- Open Access
Nanocomposite hydrogel actuators hybridized with various dimensional nanomaterials for stimuli responsiveness enhancement
© The Author(s) 2019
- Received: 4 April 2019
- Accepted: 2 May 2019
- Published: 10 June 2019
Hydrogel actuators, that convert external energy, such as pH, light, heat, magnetic field, and ion strength, into mechanical motion, have been utilized in sensors, artificial muscles, and soft robotics. For a practicality of the hydrogel actuators in a wide range of fields, an establishment of robust mechanical properties and rapid response are required. Several solutions have been proposed, for example, setting porous and anisotropy structures to hydrogels with nanocomposite materials to improve the response speed and deformation efficiency. In this review paper, we focused on hydrogel actuators including various nanocomposite by categorizing the dimensional aspects of additive materials. Moreover, we described the role of diverse additive materials in terms of the improvement of mechanical property and deformation efficiency of the hydrogel actuators. We assumed that this review will provide a beneficial guidance for strategies of developing nanocomposite hydrogel actuators and outlooks for the future research directions.
- Nanocomposite materials
- Soft materials
Hydrogels, which are a three-dimensional (3D) network of cross-linked hydrophilic polymer chains with high water content (up to 90 wt%), are highly elastic and soft materials. If these hydrogels contain stimuli-responsive polymer, they can produce drastic changes in their volume in response to environmental stimuli, such as heat, light, and magnetic and electric fields. Particularly, hydrogel actuators, converting the energy received from outside into mechanical motion, can exhibit soft and flexible motions similar to that of living creatures. Additionally, actuators with rigid materials (e.g., metals) require joints to connect the rigid parts together, whereas hydrogel actuators do not require these joints. Owing to the flexibility, biocompatibility, and stimuli sensitivity advantages of hydrogels, they can be utilized in a wide variety of applications, including drug delivery [1–6], smart window [7, 8], and soft actuators [8–15].
There are various types of external stimuli including pH [16, 17], light [18, 19], heat , magnet field [21, 22], and ion strength [23, 24]. Stimuli-responsive polymers containing hydrogels change their hydrophilicity in response to these external stimuli to ensure that hydrogels demonstrate macroscopic shrinking or swelling by expelling or absorbing water molecules. However, typical hydrogels exhibit a sluggish mechanical change in response to these external stimuli because their volume phase transitions are associated with diffusion and mass transport of solvent molecules in both the interior and exterior of the hydrogel network. Generally, stimuli are externally applied to inside, and the surface of the hydrogel forms skin layers that interfere with the permeation of solvent molecules. These skin layers are also one of the main factors causing the delay in shrinking or swelling of the hydrogel . Reducing the size of the gel or introducing a porous structure in the hydrogel network could accelerate the response rate [26–28].
Synthetic hydrogels typically comprise randomly oriented 3D polymer networks, physically or chemically cross-linked polymers. Meanwhile, biological systems employ anisotropic structures in hierarchically integrated building units. These anisotropic structures often play a crucial role in biological systems for performing a specific function, as represented by muscle tissue containing unidirectionally oriented actin–myosin units. If a synthetic polymer system can be used to achieve these well-oriented structures, developing highly efficient directional action of the hydrogel, which is similar to muscle contraction, and new biomimetic materials would be possible.
Various metal nanoparticles have been utilized as useful 0D additives. For example, gold nanoparticles (AuNPs) and iron oxide nanoparticles (IONPs) are generally known to generate thermal energy owing to the surface plasmon resonance (SPR) and magneto-thermal effects, respectively. Additionally, rare-earth oxide nanoparticles (REO NPs), such as ytterbium oxide, neodymium oxide, and poly-dopamine nanoparticle (PDA-NPs), have been embedded in the hydrogels to exploit their particular functions. Nanofibers and carbon nanotubes (CNTs) are representative materials of 1D additives. The aligned nanofibers assist the hydrogel actuator to deform directionally. CNTs can also generate thermal energy by absorbing near-infrared (NIR) light and possess high electric and thermal conductivity. In the case of 2D additives, photo-thermal reactive and electro-conductive graphene oxides (GOs) and transition metal dichalcogenides (TMDs) have been utilized extensively. Titanate nanosheets (TiNSs) are implanted in the hydrogels for the electrostatic repulsion between TiNSs. Furthermore, other types of nanosheets, such as fluorohectorite liquid crystal nanosheets (FHT LC NSs) and alumina platelets, have been employed as anisotropic reinforcements for the directional deformation.
In this review, the selected examples of the recently reported nanocomposite hydrogel actuators are classified into the types of additives used. Moreover, the roles of each additive in the enhanced performance of the hydrogel actuators are discussed (Fig. 1). We expect that this review paper provides rational strategies for the development of artificial muscles by using functional nanomaterials and the hydrogels.
