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Stimulus-responsive gradient hydrogel micro-actuators fabricated by two-photon polymerization-based 4D printing

  • Tongqing Li , Gary Chi-Pong Tsui EMAIL logo , Chi-Ho Wong , Chak-Yin Tang , Kai Tang and Youhua Tan
Published/Copyright: March 17, 2025
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Nanotechnology Reviews
From the journal Nanotechnology Reviews Volume 14 Issue 1

Abstract

The growing field of 4D printing has spurred extensive exploration into applications of stimulus-responsive materials, such as hydrogels for micro-actuators. However, the hydrogel-based micro-actuators fabricated by one-step, single-material printing are typically bilayer, and their actuation capabilities are limited. This study proposes a novel gradient printing strategy via two-photon polymerization (2PP) based 4D printing to enhance the actuation performance of stimulus-responsive hydrogel micro-actuators. The feasibility of this approach was demonstrated by investigating the shrinkage rates and elastic moduli of the poly(N-isopropylacrylamide) (PNIPAm) hydrogel micro-cuboids printed at different laser doses using the confocal laser scanning microscope and atomic force microscopy based nano-indentation respectively. The 2PP-based gradient printing was used to fabricate bilayer and trilayer PNIPAm hydrogel micro-actuators, with the laser dose programmed to modulate the crosslinking degree of each layer. These micro-actuators were actuated by near-infrared (NIR) light in the gold nanorods (AuNRs) solutions. The effects of the NIR light powers, micro-actuator sizes, and layer thicknesses on the actuation behaviors were systematically investigated. Compared with 12-µm-thickness bilayer micro-actuation, the introduction of the transitional layer into the gradient trilayer one significantly enhanced the actuation amplitude and speed (the bending angle and curvature increased by about 150 and 70%, respectively, and the cycle time of actuation and recovery shortened by 35%). These advancements have significant implications for printing microscale gradient materials and enhancing their applications.

Keywords: 4D printing; stimulus-responsive hydrogels; gradient printing; two-photon polymerization

1 Introduction

Micro-actuators, which can convert external stimuli into mechanical motion or deformation, show promise for diverse applications such as micro-robotics, biomedical devices, and soft microelectronics [1]. However, it is still challenging to fabricate soft micro-actuators with defined 3D structures and high actuation performance. 4D printing, an evolution of 3D printing, incorporates the temporal dimension and enables materials to morph, self-assemble, or respond to external stimuli over time [2,3]. 4D printing at the microscale allows the fabrication of dynamic micro-structures with controllable functionalities [4], offering significant implications across disciplines such as biomedicine [5,6] and robotics [7]. The 4D-printed micro-actuators, with intricate motion and high actuation performance, can be fabricated by high-precision 4D printing technologies, such as two-photon polymerization (2PP) [8]. This precision 4D printing technology advances the development of intricate devices with tailored functionalities, particularly beneficial in microrobots [9], microfluidics [10], and electronics [11,12,13].

Compared to other soft materials used in the 4D printing of micro-actuators, such as shape memory polymers and liquid crystal elastomers, hydrogels have gained considerable attention and have been widely applied for sensors [14], drug delivery [15], biomedical [16], and soft robots [17,18,19] due to their unique biocompatible and tunable properties. These 3D crosslinked polymer networks, swollen with water, exhibit high biocompatibility, flexibility, and responsiveness to environmental changes [20,21]. Stimulus-responsive hydrogels, in particular, can undergo significant reversible changes in volume, shape, or mechanical properties in response to stimuli such as pH [22], temperature [23], light [24], electricity [25], and magnetic field [26]. Among the stimulus-responsive hydrogels, the poly (N-isopropylacrylamide) (PNIPAm) hydrogel is inherently temperature-responsive, and it can undergo a phase transition near its lower critical solution temperature (LCST) which is close to the physiological temperature. This phase transition enables the PNIPAm hydrogel to expand or contract when the temperature changes around its LCST, making it well suitable for applications requiring reversible and repeatable actuation. Moreover, it can incorporate photothermal materials, such as gold nanorods (AuNRs), to fabricate light-responsive hydrogels. This light-driven hydrogel can be stimulated by light stimuli in a contactless manner by converting light into localized heat [25], unlike other types of stimulus-responsive hydrogels that require precise and complex stimulus control. These unique properties make the PNIPAm hydrogel become the suitable material for actuators [27]. By harnessing the responsiveness of hydrogels and 4D printing, the actuators can be engineered to perform intricate motion and shape transformations, mimicking biological systems. The tunable responsiveness and biocompatibility of hydrogel actuators further expand their utility in biomedical devices and soft robotics [28,29].

Various macroscale hydrogel actuators have been fabricated by 4D printing technologies such as direct ink writing (DIW) and stereolithography (SLA). However, at the microscale, the fabrication of hydrogel actuators presents several challenges, particularly in achieving precise and facile fabrication and enhanced actuation performance [30,31]. Current strategies, such as multi-material and multi-step printing, aim at printing heterogeneous-structured hydrogels [32,33]. For instance, bilayer soft micro-actuators made of PNIPAm and commercial IP-L were printed using the multi-material strategy [34]. This method encounters issues like complex fabrication processes and poor interfacial adhesion, resulting in interfacial delamination during actuation in bilayer actuators [35,36,37]. To address the interfacial problems, researchers have explored adding an intermediate layer to fabricate trilayer actuators [38,39]. For instance, the inclusion of a poly(2-ethylhexyl acrylate) interfacial layer between polyacrylamide and polyethylene terephthalate layers has been proven to improve the actuation performance of the trilayer actuator [39]. Similarly, gradient structures exhibit transitional material properties across the heterogeneous structure and can alleviate interfacial adhesion problems [40,41]. Since the trilayer and gradient structure has been demonstrated to enhance the actuation performance of macro-actuators, the strategy of fabricating gradient trilayer structure would be a promising method for the actuation enhancement of micro-actuators. To fabricate such gradient trilayer micro-hydrogels, among the current 3D printing technologies such as DIW, SLA, and 2PP, 2PP technology has a high printing resolution (about 100 nm) beyond the diffraction limit achieved by focusing an ultrashort femtosecond laser pulse into a voxel of photosensitive material to initiate polymerization through two-photon absorption. It also allows programming the laser doses to fabricate heterogeneous microstructures in a single-material and one-step printing manner [42,43,44]. The laser dose can be tuned via 2PP-based printing through variations of the scanning speed and laser power [45] or the slicing and hatching distances to fabricate heterogeneous-structured bilayer micro-actuators [46]. The laser dose variation produces a difference in the crosslinking densities of hydrogel, thus causing a difference in swelling or shrinking capabilities for micro-actuator layers. Therefore, 2PP was used to fabricate gradient trilayer micro-actuators to improve the actuation performance and simplify the fabrication process. By leveraging the 2PP-based gradient printing strategy and trilayer microstructure, it is easy to print hydrogel micro-actuators with enhanced actuation performance.

In this article, we present a facile approach to fabricating stimulus-responsive hydrogel micro-actuators utilizing 2PP-based gradient printing (Figure 1). The gradient printing strategy was developed to achieve precise spatial control of the laser dose, enabling the fabrication of intricate structures with tailored material properties. Leveraging this strategy, the gradient bilayer-structured and multilayer-structured hydrogel stripes with varying sizes were designed. The crosslinking variations within the bilayer and gradient multilayer stripes were achieved by programming the laser dose. A series of gradient PNIPAm hydrogel micro-actuators were printed by a 780-nm femtosecond laser and subsequently actuated by 808-nm near-infrared (NIR) light in the AuNRs solutions. To the best of our knowledge, printing gradient micro-structures remains a challenge and has not been applied in the 4D printing of hydrogel micro-actuators. This research aims to fabricate gradient hydrogel micro-actuators with improved actuation performance using stimulus-responsive materials and a gradient printing strategy. The gradient-printed trilayer hydrogel micro-actuators demonstrated enhancement in the light-actuated performance, including both bending amplitude and speed. This printing method highlights its versatility and potential applications in fabricating diverse materials and gradient microstructures.

