslinked poly(AM-co-AA) hydrogels, incorporating SA and electrolyte fillers such as sodium chloride (NaCl), calcium chloride dihydrate (CaCl2·2H2O), and aluminum trichloride hexahydrate (AlCl3·6 H₂O) to enhance the electrolyte properties. The resulting electrolyte hydrogels undergo electrical and mechanical testing to assess their potential as strain sensors. Additionally, real-time tests, such as finger bending, are conducted to evaluate their responsiveness in monitoring human activities, highlighting the hydrogels’ suitability for wearable sensing applications.

Acrylamides (AM), N,N′-methylenebisacrylamide (MBA), and lithium phenyl-2,4,6 trimethylbenzoylphosphinate (LAP) were purchased from Sigma-Aldrich. Acrylic acid (AA) and sodium alginate (SA) were purchased from Loba chemie PVT LTD. Sodium chloride (NaCl), calcium chloride dihydrate (CaCl2·2H2O), and aluminum trichloride hexahydrate (AlCl3·6H2O) were purchased from Kemaus. Deionized water (DI water) was used to prepare the conductive electrolyte hydrogel in the experiments.

The poly(AM-co-AA) hydrogel was prepared using a one-pot method, as outlined in Table 1; Fig. 1. Acrylamide and acrylic acid (in a 95:5 ratio) were mixed in deionized water at room temperature, achieving a total solid content (TSC) of 20 wt% for 15 min. Following this, 0.25 wt% of MBA was added to the mixture, which was stirred continuously for an additional 15 min. Next, sodium acrylate (SA) was introduced at 1 wt% of TSC, along with various electrolytes (NaCl, CaCl₂, and Al₂Cl₃), maintaining a fixed electrolyte content of 5 wt% in the mixture for 30 min. Finally, 0.25 wt% of LAP was added to the electrolyte mixture, which was mixed for another 15 min to obtain an electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel suitable for 3D-LCD printing.

An electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel was printed using the Phrozen Sonic Mini 4 K Resin 3D Printer (PHROZEN TECH CO., LTD., Taiwan), a bottom-up LCD printer. The UV light wavelength is 405 nm (ParaLED Matrix 2.0) with a UV lamp power of 40 watts. The hydrogel mixture was poured into the printer tray, and printing commenced with parameters set to a layer height of 0.05 μm, curing time of 45 s, and printing speed of 60 mm/min. After printing, the samples were washed with DI water to remove any residual compounds to obtain 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel.

Schematics of the 3D printed electrolyte MBA crosslinked poly(AM-co-AA) hydrogel. This figure was created by the authors.

The chemical structure of electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel sample was measured by Thermo Fisher Scientific iZ10 FTIR spectrometer with attenuated total reflectance (ATR) transmission mode (Thermo Fisher Scientific Inc., USA) in wavenumber range from 4000 to 500 cm− 1 at a resolution of 4 cm− 1 over 32 scans.

The internal structure and morphology of 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel was observed using SEM. The hydrogel sample was immersed in liquid nitrogen for brittle fracture and stored immediately in a refrigerator after freeze-drying by CHRIST Alpha1-2 LDplus freeze dryer (Sigma Laborzentrifugen GmbH, Germany) for 48 h. The freeze-drying hydrogel was sputter coated with gold. The morphology of hydrogel fracture was observed by SEM microscope (Hitachi SU3500-Horiba X-maxN, Hitachi High-Technologies Corporation, Japan).

The 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel samples, each measuring 10 × 10 × 2 mm3, were prepared using a 3D-LCD printer. All samples were first dried to a constant weight (W₀). The dried samples were then extracted in 2000 ml of DI water until a constant weight was achieved, after which they were dried at 60 ºC to a constant weight (W₁)22. The gel content was calculated using the following equation:

All samples of 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel (10 × 10 × 2 mm3) were dried to a constant weight (Wdried). The dried hydrogels were then immersed in DI water at room temperature for 24 h. The swollen hydrogels were removed from the DI water, and excess water on the surface of the swollen hydrogels was blotted with filter paper. Subsequently, the swollen hydrogels were weighed (Wswollen). Water absorption was determined using the following Eq. (2):

The water absorption experiment was performed in triplicate, and the average value was calculated23.