Figure 1 Schematic of fabrication and light-actuation of hydrogel micro-actuators. (a) Schematic of fabrication of hydrogel micro-actuators by 2PP using gradient printing; (b) gradient hydrogel micro-actuators immersed in the AuNR solution; and (c) 808-nm NIR light-actuation of gradient hydrogel micro-actuators.
Figure 1

Schematic of fabrication and light-actuation of hydrogel micro-actuators. (a) Schematic of fabrication of hydrogel micro-actuators by 2PP using gradient printing; (b) gradient hydrogel micro-actuators immersed in the AuNR solution; and (c) 808-nm NIR light-actuation of gradient hydrogel micro-actuators.

2 Materials and methods

2.1 Materials

N,N′-methylenebisacrylamide (MBA, 98%), 3-(trimethoxysilyl)propyl acrylate (TMSPA, 92%), and lithium phenyl(2,4,6-trimethylbenzoyl) phosphinate (LAP, 95%) were purchased from Sigma-Aldrich. Acrylic acid (AAc, 99.5%) was obtained from Thermo Scientific Chemicals, while methacryloxyethyl thiocarbamoyl rhodamine B was procured from Polysciences, Inc. Ethylene glycol (>99%), acetone (99.5%), isopropanol (>99.5%), n-hexane (ACS), ethanol (>99.8%) and chloroform (>99.8%) were purchased from Anaqua Company Limited, Hong Kong. Poly(ethylene glycol) methyl ether thiol (mPEG-SH, Mn ≈ 2000) was procured from Sigma-Aldrich. All chemicals were used as received without additional purification or treatment. N-isopropylacrylamide (NIPAm, 97%) was purchased from Sigma-Aldrich, recrystallized in hexane, and stored at 4°C. Gold nanorods (AuNRs) (0.1 mg/mL, aspect ratio = 4:1) stabilized by cetyltrimethylammonium bromide (CTAB) were acquired from Xianfeng, China.

2.2 Preparation of photoresist and modification of AuNRs

The PNIPAm hydrogel photoresist was prepared by dissolving N-isopropylacrylamide (45 wt%), MBA (4 wt%), AAc (0.5 wt%), and LAP (1 wt%) with ethylene glycol (48 wt%). Methacryloxyethyl thiocarbamoyl rhodamine B (0.5 wt%) was also added for fluorescence imaging. The mixture was filtrated by 0.22 µm syringe filters. The as-prepared photoresist was put in a brown reagent bottle, stored at room temperature, and shielded from visible light to prevent undesired polymerization.

To obtain PEG-modified AuNRs, the CTAB-stabilized AuNRs were modified by HS–PEG–OCH3 according to the previously reported article [47]. The details of the procedures are presented in Supplementary Materials.

2.3 Design and fabrication of hydrogel micro-actuators

2.3.1 Design of 3D structures for the hydrogel micro-actuators

To demonstrate the effectiveness of gradient printing in enhancing actuation performance, the hydrogel micro-actuators were designed as bilayer and trilayer hydrogel stripes, utilizing the temperature-responsive property of the PNIPAm hydrogel [48], and the one-step, single-material gradient printing strategy. To achieve complex deformations like bending, the gradient printing strategy was employed to print heterogenous-structured PNIPAm hydrogel as the homogenous-structured PNIPAm hydrogel exhibits isotropic expansion or contraction. The gradient printing enables the fabrication of micro-actuators with gradient crosslinking across layers, driving the micro-actuators to bend [49]. The gradient crosslinking of the layers can be controlled by the gradient laser dose, which can be modulated by printing parameters such as laser power and scanning speed using the 2PP technology. Layers exposed to higher laser doses exhibit higher crosslinking, reduced shrinkage rates, and greater stiffness, while lower laser doses produce the opposite effect. However, excessive crosslinking can restrict deformation due to the low shrinkage rate and high rigidity. Additional design factors, including layer thickness and length, can influence the actuation speed and amplitude. By tailoring printing parameters, such as laser dose modulation, and structural factors, such as size and layer arrangement, the actuation performance of bilayer and trilayer micro-actuators can be systematically improved.

The gradient bilayer hydrogel micro-actuator was designed as a strip with upper and lower layers having different cross-linking densities (Figure 1). In contrast, the gradient trilayer hydrogel micro-actuator has a thinner layer thickness, and the laser dose variation between adjacent layers is less apparent than that of the bilayer ones. To study the effect of varying thicknesses and structures of the micro-actuators on their actuation performance, five micro-actuators with varying sizes and layer thicknesses were designed and labeled as micro-actuators a, b, c, d, and e (Table 1). The micro-actuators a, b, and d have gradient bilayer structures, while micro-actuators c and e possess gradient trilayer structures. The micro-actuator a has a length of 150 µm, while the others are 225 µm in length, with all micro-actuators having the same width. The thicknesses of micro-actuators a, b, and c are 12 µm, whereas the micro-actuators d and e are 6 µm thick. Based on the overall thickness and number of layers, the layer thicknesses of the micro-actuators a, b, c, d, and e were 6, 6, 4, 3, and 2 µm, respectively.

Table 1

The size information of micro-actuators a–e

Name of micro-actuators a b c d e
Length (µm) 150 225 225 225 225
Width (µm) 20 20 20 20 20
Thickness (µm) 12 12 12 6 6
Layer thickness (µm) 6 6 4 3 2

2.3.2 Fabrication of hydrogel micro-actuators

The 3D structures of hydrogel micro-actuators with both bilayer and tri-layer structures were designed using SolidWorks 2016, and the resulting STL files were processed with Describe software to generate GWL files compatible with the Nanoscribe system. The 3D printing of the PNIPAm hydrogel micro-structures was conducted using a Photonic Professional DLW system (Nanoscribe GmbH, Germany). The printing was under the oil immersion mode using a 63× objective lens (numerical aperture of 1.4, Zeiss, Germany). A femtosecond laser with an emission wavelength of 780 nm served as the laser source. The pulse width and repetition rate were 120 fs and 100 MHz, respectively. The photoresist was exposed to the laser beam according to CAD design, resulting in polymerization and crosslinking at the focal point to form PNIPAm micro-gels. Slicing and hatching distances were adjusted within a 200–400 nm range to optimize the printing parameters.

To prevent the PNIPAm hydrogel structure from being damaged by overexposure or incomplete curing due to underexposure, the laser power and scanning speed were first optimized to laser dose range to print for the printing of PNIPAm photoresists by Nanoscribe [50]. After the preliminary investigations on these two parameters, the laser powers from 25 to 45 mW, and the scanning speeds from 5,000 to 15,000 μm s−1 were used to print PNIPAm hydrogel micro-cuboids for further observing the printed structures. The PNIPAm hydrogel micro-stripes of varying widths were printed to investigate the resolution of this hydrogel (Figure S1).

To fabricate the gradient bilayer and trilayer hydrogel micro-actuators, a user-defined subroutine programming the laser power and scanning speed for different layers across the thickness of the micro-actuators with the aid of Describe software of the Nanoscribe System. For the laser power for bilayer micro-actuators a, b, and d, the bottom layer was printed at 40 mW and the upper layer at 30 mW, respectively, which are determined based on the results of shrinkage rates and elastic moduli of the hydrogel micro-cuboids.