The mechanical properties of 3D-printed hydrogel samples were tested following ASTM D638. All samples were printed as dumbbell-shaped specimens (ASTM D638 Type V) using a 3D-LCD printer. The Universal Testing Machine (ProLine-ZwickRoell GmbH & Co. KG, Germany) was equipped with a 500 N load cell, and the crosshead distance was set to 30 mm with a testing speed of 50 mm/min. Tensile strength, elongation at break, and Young’s modulus of the hydrogel were calculated as averages of five samples for each hydrogel material formulation. Young’s modulus was determined from the stress-strain curve as the slope at 10% strain, and the area under the stress-strain curve was integrated to obtain the toughness value.

A potentiostat/galvanostat/impedance analyzer by PSTrace PalmSens4 (PalmSens BV, Netherlands) was used to measure the electrical properties of a 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogels.

For strain sensing, 3D-printed hydrogels were created as dumbbell-shaped specimens (ASTM D638 Type V). These hydrogels were tested using a universal testing machine that applies tension to the specimen at a speed of 50 mm/min. The recorded signals were analyzed in real-time with a PSTrace PalmSens4, using two adjoining electrodes under a constant AC voltage of 10 V. The relative resistance change was calculated using Eq. (3). This experiment demonstrates that the advantage of 3D printing is its ability to customize wearable conductive hydrogels.

Additionally, 3D-printed hydrogel samples were printed in a rectangular shape of.

12 × 90 × 2 mm3 to test electrical conductivity under a constant AC voltage of 10 V.

To monitor changes in electrical resistance during finger movements, hydrogel samples of the same shape were also tested under a constant AC voltage of 3 V and 10 V, respectively. The signals were recorded with a PSTrace PalmSens4 and analyzed using two adjoining electrodes. The electrical conductivity and the relative resistance change were calculated using Eqs. (3) and (4).

where R0 and R were the resistance without and with applied strain, respectively.

where d represented the length between adjacent electrodes. R and A were the resistance and the cross-sectional area of hydrogel samples. respectively.

The appearance of the 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel is shown in Fig. 2A. Before the addition of electrolyte fillers, the samples labeled as 0SA and 1SA exhibit a transparent appearance. Upon incorporating electrolyte fillers, a uniform and well-distributed dispersion of electrolyte fillers is observed, as illustrated in Fig. 2A and further supported by Fig. S1. With LCD-3D printing technique, MBA crosslinked poly(AM-co-AA) hydrogel can be fabricated into intricate designs, such as a dot-patterned sheet, as depicted in Fig. 2B. Additionally, the hydrogel was printed into a rectangular shape with dimensions of 12  ×  90  ×  2 mm3 and approximate mass of 1.60 g to demonstrate its ability to undergo various deformations, including stretching, twisting, and knotting, as illustrated in Fig. 2C. As a result, the 1SA_5NaCl hydrogel exhibited strong adhesive properties to a range of substrates, including glass, ceramic, metal, silicone, polyethylene covers, PVC sheets, nitrile gloves, and skin. The hydrogel’s load-bearing capacity was also impressive. By maintaining the same rectangular dimensions of 12 × 90 × 2 mm³, it successfully carried weights of 10 g, 50 g, and withstanding up to 100 g, as shown in Fig. 2D and E.

Photographs of (A) the appearance of a 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogels, (B) 3D-printed 1SA_5NaCl dot-patterned hydrogel sheet (C) its mechanical performance under different types of deformation, (D) adhesion differences on various substrates, and (E) loading of the 1SA_5NaCl hydrogel with different weights.

As illustrated in Fig. 2D and E, the adhesion strength of MBA crosslinked poly (AM-co-AA) hydrogels is influenced by key factors, including tensile strength, elongation at break, and the degree of crosslinking. Typically, higher tensile strength improves adhesion, as it helps maintain structural integrity under stress. Toughness, represented by the area under the stress-strain curve, is another critical factor. Increased toughness enhances the hydrogel’s resistance to deformation, thereby improving adhesion performance. The degree of crosslinking also plays a pivotal role, affecting both mechanical strength and adhesion. While higher crosslinking density typically improves tensile properties, it can reduce adhesion by limiting polymer segment mobility and reducing the availability of functional groups required for bonding. Thus, optimizing adhesion strength involves balancing crosslinking density with mechanical properties such as tensile strength and toughness24,25,26.

The morphology of the 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel cross-section—specifically, hydrogel samples without SA (0SA), with 1 wt% SA (1SA), and with 5 wt% NaCl (1SA_5NaCl)—was analyzed using SEM after freeze-drying the samples at -80 ºC for 48 h. The results are presented in Fig. 3.

SEM micrographs of the cross-section surface of the hydrogel: (A) 0SA, (C) 1SA, (E) 1SA_5NaCl at 500x magnification, and (B) 0SA, (D) 1SA, (F) 1SA_5NaCl at 1000x magnification.

As shown in Figs. 3A and B, the SEM micrograph of 0SA hydrogel revealed larger pore sizes and thinner pore walls, which unfortunately result in higher electrical resistance under external loads10,27,28. The monomers AM and AA undergo radical polymerization under UV irradiation to form a gel network. A significant number of hydrogen bonds are established between the amine groups of AM and the carboxyl groups of AA. This combination of hydrogen bonding and covalent bonding leads to the long chains of MBA crosslinked poly(AM-co-AA) interweaving to create a densely entangled physical network. In addition to covalent crosslinking, hydrogen bonds are also formed between the polymer units and water molecules. The surface of the hydrogel samples possesses a highly porous structure with numerous pores and holes. The open pores observed in the swollen hydrogel serve as effective reservoirs for water and fluids, provided a mechanism exists for transporting water into these pores. The porous structural properties of the hydrogel result in a larger specific surface area, which enhances solution uptake and facilitates interactions between the hydrogel and the solution.

When 1 wt% SA is added, the SEM micrographs shown in Fig. 3C and D reveal the formation of coordinate (covalent) bonds between Na+ in SA and the carboxyl groups within the poly(AM-co-AA) network. This interaction creates additional independent physical crosslinking. These coordinate bonds influence the molecular segments, reducing the density of the three-dimensional network of the hydrogel. The SA network is linked by weak hydrogen bonds between SA molecules, while the MBA crosslinked poly(AM-co-AA) network is crosslinked by physical entanglements. The pore sizes in the SA-containing hydrogel are smaller compared to those in the MBA crosslinked poly(AM-co-AA) hydrogel. This difference arises because the MBA crosslinked poly(AM-co-AA) and SA hydrogel contains two distinct networks: the MBA crosslinked poly(AM-co-AA) network, formed through physical entanglements, and the SA network, formed via hydrogen bonds28.

The addition of 5 wt% NaCl to the MBA crosslinked poly(AM-co-AA) hydrogel containing 1 wt% SA (designated as 1SA_5NaCl) results in a distinct morphological transformation. Strong crosslinking interactions occur between Na + and the carboxyl (C = O) groups of AM, AA, MBA, and SA, as well as between the hydroxyl or amino groups in SA and MBA27. This leads to a reduction in pore size and a significant increase in pore wall thickness. The presence of NaCl facilitates ionic interactions between Na + and the -COO- groups, enhancing the formation of physical crosslinking points within the hydrogel11,29. Additionally, the incorporation of Na + introduces coordinate bonds, leading to improved mechanical properties of the polymer network through ionic coordination and physical interactions30.

The characteristics of the electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel samples were confirmed by FTIR spectroscopy. Figure 4 presents the FTIR spectra of three samples: 0SA (without SA), 1SA (containing 1 wt% SA), and 1SA_5NaCl (containing 1 wt% SA and 5 wt% NaCl).