The trilayer hydrogel micro-actuators c and e were printed with the laser power decreasing from 40 to 30 mW from the bottom to the top. All these micro-actuators were printed at a printing speed of 10,000 µm/s.

Before the hydrogel micro-actuators were printed, the glass coverslips were cleaned with acetone and treated with TMSPA solutions to enhance the adhesion between the glass substrates and the printed hydrogel structures. After printing, the samples were immersed in isopropanol for 24 h to remove the uncured photoresist. Subsequently, isopropanol was replaced with water to obtain the PNIPAm hydrogel micro-actuators.

2.4 Characterization and testing

2.4.1 Confocal imaging of PNIPAm hydrogel micro-cuboids

The swelling and shrinking behaviors of homogeneous PNIPAm micro-cuboids, printed at printing speeds ranging from 5,000 to 10,000 μm/s and laser powers from 20 to 40 mW, were investigated using a confocal laser scanning microscope (CLSM) (Leica TCS SP8, Leica, Germany). The micro-hydrogel sample printed on the glass coverslip was immersed in ultrapure water for 24 h before characterization. The petri dish containing the sample was placed in a temperature-controlled chamber (Tokai Hit chamber). Initially, the sample was imaged at room temperature, followed by heating and stabilization in the chamber at 37°C for 20 min to acquire confocal images of the shrunk hydrogel micro-cuboids. Image analysis was performed using ImageJ software.

2.4.2 Micromechanical characterization by atomic force microscopy (AFM)

The micromechanical properties of the PNIPAm hydrogel micro-cuboids were determined by atomic force microscope (BioScope Catalyst, Bruker). The AFM was integrated with an inverted microscope (Nikon, Eclipse Ti) equipped with a 20× objective to enable visualization of the probe, facilitating precise control of tips and sample positioning. Silicon cantilevers (MLCT, Bruker) with a spring constant k of 0.03 N/m were employed for the measurements, with setup and calibration of the AFM conducted before testing. Specifically, the contact model and ScanAsyst in the fluid model were selected to evaluate hydrogel stiffness in aqueous environments. The laser positioning, AFM positioning, and cantilever calibration were carried out sequentially. Young’s modulus (E) of the hydrogel sample was determined by analyzing the force (F) generated during the indentation between the probe and hydrogel. The force was calculated using the formula: F = k × δ, where k represents the spring constant of the cantilever, and δ denotes the deflection of the cantilever. Data analysis was performed using the software Nanoscope Analysis 1.7. The Hertzian model was employed to fit force–indentation curves generated by the AFM for cantilevers with a pyramidal indenter. Young’s moduli of hydrogel micro-cuboids were obtained from the force–indentation curves via the Nanoscope Analysis.

2.4.3 Characterization of AuNRs by UV–Vis and transmission electron microscope (TEM)

To confirm the light absorption properties of the PEG-modified AuNRs at 808 nm for the light actuation of PNIPAm micro-hydrogels, both the PEG-modified AuNRs and the CTAB-modified AuNRs were characterized using a UV–Vis–NIR spectrometer (PerkinElmer, USA). The light wavelength range for the testing was set from 400 to 1,180 nm.

The morphologies of CTAB-stabilized AuNRs and PEG-modified AuNRs were examined using the TEM. The samples were prepared by depositing the AuNR solutions on carbon-coated copper grids, followed by drying at room temperature. TEM imaging was conducted using the Jeol JEM-2011 microscope (Japan), operating at an accelerating voltage of 120 kV. The sizes of these AuNRs, including their lengths and diameters, were analyzed using ImageJ.

2.4.4 NIR light actuation of hydrogel micro-actuators

An 808-nm laser (LSR808h-4W-FC; Lasever Inc., Ningbo, China) served as the NIR light source. The coverslip with the printed hydrogel sample was placed in the petri dish and immersed in the solution of PEG-modified AuNRs. The NIR light was used as the stimulus for actuation because of its deep penetration, minimal phototoxicity, and potential applications in biomedical fields, while UV light poses high risks of photodamage and limited penetration depth. The light source was placed 20 cm away from the sample. The laser power for the actuation of micro-actuators a–c ranged from 1.50 to 2.5 W for investigating the effects of laser power on the actuation behaviors. A FLIR C3-X (FLIR System OU, Estonia) thermal camera was used to record the temperature changes during the actuation. To compare the actuation behaviors of micro-actuators a–e, the laser power of NIR light was kept the same at 2.0 W (the power density, 1.34 W cm−2). Observation of the light actuation of these micro-actuators was conducted using an optical microscope, and the videos were recorded using a CCD camera. The actuation behaviors, including the measurement of bending angles (α), curvatures (κ), and calculation of the actuation time (t a ) and recovery time (t r ), were analyzed using ImageJ from the recorded videos, based on five cycles of actuation and recovery.

3 Results and discussion

3.1 Modification of AuNRs by PEG

The optical properties and morphologies of the PEG-modified AuNRs were characterized using UV–Vis spectroscopy and TEM. Figure 2 presents the characterization results of the AuNRs before and after modification. In the UV–Vis spectra of CTAB-stabilized AuNRs and PEG-modified AuNRs (Figure 2(a)), both spectra exhibit strong absorption peaks around 800 nm, indicative of the localized surface plasmon resonance characteristic of AuNRs. However, the absorption peak of PEG-modified AuNRs appears slightly shifted compared to CTAB-stabilized AuNRs, suggesting alterations in the optical properties due to surface modification. From the TEM images of CTAB-stabilized AuNRs and PEG-modified AuNRs (Figure 2(b) and (c)), the average aspect ratio of the PEG-modified AuNRs is approximately 3.86, slightly lower than that of CTAB-stabilized AuNRs (Table S1). This reduction in aspect ratio may result from the binding of PEG molecules to the AuNR surface, leading to alterations in nanorod dimensions. Nevertheless, despite these changes, the PEG-modified AuNRs retain their nanorod morphology and exhibit high light absorption at approximately 800 nm. Overall, these results confirm the successful modification of AuNRs with PEG, highlighting the potential for tailoring their optical and morphological properties for the application of NIR light actuation.

Figure 2 Characterization of the light-absorption properties and morphologies of PEG-modified and CTAB-stabilized AuNRs: (a) UV–Vis of CTAB-stabilized AuNRs and PEG-modified AuNRs; (b) TEM image of the CTAB-stabilized AuNRs; and (c) TEM image of the PEG-modified AuNRs.
Figure 2

Characterization of the light-absorption properties and morphologies of PEG-modified and CTAB-stabilized AuNRs: (a) UV–Vis of CTAB-stabilized AuNRs and PEG-modified AuNRs; (b) TEM image of the CTAB-stabilized AuNRs; and (c) TEM image of the PEG-modified AuNRs.

3.2 Relationship between laser dose and swelling/deswelling ability

The effect of printing laser dose on the swelling and deswelling behaviors of PNIPAm hydrogel micro-cuboids fabricated via 2PP was investigated using CLSM. Figure 3 shows the micro-cuboids printed at different laser powers and scanning speeds, along with their corresponding shrinkage rates. In Figure 3(a) and (b), the laser power for printing each micro-cuboid increases from 20 to 45 mW from the left to the right, while the scanning speed increases from 5,000 to 10,000 µm/s from the bottom to the top. The sizes of micro-cuboids printed at 20 mW are slightly smaller than the designed size and are less bright compared to those printed at higher laser powers (Figure 3(a)). This is caused by incomplete curing due to the lower laser dose below the polymerization threshold of the hydrogel photoresist or the weak fluorescent signal below the LCST, especially for the edge regions. When the temperature of the sample reaches 37°C, the micro-cuboids in Figure 3(b) become smaller and brighter than those in Figure 3(a). This is because when the temperature of the sample exceeds the LCST of PNIPAm hydrogels, the hydrogel networks contract and expel water, causing the incorporated fluorescence molecules to aggregate closer, which increases the fluorescence signal intensity [51]. By analyzing the volumes of micro-cuboids at room temperature and 37°C, the volumetric changes of the PNIPAm hydrogel micro-cuboids produced at different printing settings were obtained. The relationships among shrinkage rates, scanning speed, and laser power are presented in Figure 3. In general, the shrinkage rate of PNIPAm micro-cuboids increases noticeably with decreasing laser power and slightly with increasing scanning speed. The maximum difference in the shrinkage rate reaches over 40% between the one printed at 45 mW, 5,000 µm/s and the one printed at 25 mW, 10,000 µm/s. This result reveals that both higher laser power and lower scanning speed contribute to increased exposure of the photoresist to the laser, resulting in denser crosslinking of the PNIPAm hydrogel and less pronounced swelling and deswelling behaviors. These findings demonstrate that variation in printing settings, particularly laser power, can effectively modulate the swelling and deswelling capabilities of the PNIPAm hydrogels. This fundamental understanding underpins the feasibility of one-step printing of stimulus-responsive micro-actuators with tailored functionalities.

Figure 3 The shrinkage behavior of PNIPAm hydrogel micro-cuboids printed at different laser doses. The CLSM images of PNIPAm hydrogel micro-cuboids (a) at room temperature and (b) at 37°C; and (c) the shrinkage rates of PNIPAm hydrogel micro-cuboids printed at different laser power and scanning speed.
Figure 3

The shrinkage behavior of PNIPAm hydrogel micro-cuboids printed at different laser doses. The CLSM images of PNIPAm hydrogel micro-cuboids (a) at room temperature and (b) at 37°C; and (c) the shrinkage rates of PNIPAm hydrogel micro-cuboids printed at different laser power and scanning speed.

3.3 Relationship between micro-mechanical properties and laser doses

The micro-mechanical properties of PNIPAm hydrogel micro-cuboids printed at varying laser powers and scanning speeds were investigated using AFM-based nanoindentation. A series of PNIPAm hydrogel micro-cuboids arranged in a 3 × 3 array was printed using a laser power varied from 25 to 45 mW and scanning speeds ranging from 5,000 to 10,000 µm/s (Figure S2). The nanoindentation analysis was performed on these printed hydrogel micro-cuboids. The force–separation curves obtained from nanoindentation experiments were analyzed using the Hertzian model to determine Young’s moduli. The force–separation curves obtained from curve fitting show the correlation between applied force and indentation depth under various printing conditions (Figure 4(a)). Young’s moduli of the printed hydrogels vary with laser doses, including scanning speed and laser power (Figure 4(b)).

Figure 4 Elastic moduli of PNIPAm hydrogel micro-cuboids printed at different laser doses: (a) The fitted force–separation curves of PNIPAm hydrogel micro-cuboids printed at the scanning speed from 5,000 to 10,000 µm/s and the laser power from 25 to 45 mW; and (b) Young’s moduli of the hydrogel micro-cuboids.
Figure 4

Elastic moduli of PNIPAm hydrogel micro-cuboids printed at different laser doses: (a) The fitted force–separation curves of PNIPAm hydrogel micro-cuboids printed at the scanning speed from 5,000 to 10,000 µm/s and the laser power from 25 to 45 mW; and (b) Young’s moduli of the hydrogel micro-cuboids.

Young’s moduli of the hydrogels significantly increase with the laser power while showing a slight decrease with the scanning speed (Figure 4(b)). For instance, at a fixed scanning speed of 10,000 µm/s, when the laser power increases from 25 to 45 mW, Young’s modulus of the hydrogels significantly increases from 1.2 to 90.4 kPa (Table S2). Moreover, maintaining a constant laser power at 35 mW while decreasing scanning speed from 10,000 to 5,000 µm/s can lead to an increase in Young’s modulus from 6.7 to 18.7 kPa but with a lesser amount. The increase in Young’s moduli of the hydrogels can be attributed to their enhanced crosslinking induced by higher laser doses [45,52], thus demonstrating the potential for tailoring the mechanical behavior of hydrogels through precise control over printing parameters such as laser power.

3.4 NIR-light actuation behaviors of PNIPAm hydrogel micro-actuators

The actuation performance of hydrogel macro-actuators can be predicted and optimized for design purposes using simulation models via various methods such as residual neural networks and genetic algorithms [53,54]. However, predicting the actuation performance of micro-actuators by simulation would be challenging due to the difficulty of determining accurate model parameters such as viscoelastic properties of the micro-hydrogels. In this study, the actuation performance of the gradient hydrogel micro-actuators was assessed experimentally by investigating various factors, including the power of NIR light during the actuation, gradient structures, and sizes. The actuation behaviors investigated include the α, κ, t a , and t r . α was measured as the angle between the bent state and the original state (Figure 5(a)) [55,56]. The difference in bending angle (Δα) from the initial state to a certain state of actuation or recovery was calculated to present the shape change degree during the actuation and recovery. Moreover, to determine the shape-changing capability of the micro-actuators, the maximum values of the difference in bending angle change (Δα max) were compared. Meanwhile, t a and t r were adopted to evaluate the shape-morphing speed. The t a refers to the duration from exposure to NIR light until Δα reaches its maximum, while t r denotes the time taken for the micro-actuator to return to its original state upon the light being turned off. Additionally, the cycle time of actuation and recovery (t c = t a + t r ) was used to evaluate the overall actuation and recovery speed.

Figure 5 (a) Schematic for measurement of the α and the Δα max of PNIPAm hydrogel micro-actuators a–c at different power levels; (b) variation of the t a and t r at different power levels; (c)–(e) thermal images of micro-actuator a during the NIR-light actuation under the light power of (c) 2.50 W, (d) 2.0 W, and (e) 1.5 W and recovery (images (i), (ii), and (iii) refer to the state of samples at the time of 0.0 s, t a , and t c [or t a + t r ]); and (f) temperature changes of micro-actuator a during the NIR light actuation at 2.50, 2.0, and 1.5 W (the solid lines and dotted lines indicate the actuation under the NIR light and the recovery process with the NIR light off, respectively).
Figure 5

(a) Schematic for measurement of the α and the Δα max of PNIPAm hydrogel micro-actuators a–c at different power levels; (b) variation of the t a and t r at different power levels; (c)–(e) thermal images of micro-actuator a during the NIR-light actuation under the light power of (c) 2.50 W, (d) 2.0 W, and (e) 1.5 W and recovery (images (i), (ii), and (iii) refer to the state of samples at the time of 0.0 s, t a , and t c [or t a + t r ]); and (f) temperature changes of micro-actuator a during the NIR light actuation at 2.50, 2.0, and 1.5 W (the solid lines and dotted lines indicate the actuation under the NIR light and the recovery process with the NIR light off, respectively).