FTIR spectra of electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel.

In Fig. 4A, the absorption bands at 3346 and 3197 cm−1 correspond to O-H and N-H stretching vibrations, respectively31,32,33. The peaks at 1662 and 1620 cm⁻¹ are attributed to C=O stretching vibrations and N–H bending vibrations, respectively6,34,35.

In Fig. 4B, the –CH₂– bending vibrations are observed at 1454 and 1356 cm−1, while C–N stretching gives an absorption peak at 1432 cm−19 ,36,37,38. The peak at 1320 cm−1 is attributed to N-H stretching vibrations in AM and MBA39. The absorption peaks at 1284, 1242, and 1208 cm−1 are related to C-O stretching of the unsaturated carboxylic acid group in PAA40,41. The C–N stretching vibrations in AM and MBA are observed at 1163 cm⁻¹42. The peaks at 1113 cm−1 correspond to C–O stretching in the CH-OH structure of SA29. The characteristic absorption bands of SA at 1055 and 1032 cm−1 (stretching vibrations of C–OH groups) and C–O stretching in the C–O–C structure are noticeably weakened after the reaction29,43,44. The peak at 987 cm⁻¹ is attributed to the polymerization of acrylamide and acrylic acid with MBA45. The disappearance of =CH and =CH₂ out-of-plane vibrations in AM, AA, and MBA at 871 and 810 cm⁻1 is also noted46.

Figure 5 illustrates the gel content and water absorption capacity of the 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogels. As shown in Fig. 5A, all 3D-printed hydrogel samples exhibited gel contents exceeding 90%. The presence of SA and MBA enables both physical and chemical crosslinking reactions, resulting in a highly crosslinked hydrogel structure that is insoluble in water8,22. However, when electrolyte fillers were incorporated into the MBA crosslinked poly(AM-co-AA) hydrogels, the gel content decreased as the charge density of the electrolyte filler increased47.

Gel content (A) and water absorption (B) of 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel.

The water absorption behavior of the 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel was characterized by rapid swelling during the initial stage (5–120 min), reaching equilibrium after 240 min, as shown in Fig. 5B. During the initial stage, the 0SA hydrogel exhibited the highest water absorption, whereas the 1SA_5AlCl3 hydrogel showed the lowest water absorption. This trend suggests that the improved polymer network structure facilitates the formation of uniform voids for water retention. The presence of electrolyte particles within the hydrogel network structure polymer chain entanglement and reduced hydrogen bonding among hydrophilic groups, decreasing the degree of physical crosslinking and thereby increasing the swelling capacity. Upon exposure to the water, the electrolyte ions in the hydrogel network dissociated into cations and anions9,43,44,48. Moreover, the 3D-printed hydrogel contained numerous hydrophilic groups, such as –COO–, COOH, and NH2. Sodium carboxylate groups became protonated, strengthening hydrogen bonding interactions among –COOH (from acrylic acid) and –CONH2 (from acrylamide) groups. These interactions contributed to enhanced physical crosslinking within the hydrogel network22,31,49,50.

As the swelling time increased from 240 to 1,440 min, the 1SA_5NaCl hydrogel exhibited the highest water absorption, while the 1SA_5AlCl3 hydrogel showed the lowest, as depicted in Fig. 5B. The water absorption of the electrolyte hydrogels varied with different chloride salts in the order NaCl > CaCl2 > AlCl3. This trend indicates that water absorption decreases as the cationic charge increases, progressing from Na+ to Ca2+ to Al3+. The reduced water absorption in the presence of multivalent cations is likely due to their stronger complexation with the -COO- groups in the polymer matrix. The observed dependence of swelling behavior on ionic strength and ionic charge confirms the salt sensitivity of the prepared hydrogels42,47.