3.4.1 Effects of laser power of NIR light on actuation behaviors

The power of the NIR laser could affect actuation behaviors such as α, t a , and t r . The bending behaviors of the gradient bilayer micro-actuators a and b and the gradient trilayer micro-actuator c were investigated across a range of laser powers from 2.50 to 1.50 W. When the laser power increases from 1.50 to 1.75 W, the Δα max increases apparently, but when the laser power further increases, there is only a slight increase in Δα max (Figure 5(a)). Meanwhile, in Figure 5(b), with the increase of laser power, their t a values decrease while the t r values increase. Particularly, the t c values of these micro-actuators reach their minimum values at the laser power of 2.00 W, which can serve as the optimum laser power for the micro-actuator to achieve a larger Δα max and a shorter t c as compared with other conditions.

The influence of laser power on the actuation behaviors of the 4D-printed micro-actuators is related to the photothermal performance and thermal dissipation of the AuNRs solution. To investigate this relationship, micro-actuator a was selected to observe the temperature changes during its NIR-light actuation and recovery at three different laser powers (Figure 5(c)–(e)). Figure 5(c)(i)–(e)(i) correspond to the states before exposure to NIR light (i.e., t = 0.0 s), with the maximum temperatures in the center of the light spot recorded around 24.0°C. Under the NIR light exposure, respectively, when maximum bending angle achieving their maximum bending angles (Figure 5(c)(ii)–(e)(ii)), the temperatures reach a maximum at 52.7, 46.5, and 34.7°C for the light power of 2.5, 2.0, and 1.5 W, respectively. Upon turning off the NIR light, the temperature gradually declines to near room temperature. Notably, as the laser power is reduced from 2.50 to 1.50 W, the peak temperature decreases from 52.7 to 34.7°C (Figure 5(f)). Concurrently, t a increases from approximately 3.0 to 12.0 s, while t r decreases from nearly 7.0 to 5.0 s.

These results suggest that a higher NIR laser power accelerates the actuation but reduces the recovery time. Conversely, low laser powers, such as 1.75 and 1.50 W, fail to fully actuate the micro-actuators to their maximum bending angles. Particularly at the laser power of 1.50 W, the maximum temperature in the central spot of the light-exposed area is 34.7°C, just above the LCST of PNIPAm hydrogels. The temperature gradually decreases from the center to the edge of the light spot due to the gradient distribution of laser energy density and heat transfer in the aqueous solution [57]. Consequently, although the micro-actuator exhibits slight bending behavior, the maximum temperature remains higher than the LCST of PNIPAm [58]. The heat generated by the photothermal effects of AuNRs dissipates in the outer area, preventing further shrinkage of the PNIPAm hydrogel micro-actuator to its most bent state. When the laser power is above 1.75 W, the center temperature of the micro-actuator would be much higher than the LCST of PNIPAm, so it rapidly shrinks and leads to its maximum bending. However, at high laser power levels such as 2.25 and 2.50 W, the high temperatures when the micro-actuator reaches its maximum bending angle result in a slow cooling and recovery. Therefore, to achieve both a higher bending angle and a shorter cycle time of actuation and recovery for the micro-actuator, a power of 2.00 W was chosen as the optimum NIR laser power for investigating the effects of size and layer thickness on the actuation behaviors of micro-actuators a–e.

3.4.2 Effects of size and layer thickness on actuation behaviors

Although the NIR laser power could affect the actuation behaviors of the hydrogel-based micro-actuators, their actuation capabilities were determined by their sizes and layer thickness. The effects of length, overall thickness, and layer thickness on the actuation behaviors, including Δα max, t a , t r , and Δκ max (the maximum value of the difference of the curvature between the original state and the most bent state) of micro-actuators a–e under the NIR laser power of 2.00 W were investigated. Since the side-view optical images of the micro-actuator cannot depict the bending behavior (Figure S3), the top-view optical images of the micro-actuators a–e in Figure 6(a)–(e) are used to evaluate their bending degree during the one-cycle NIR-light actuation and recovery. The actuation behavior of each micro-actuator over two cycles is presented in Video S1. The performance was evaluated over five consecutive actuation–recovery cycles, with no noticeable decline in actuation efficiency observed (Figure S4). The as-printed hydrogel micro-actuators show different degrees of bending angles during exposure to the NIR laser. Their bending results from their heterogeneous structures with the differences in crosslinking density induced by varying laser doses during printing. It is found that the bending direction is dominated by the elastic modulus [59], with all these micro-actuators bending toward the layer printed with a higher laser dose and, namely, with a higher elastic modulus. Although the densely crosslinked PNIPAm hydrogel layer exhibits lower shrinkage, its Young’s modulus is significantly higher than that of the sparsely crosslinked layer, as indicated by the nanoindentation results in Section 3.3. Within a hydrogel micro-actuator, the layer with a high elastic modulus shows much higher bending stiffness than the one with a low elastic modulus. If their bending is regarded as the bending of a cantilever beam, the bending behavior can be explained by the equation of bending

(1) F δ = CEI L 3 ,

where F is the actuation force, C is the constant, δ is the deflection, E is the elastic modulus, I is the second moment area or the moment of inertia, and L is the length of the micro-actuator.

Figure 6 3D models, NIR-light actuation, and recovery of micro-actuators with varying sizes and layer thicknesses. (a)–(e) corresponds to micro-actuators a–e; (a)(i)–(e)(i) 3D models for printing of micro-actuators a–e; (a)(ii)–(e)(ii) the NIR-light actuation process of micro-actuators a–e; (a)(iii)–(e)(iii) the recovery process of micro-actuators a–e with the NIR light off.
Figure 6

3D models, NIR-light actuation, and recovery of micro-actuators with varying sizes and layer thicknesses. (a)–(e) corresponds to micro-actuators a–e; (a)(i)–(e)(i) 3D models for printing of micro-actuators a–e; (a)(ii)–(e)(ii) the NIR-light actuation process of micro-actuators a–e; (a)(iii)–(e)(iii) the recovery process of micro-actuators a–e with the NIR light off.

Since the cross-section of these micro-actuators is a rectangle, the moment of inertia can be given by

(2) I = b × h 3 12 ,

where b and h are the width and the thickness of the micro-actuator, respectively [60].

For a single micro-actuator, the moment of inertia and lengths of different layers are the same. From equation (1), under the same load, the layer with high elasticity would show less displacement, so the layer with high elasticity lies in the inner layer of the bent micro-actuator. For the other aspect, the shrinkage of the hydrogel network, the hydrogel layer printed at a lower laser dose exhibits a higher volumetric contraction rate, which can occur along both length and thickness. Although the lengthwise shrinkage of the sparsely crosslinked layer is partially constrained by the high bending stiffness of the densely crosslinked layer, the contraction in thickness compensates for the overall shrinkage. This revealed that the bending mechanism is essential for designing and printing the gradient micro-actuators with specified bending directions, particularly in complex hydrogel structures.

The variation in parameters evaluating the bending behaviors of micro-actuators a–e, including the Δα, Δκ during the actuation and recovery, as well as Δα max, Δκ max, t a , and t r , are shown in Figure 7. The values of Δα max, Δκ max, t a , and t r of these micro-actuators are also listed in Table 2. The differences in the actuation behaviors of the micro-actuators resulting from the variation in their length, overall thickness, and layer thickness are elucidated below.

Figure 7 The actuation behaviors of micro-actuators a–e: (a) the bending angle changes and (b) the curvature changes of micro-actuator a–e during the NIR-light actuation and recovery; (c) the comparison of maximum bending angle and curvature of micro-actuator a–e; and (d) the comparison of actuation time and recovery time of micro-actuator a–e.
Figure 7

The actuation behaviors of micro-actuators a–e: (a) the bending angle changes and (b) the curvature changes of micro-actuator a–e during the NIR-light actuation and recovery; (c) the comparison of maximum bending angle and curvature of micro-actuator a–e; and (d) the comparison of actuation time and recovery time of micro-actuator a–e.