The 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogels were printed as dumbbell-shaped specimens using a 3D-LCD printer, as shown in Fig. 6A. These specimens were prepared with varying concentrations of SA (0% (0SA) and 1 wt% (1SA)). Additionally, different electrolytes were incorporated (NaCl (1SA_5NaCl), CaCl2 (1SA_CaCl2), and AlCl3 (1SA_5AlCl3)). The specimens were then subjected to tensile testing, with the results summarized in Table 2 and illustrated in Fig. 6.

Mechanical properties of 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogels.

Figure 6B shows the stress-strain curves for all 3D-printed hydrogel samples. The stress-strain curve for the 1SA sample exhibits the highest values among all tested samples, with a tensile strength of 454.33 ± 11.71 kPa. The introduction of Na+ into the hydrogel, which can cross-link with poly(AM-co-AA), further enhances its performance, improving both flexibility and tensile properties7,29. However, the incorporation of different electrolytes – classified as monovalent (Na+), divalent (Ca2+), and trivalent (Al3+) – into the 3D-printed hydrogels results in a notable reduction in their stress-strain behavior. As shown in Fig. 6C, the 1SA_NaCl hydrogel exhibits higher Young’s modulus and toughness compared to the 1SA_CaCl2 and 1SA_AlCl3 hydrogels. Excessive electrolytes can cause the hydrogel network to become uneven, significantly reducing tensile properties. The disruption of ionic cross-linking between Na+, Ca2+, and Al3+ with MBA crosslinked poly(AM-co-AA) and SA also results in some energy dissipation. The hydrogel’s ability to dissipate energy effectively is further supported by increased ionic coordination and hydrogen bonding, which promote restructuring and reorganization within the network7,19. Additionally, MBA enhances the hydrogel’s mechanical stability by forming covalent bonds with multiple polymer segments, thereby densifying and stabilizing the network structure28.

The mechanical properties of alginate hydrogels are significantly influenced by the type of ionic crosslinking agent used. Numerous crosslinking strategies using cations such as sodium, calcium, iron, barium, zinc, aluminum, and strontium have been extensively reported in the literature51,52,53,54,55.

The relative resistance of 3D-printed hydrogels was measured using tensile strain sensing under a constant AC voltage of 10 V, with a testing speed of 50 mm/min, as shown in Fig. 7A. The results indicated that the relative resistance increased with tensile strain. Notably, the resistance change rate of the 3D-printed MBA crosslinked poly(AM-co-AA) hydrogels exhibited a strong linear relationship across different strain ranges. Among the various electrolyte fillers tested (i.e., NaCl, CaCl2, AlCl3), the 1SA_CaCl2 sample displayed the highest slope of relative resistance change, while the 0SA sample had the lowest. This suggests that the interaction between the electrolyte filler and the MBA crosslinked poly(AM-co-AA) chains forms an effective conductive channel56. Due to the abundance of electrolyte ions, the synthesized stretchable hydrogels can be utilized as ionic hydrogel sensors with strain-sensitivity properties7,27.

The relative resistance changes of the 3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogels as tensile strain (A) and the electrical conductivity of 3D-printed hydrogels (B).

3D-printed hydrogel samples were tested for electrical conductivity under a constant AC voltage of 10 V, as shown in Fig. 7B. The electrical conductivity of the 1SA_5NaCl hydrogel is higher than that of the 1SA_5CaCl2 and 1SA_5AlCl3 hydrogels, which can be attributed to the diffusion of free ions within the hydrogel and the NaCl has enhanced freely movable ions in the hydrogel matrix7,37,54,57. Unsurprisingly, the conductivities of the 0SA and 1SA samples are lower than those of the electrolyte-conductive MBA crosslinked poly(AM-co-AA), as there are no free ions in those formulations.

3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogel for detecting human finger bending motion at varying speeds (fast and slow) and voltages (3 V and 10 V).

3D-printed electrolyte-conductive MBA crosslinked poly(AM-co-AA) hydrogels were tested to monitor changes in electrical resistance during finger movements, under constant AC voltages of 3 V and 10 V. The relative resistance of the 3D-printed hydrogel is shown in Fig. 8.

Figures 8A–E illustrates the relative resistance of 3D-printed hydrogels under constant AC voltages of 3 V. A bending angle of 90 degrees in finger joint flexion corresponds to a more significant change in the resistance response. When the finger returns to its initial position, the resistance immediately reverts to its original state, similar to the behavior observed under constant AC voltages of 10 V (cf. Figures 8F-J). The 1SA_5NaCl sample exhibited the highest signal of relative resistance signal when excessive deformation of the hydrogel sensor caused fluctuations in the shuttle interval. As a result, significant changes in resistance were observed under a constant AC voltage of 3 V58 (cf. Fig. 8C). Furthermore, Fig. 8H highlights the stability of the relative resistance signal when a constant AC voltage of 10 V was applied. Additionally, the speed of finger bending influences the stability of the relative resistance signal; faster-bending leads to a more stable signal compared to slower motions7,12,20,33,59.

The strain-sensing characteristics of 3D-printed MBA crosslinked poly(AM-co-AA) hydrogels offer significant advantages, making them promising materials for wearable flexible sensors in human applications. This is a major benefit for the future development of wearable electronic devices.

In this study, a transparent, electrically conductive MBA crosslinked poly(AM-co-AA) hydrogel was successfully fabricated using LCD-3D printing. LAP served as the photoinitiator to drive the polymerization of poly(AM-co-AA), with MBA and SA providing crosslinking, along with electrolyte fillers (NaCl, CaCl₂, and AlCl₃) to create dual networks composed of hydrogen bonds and covalent bonds between molecular chains that incorporate the electrolytes. The 3D-printed 1SA_5NaCl hydrogel demonstrated a significant water adsorption capacity of over 800%, influenced by NaCl incorporation, as confirmed by water absorption tests and SEM analysis. This hydrogel exhibited favorable mechanical and electrical properties, enabling precise monitoring of finger movements in strain sensor applications. These findings highlight the hydrogel’s potential for wearable soft strain sensors and health monitoring systems.

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy concerns.

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This research project is supported by National Research Council of Thailand (NRCT) and Chulalongkorn University. We also acknowledge the partial financial support from Thailand Science Research and Innovation Fund Chulalongkorn University. Phanthanyaphon Tsupphayakorn-aek would like to acknowledge the Second Century Fund (C2F) for the support.

Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok, Thailand

Phanthanyaphon Tsupphayakorn-aek & Chuanchom Aumnate

Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand

Nuttapol Risangud

Center of Excellence in Responsive Wearable Materials, Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok, Thailand

Manunya Okhawilai & Chuanchom Aumnate

Center of Excellence in Polymeric Materials for Medical Practice Devices, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand

Manunya Okhawilai

Faculty of Commerce and Accountancy, Chulalongkorn University, Bangkok, Thailand

Worapong Leewattanakit

Department of Mechanical Engineering, University of Wisconsin–Madison, Madison, WI, 53706, USA

Lih-Sheng Turng

Wisconsin Institute for Discovery, University of Wisconsin–Madison, Madison, WI, 53715, USA

Lih-Sheng Turng

Hub of Waste Management for Sustainable Development, Center of Excellence on Hazardous Substance Management, Chulalongkorn University, Bangkok, Thailand

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P.T.: Methodology, Formal analysis, Investigation, Writing—original draft, N.R.: Writing—review and editing, M.O.: Writing—review and editing, W.L.: Writing-review and editing, L.-S.T.: Writing-review and editing, C.A.: Conceptualization, Visualization, Writing—review and editing, Supervision.

Correspondence to Chuanchom Aumnate.

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Tsupphayakorn-aek, P., Risangud, N., Okhawilai, M. et al. One-step 3D printing of flexible poly(acrylamide-co-acrylic acid) hydrogels for enhanced mechanical and electrical performance in wearable strain sensors. Sci Rep 15, 11900 (2025). https://doi.org/10.1038/s41598-025-97120-1

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Received: 11 November 2024

Accepted: 02 April 2025

Published: 07 April 2025

DOI: https://doi.org/10.1038/s41598-025-97120-1

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