Table 2

Comparison of maximum Δ bending angle, maximum Δ curvature, actuation time, and recovery time of micro-actuators a–e

Name of micro-actuators Maximum Δ bending angle (Δα max, 0) Maximum Δ curvature (Δκ max, µm−1) Actuation time (t a , s) Recovery time (t r , s)
a 25.0 ± 1.73 0.0093 ± 0.0016 3.7 ± 0.25 5.2 ± 0.34
b 39.2 ± 1.99 0.0138 ± 0.0018 5.1 ± 0.58 6.3 ± 0.42
c 101.3 ± 0.98 0.0233 ± 0.0021 3.1 ± 0.16 4.2 ± 0.50
d 157.9 ± 1.47 0.0204 ± 0.0028 10.5 ± 0.67 26.3 ± 2.53
e 184.6 ± 1.21 0.0241 ± 0.0029 9.8 ± 0.59 18.8 ± 1.76
3.4.2.1 Effect of length variation

The gradient bilayer micro-actuators a and b have lengths of 150 and 225 µm, respectively, with the lower and upper layers printed at the laser dose of 40 and 30 mW. The longer micro-actuator b shows higher values of Δα max and Δκ max as compared with the shorter micro-actuator a, namely increasing from 25.0° to 39.2° and from 0.0093 to 0.0138 µm−1, respectively. The higher curvature Δκ max of the longer micro-actuator b during actuation is attributed to the increased arc length of its bent state compared with micro-actuator a [61]. However, the length increase also results in longer actuation and recovery, with the t a value rising from 3.7 to 5.1 s, and the t r value increasing from 5.2 to 6.3 s separately. Given that both micro-actuators a and b have identical thickness and their upper and lower layers were printed at the same laser dose, the shrinkage speed is also uniform because of the consistent crosslinking in each layer and the water repulsion properties of the PNIPAm hydrogel network [62]. The increased actuation and recovery time for the longer micro-actuator is due to the greater distance the free end needs to travel to reach the maximum bending angle and return to the original position. These results indicate that increasing the length is a method to fabricate the micro-actuator with a large bending angle despite slightly slowing down the actuation and recovery.

3.4.2.2 Effect of thickness variation

The effect of thickness variation on the actuation behaviors of the micro-actuators b–e, all with a length of 225 µm, is summarized as shown in Table 2. When the thickness decreases from 12 to 6 µm, both the gradient bilayer micro-actuator d shows higher values of Δα max and Δκ max as compared with the gradient bilayer micro-actuator b, namely increasing from 39.2° to 157.9° and from 0.0138 to 0.0204 µm−1, respectively. Similarly, the gradient trilayer micro-actuator e also exhibits higher values of Δα max and Δκ max as compared with the gradient trilayer micro-actuator c, namely rising from 101.3° to 184.6° and from 0.0233 to 0.0241 µm−1, correspondingly. The thicker gradient bilayer micro-actuators b and trilayer micro-actuator c show lower maximum bending angles and curvature correspondingly as compared with the micro-actuator d and e because of the higher moment of inertia for a thicker cross-section according to equation (2). However, the decrease in the overall thickness also slows the actuation and recovery process, specifically, the t a and t r values for the gradient bilayer micro-actuators increasing from 5.1 to 10.5 s and from 6.3 to 26.3 s, respectively, and the t a and t r values for the gradient trilayer micro-actuators rising from 3.1 to 9.8 s and from 4.2 to 18.8 s, respectively. The thinner gradient bilayer and trilayer micro-actuators b and c show slower actuation and recovery mainly because of the longer trajectory for their free ends to reach the maximum bending angles.

3.4.2.3 Effect of varying layer thickness

The impact of varying layer thickness on their actuation behaviors is assessed by comparing 12-µm-thickness micro-actuators b and c, as well as the 6-µm-thickness micro-actuators d and e. The individual layer thicknesses of micro-actuators b, c, d, and e are 6, 4, 3, and 2 µm, respectively, exceeding the resolution (∼0.8 µm) of the printed PNIPAm hydrogel stripes (Figure S1). The bilayer gradient micro-actuators b and d were separately printed at 40 and 30 mW for the lower and upper layers, while the laser powers for the printing of lower, intermediate, and upper layers of triple-layered gradient micro-actuators c and e were 40, 35, and 30 mW, respectively. The variation from a bilayer to trilayer gradient structure leads to a significant increase in Δα max, namely from 39.2° to 101.3° for micro-actuators b and c and from 157.9° to 184.6° for the micro-actuators d and e, respectively. Similarly, the Δκ max increases from 0.0138 to 0.0233 µm−1 for the micro-actuators b and c and from 0.0204 to 0.0241 µm−1 for the micro-actuators d and e, respectively. The t a are reduced by 2.0 and 0.7 s, and t r decreases by 2.0 and 7.5 s, correspondingly. For the 12-µm-thickness micro-actuators b and c, when the layer thickness decreases from 6 to 4 µm, the Δα max and Δκ max increase by over 150 and nearly 70%, respectively, and the cycle time of actuation and recovery shortens by about 35%. Similarly, for the 6-µm-thickness micro-actuators d and e, when the layer thickness decreases from 3 to 2 µm, the Δα max and Δκ max increase by 17 and 18%, respectively, and the cycle time of actuation and recovery shortens by about 22%. These results indicate that decreasing layer thickness enhances the bending angle and accelerates both the actuation and recovery processes. This is because a more gradual crosslinking gradient within the trilayer hydrogel structure leads to a more uniform and continuous stress distribution [63]. The gradient trilayer structure, with an intermediate layer, also facilitates a gradual transition in crosslinking density and elastic modulus across layers, reducing internal stress and improving actuation amplitude and speed. In contrast, the sharp change in the elastic modulus at the interface in the gradient bilayer structure increases the internal stress, resulting in rougher and slower actuation and recovery [64]. It is noted that the micro-actuator e exhibits the largest bending degree change (over 180°) among these micro-actuators as the layer thickness is the minimum at 2 µm and the crosslinking within the micro-structure varies at 2-µm level. Theoretically, the minimum layer thickness can be designed as the hatching distance (generally less than 1 µm) to fabricate the gradient structure with the sub-microscale crosslinking gradient and multilayer gradient microstructures.

Overall, increasing the length and decreasing the overall thickness of the micro-actuators enhance the bending angles and curvature but slow down the actuation and recovery. Reducing the layer thickness in gradient hydrogel structures is also favorable for increasing the bending angles and expediting the actuation and recovery processes. This study was among the first attempts to demonstrate the applicability of the programming-based gradient crosslinking strategy for fabricating the hydrogel micro-actuators with enhanced bending performance achieved in the trilayer structure over the bilayer one. Moreover, using this gradient printing strategy, the micro-actuators could be fabricated in a one-step and one-material process. Compared with multi-material printing, this gradient printing method simplifies the fabrication process and alleviates issues of the interfacial adhesion between different materials.

4 Conclusion

In conclusion, the 2PP-based 4D printing coupled with the innovative gradient printing strategy has been demonstrated to be capable of fabricating stimulus-responsive hydrogel micro-actuators with different bending performance. With the developed gradient printing strategy, the micro-actuators with gradient variations in cross-linking densities and, thus, mechanical behaviors across their thickness were realized by creating a user-defined subroutine programming the laser dose and scanning speed of different layers. Among the various studied factors, including NIR laser power, length, thickness, and the number of layers of the micro-actuators, it was found that the number of layers played the most significant role in enhancing the actuation performance. From the bilayer to the gradient trilayer, the maximum bending angle and curvature change increased by over 150% and nearly 70%, respectively, and the cycle time of actuation and recovery shortened by about 35%. This gradient printing strategy overcomes the challenges associated with multi-material printing, such as complex printing procedures and interfacial adhesion issues. The as-printed gradient micro-actuators highlight the versatility of this technique, opening avenues for applications in fields such as soft robotics, biomedical devices, and microfluidics. Furthermore, this technique holds great potential for developing universal strategies for printing gradient multilayer microstructures across various materials. Future research could explore the gradient printing of other materials and micro-actuators with complex functions and other materials, as well as the applications of the micro-actuators.

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  1. Funding information: The authors would like to express sincere thanks to the financial support from the Research Committee of The Hong Kong Polytechnic University (Project codes: RHFV and G-UARR). The work described in this article was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project Nos. PolyU15211221 and PolyU15212523).

  2. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  3. Conflict of interest: The authors state no conflict of interest.

  4. Data availability statement: The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Received: 2024-09-17
Revised: 2024-12-03
Accepted: 2025-02-12
Published Online: 2025-03-17

© 2025 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/ntrev-2025-0145
Keywords for this article
4D printing; stimulus-responsive hydrogels; gradient printing; two-photon polymerization
Creative Commons
BY 4.0

Articles in the same Issue

  1. Research Articles
  2. MHD radiative mixed convective flow of a sodium alginate-based hybrid nanofluid over a convectively heated extending sheet with Joule heating
  3. Experimental study of mortar incorporating nano-magnetite on engineering performance and radiation shielding
  4. Multicriteria-based optimization and multi-variable non-linear regression analysis of concrete containing blends of nano date palm ash and eggshell powder as cementitious materials
  5. A promising Ag2S/poly-2-amino-1-mercaptobenzene open-top spherical core–shell nanocomposite for optoelectronic devices: A one-pot technique
  6. Biogenic synthesized selenium nanoparticles combined chitosan nanoparticles controlled lung cancer growth via ROS generation and mitochondrial damage pathway
  7. Fabrication of PDMS nano-mold by deposition casting method
  8. Stimulus-responsive gradient hydrogel micro-actuators fabricated by two-photon polymerization-based 4D printing
  9. Physical aspects of radiative Carreau nanofluid flow with motile microorganisms movement under yield stress via oblique penetrable wedge
  10. Effect of polar functional groups on the hydrophobicity of carbon nanotubes-bacterial cellulose nanocomposite
  11. Review in green synthesis mechanisms, application, and future prospects for Garcinia mangostana L. (mangosteen)-derived nanoparticles
  12. Entropy generation and heat transfer in nonlinear Buoyancy–driven Darcy–Forchheimer hybrid nanofluids with activation energy
  13. Green synthesis of silver nanoparticles using Ginkgo biloba seed extract: Evaluation of antioxidant, anticancer, antifungal, and antibacterial activities
  14. A numerical analysis of heat and mass transfer in water-based hybrid nanofluid flow containing copper and alumina nanoparticles over an extending sheet
  15. Investigating the behaviour of electro-magneto-hydrodynamic Carreau nanofluid flow with slip effects over a stretching cylinder
  16. Electrospun thermoplastic polyurethane/nano-Ag-coated clear aligners for the inhibition of Streptococcus mutans and oral biofilm
  17. Investigation of the optoelectronic properties of a novel polypyrrole-multi-well carbon nanotubes/titanium oxide/aluminum oxide/p-silicon heterojunction
  18. Novel photothermal magnetic Janus membranes suitable for solar water desalination
  19. Green synthesis of silver nanoparticles using Ageratum conyzoides for activated carbon compositing to prepare antimicrobial cotton fabric
  20. Activation energy and Coriolis force impact on three-dimensional dusty nanofluid flow containing gyrotactic microorganisms: Machine learning and numerical approach
  21. Machine learning analysis of thermo-bioconvection in a micropolar hybrid nanofluid-filled square cavity with oxytactic microorganisms
  22. Research and improvement of mechanical properties of cement nanocomposites for well cementing
  23. Thermal and stability analysis of silver–water nanofluid flow over unsteady stretching sheet under the influence of heat generation/absorption at the boundary
  24. Cobalt iron oxide-infused silicone nanocomposites: Magnetoactive materials for remote actuation and sensing
  25. Magnesium-reinforced PMMA composite scaffolds: Synthesis, characterization, and 3D printing via stereolithography
  26. Bayesian inference-based physics-informed neural network for performance study of hybrid nanofluids
  27. Numerical simulation of non-Newtonian hybrid nanofluid flow subject to a heterogeneous/homogeneous chemical reaction over a Riga surface
  28. Enhancing the superhydrophobicity, UV-resistance, and antifungal properties of natural wood surfaces via in situ formation of ZnO, TiO2, and SiO2 particles
  29. Synthesis and electrochemical characterization of iron oxide/poly(2-methylaniline) nanohybrids for supercapacitor application
  30. Impacts of double stratification on thermally radiative third-grade nanofluid flow on elongating cylinder with homogeneous/heterogeneous reactions by implementing machine learning approach
  31. Synthesis of Cu4O3 nanoparticles using pumpkin seed extract: Optimization, antimicrobial, and cytotoxicity studies
  32. Cationic charge influence on the magnetic response of the Fe3O4–[Me2+ 1−y Me3+ y (OH2)] y+(Co3 2−) y/2·mH2O hydrotalcite system
  33. Pressure sensing intelligent martial arts short soldier combat protection system based on conjugated polymer nanocomposite materials
  34. Magnetohydrodynamics heat transfer rate under inclined buoyancy force for nano and dusty fluids: Response surface optimization for the thermal transport
  35. Fly ash and nano-graphene enhanced stabilization of engine oil-contaminated soils
  36. Enhancing natural fiber-reinforced biopolymer composites with graphene nanoplatelets: Mechanical, morphological, and thermal properties
  37. Performance evaluation of dual-scale strengthened co-bonded single-lap joints using carbon nanotubes and Z-pins with ANN
  38. Computational works of blood flow with dust particles and partially ionized containing tiny particles on a moving wedge: Applications of nanotechnology
  39. Hybridization of biocomposites with oil palm cellulose nanofibrils/graphene nanoplatelets reinforcement in green epoxy: A study of physical, thermal, mechanical, and morphological properties
  40. Design and preparation of micro-nano dual-scale particle-reinforced Cu–Al–V alloy: Research on the aluminothermic reduction process
  41. Spectral quasi-linearization and response optimization on magnetohydrodynamic flow via stenosed artery with hybrid and ternary solid nanoparticles: Support vector machine learning
  42. Ferrite/curcumin hybrid nanocomposite formulation: Physicochemical characterization, anticancer activity, and apoptotic and cell cycle analyses in skin cancer cells
  43. Enhanced therapeutic efficacy of Tamoxifen against breast cancer using extra virgin olive oil-based nanoemulsion delivery system
  44. A titanium oxide- and silver-based hybrid nanofluid flow between two Riga walls that converge and diverge through a machine-learning approach
  45. Enhancing convective heat transfer mechanisms through the rheological analysis of Casson nanofluid flow towards a stagnation point over an electro-magnetized surface
  46. Intrinsic self-sensing cementitious composites with hybrid nanofillers exhibiting excellent piezoresistivity
  47. Research on mechanical properties and sulfate erosion resistance of nano-reinforced coal gangue based geopolymer concrete
  48. Impact of surface and configurational features of chemically synthesized chains of Ni nanostars on the magnetization reversal process
  49. Porous sponge-like AsOI/poly(2-aminobenzene-1-thiol) nanocomposite photocathode for hydrogen production from artificial and natural seawater
  50. Multifaceted insights into WO3 nanoparticle-coupled antibiotics to modulate resistance in enteric pathogens of Houbara bustard birds
  51. Synthesis of sericin-coated silver nanoparticles and their applications for the anti-bacterial finishing of cotton fabric
  52. Enhancing chloride resistance of freeze–thaw affected concrete through innovative nanomaterial–polymer hybrid cementitious coating
  53. Development and performance evaluation of green aluminium metal matrix composites reinforced with graphene nanopowder and marble dust
  54. Morphological, physical, thermal, and mechanical properties of carbon nanotubes reinforced arrowroot starch composites
  55. Influence of the graphene oxide nanosheet on tensile behavior and failure characteristics of the cement composites after high-temperature treatment
  56. Central composite design modeling in optimizing heat transfer rate in the dissipative and reactive dynamics of viscoplastic nanomaterials deploying Joule and heat generation aspects
  57. Double diffusion of nano-enhanced phase change materials in connected porous channels: A hybrid ISPH-XGBoost approach
  58. Synergistic impacts of Thompson–Troian slip, Stefan blowing, and nonuniform heat generation on Casson nanofluid dynamics through a porous medium
  59. Optimization of abrasive water jet machining parameters for basalt fiber/SiO2 nanofiller reinforced composites
  60. Enhancing aesthetic durability of Zisha teapots via TiO2 nanoparticle surface modification: A study on self-cleaning, antimicrobial, and mechanical properties
  61. Nanocellulose solution based on iron(iii) sodium tartrate complexes
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  63. Novel royal jelly-mediated green synthesis of selenium nanoparticles and their multifunctional biological activities
  64. Direct bandgap transition for emission in GeSn nanowires
  65. Synthesis of ZnO nanoparticles with different morphologies using a microwave-based method and their antimicrobial activity
  66. Numerical investigation of convective heat and mass transfer in a trapezoidal cavity filled with ternary hybrid nanofluid and a central obstacle
  67. Halloysite nanotube enhanced polyurethane nanocomposites for advanced electroinsulating applications
  68. Low molar mass ionic liquid’s modified carbon nanotubes and its role in PVDF crystalline stress generation
  69. Green synthesis of polydopamine-functionalized silver nanoparticles conjugated with Ceftazidime: in silico and experimental approach for combating antibiotic-resistant bacteria and reducing toxicity
  70. Evaluating the influence of graphene nano powder inclusion on mechanical, vibrational and water absorption behaviour of ramie/abaca hybrid composites
  71. Dynamic-behavior of Casson-type hybrid nanofluids due to a stretching sheet under the coupled impacts of boundary slip and reaction-diffusion processes
  72. Influence of polyvinyl alcohol on the physicochemical and self-sensing properties of nano carbon black reinforced cement mortar
  73. Advanced machine learning approaches for predicting compressive and flexural strength of carbon nanotube–reinforced cement composites: a comparative study and model interpretability analysis
  74. Artificial neural network-driven insights into nanoparticle-enhanced phase change materials melting for heat storage optimization
  75. Optical, structural, and morphological characterization of hydrothermally synthesized zinc oxide nanorods: exploring their potential for environmental applications
  76. Structural, optical, and gas sensing properties of Ce, Nd, and Pr doped ZnS nanostructured thin films prepared by nebulizer spray pyrolysis method
  77. The influence of nano-size La2O3 and HfC on the microstructure and mechanical properties of tungsten alloys by microwave sintering
  78. 10.1515/ntrev-2025-0187
  79. Review Articles
  80. A comprehensive review on hybrid plasmonic waveguides: Structures, applications, challenges, and future perspectives
  81. Nanoparticles in low-temperature preservation of biological systems of animal origin
  82. Fluorescent sulfur quantum dots for environmental monitoring
  83. Nanoscience systematic review methodology standardization
  84. Nanotechnology revolutionizing osteosarcoma treatment: Advances in targeted kinase inhibitors
  85. AFM: An important enabling technology for 2D materials and devices
  86. Carbon and 2D nanomaterial smart hydrogels for therapeutic applications
  87. Principles, applications and future prospects in photodegradation systems
  88. Do gold nanoparticles consistently benefit crop plants under both non-stressed and abiotic stress conditions?
  89. An updated overview of nanoparticle-induced cardiovascular toxicity
  90. Arginine as a promising amino acid for functionalized nanosystems: Innovations, challenges, and future directions
  91. Advancements in the use of cancer nanovaccines: Comprehensive insights with focus on lung and colon cancer
  92. Membrane-based biomimetic delivery systems for glioblastoma multiforme therapy
  93. The drug delivery systems based on nanoparticles for spinal cord injury repair
  94. Green synthesis, biomedical effects, and future trends of Ag/ZnO bimetallic nanoparticles: An update
  95. Application of magnesium and its compounds in biomaterials for nerve injury repair
  96. Micro/nanomotors in biomedicine: Construction and applications
  97. Hydrothermal synthesis of biomass-derived CQDs: Advances and applications
  98. Research progress in 3D bioprinting of skin: Challenges and opportunities
  99. Review on bio-selenium nanoparticles: Synthesis, protocols, and applications in biomedical processes
  100. Gold nanocrystals and nanorods functionalized with protein and polymeric ligands for environmental, energy storage, and diagnostic applications: A review
  101. An in-depth analysis of rotational and non-rotational piezoelectric energy harvesting beams: A comprehensive review
  102. Advancements in perovskite/CIGS tandem solar cells: Material synergies, device configurations, and economic viability for sustainable energy
  103. Deep learning in-depth analysis of crystal graph convolutional neural networks: A new era in materials discovery and its applications
  104. Review of recent nano TiO2 film coating methods, assessment techniques, and key problems for scaleup
  105. Antioxidant quantum dots for spinal cord injuries: A review on advancing neuroprotection and regeneration in neurological disorders
  106. Rise of polycatecholamine ultrathin films: From synthesis to smart applications
  107. Advancing microencapsulation strategies for bioactive compounds: Enhancing stability, bioavailability, and controlled release in food applications
  108. Advances in the design and manipulation of self-assembling peptide and protein nanostructures for biomedical applications
  109. Photocatalytic pervious concrete systems: from classic photocatalysis to luminescent photocatalysis
  110. Beyond science: ethical and societal considerations in the era of biogenic nanoparticles
  111. Corrigendum
  112. Corrigendum to “Synthesis and characterization of smart stimuli-responsive herbal drug-encapsulated nanoniosome particles for efficient treatment of breast cancer”
  113. Special Issue on Advanced Nanomaterials for Carbon Capture, Environment and Utilization for Energy Sustainability - Part III
  114. Efficiency optimization of quantum dot photovoltaic cell by solar thermophotovoltaic system
  115. Exploring the diverse nanomaterials employed in dental prosthesis and implant techniques: An overview
  116. Electrochemical investigation of bismuth-doped anode materials for low‑temperature solid oxide fuel cells with boosted voltage using a DC-DC voltage converter
  117. Synthesis of HfSe2 and CuHfSe2 crystalline materials using the chemical vapor transport method and their applications in supercapacitor energy storage devices
  118. Special Issue on Green Nanotechnology and Nano-materials for Environment Sustainability
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  122. Wound healing activities of sulfur nanoparticles of Allium cepa extract embedded in a nanocream formulation: in vitro and in vivo studies
  123. Retraction
  124. Retraction of “Aging assessment of silicone rubber materials under corona discharge accompanied by humidity and UV radiation”
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