2.1 0D nanocomposite hydrogel actuators
0D nanomaterials, with all their dimensions measured within the nanoscales, are commonly referred to as spherical nanoparticles or nanoclusters. The 0D nanomaterials of polymers, metals [29, 30], metal oxides [31, 32], and semiconductor materials  have particular properties (e.g., magnetic, optical, and electronic) owing to their nanoscale dimensions. By using these properties, 0D nanomaterials can be utilized as energy converters, resulting in the actuation of hydrogels. In this section, the hydrogel actuators are classified according to their embedded nanoparticle elements, such as gold [34–36], iron oxide [21, 22, 37, 38], and REO nanoparticles [39, 40].
Noble metal nanoparticles are generally known to possess strong high-energy absorption due to inherent interband transitions and to absorb SPR light. Therefore, AuNPs can convert light energy into thermal energy.
Sukhishvili et al.  demonstrated a layered nanocomposite undergoing a spatially anisotropic deformation by the light irradiation. AuNPs and AuNSs were grafted with PNIPAAm brushes and assembled using a layer-by-layer technique, as shown in Fig. 2b. By combining AuNPs and AuNSs in various layers, the shrinkage of certain regions of the hydrogel can be controlled at specific excitation wavelengths. Figure 2b also shows the microcubes of the layered composite hydrogel exhibiting partial shrinkage at light irradiation of 1.1 W/cm2 546 nm or 2 W/cm2 785 nm. At 546 nm irradiation, the shrinking of the center layer could be selectively achieved, whereas the outer layer collapsed at 785 nm irradiation. The shape change of the composite hydrogel reached the equilibrium after 15 min and was restored to its original state at approximately 5 s after the laser was turned off and was also highly reversible.
Shi et al.  also designed a photo-thermal hydrogel actuator that performed a precise finger-like one-by-one bending via light irradiation. To achieve the bending motion during light irradiation, hydrogel bilayers, comprising nanocomposite PNIPAAm hydrogel with AuNPs and non-thermoresponsive poly(acrylamide) (PAAm), were prepared by using a layer-by-layer strategy. The bending motion occurred due to the mismatch of expansion coefficients between the PNIPAAm and PAAm layers. Figure 2c illustrates that the resulting nanocomposite actuators exhibited flexible, reversible bending and non-bending movements owing to local light irradiation. When the actuator was irradiated with light at a wavelength of 411 nm, it showed a maximum curvature of 4.28 cm−1 in 24 s.
Based on these results, the PNIPAAm hydrogels with AuNPs can reveal the photo-induced actuations, wherein AuNPs rapidly and effectively converted the light energy absorbed by SPR into heat energy. Hence, these AuNP-based hydrogel actuators have been proposed to be ideal candidates for various applications, such as in light-driven soft robots and artificial muscles.
Generally, magnetic nanoparticles (e.g., IONPs) have been used in various fields, such as MRI reagent  and hyperthermia . Owing to the magnetic moment of the IONPs, they could be aligned by applying magnetic field and exhibit a magneto-thermal effect through the alternating magnetic field (AMF). When these unique characteristics are applied to the hydrogel actuators, remote-controllable actuation of the hydrogels by using a magnet could be obtained. This section discusses various studies on hydrogel actuators that are combined with IONPs.
However, most conventional magnetic hydrogels are weak and fragile. To address this limitation, Haider and Yang  developed highly stretchable and exceptionally tough magnetic hydrogels with physically and chemically cross-linked network between the dispersed alginate-coated Fe3O4 nanoparticles and PAAm (Fig. 3b). A permanent NdFeB magnet was used to activate the cylindrical hydrogel containing 20 wt% of Fe3O4. Specifically, the higher the nanoparticle content, the better the magnetic properties of the hydrogel. However, the mechanical properties of the hydrogel, such as toughness and stretchability, become worse. Thus, it remains a remarkable challenge to balance the mechanical and magnetic properties of the magnetic hydrogel.
Shen et al.  developed a soft hydrogel crawler that exhibited a directional movement in an enclosed space, similar to maggot movement. The hydrogel consisted of PNIPAAm and dispersed IONPs, and its periodic movement was demonstrated by heating the embedded IONPs through Brownian and Neel relaxation using AMF. Thus, when the AMF was turned on, the hydrogel was heated above the LCST and then collapsed. When the AMF is turned off, the hydrogel cooled down below the LCST and then restored to its original volume. If the gel was placed in a confined chamber with an asymmetric surface (e.g., ratchet), the friction coefficient in forward and backward directions varied. This function preferentially slid the hydrogel in the direction of the lowest friction (Fig. 3c).
An actuator using a non-uniform magnetization profile (Fig. 3d) was first introduced by Kim et al. . The researchers constructed the magnetically programmable polymer composite actuators by confining self-assembled IONPs in a polymer matrix. Here, the aim was to demonstrate the spatially modulated photo-patterning of the self-assembled IONPs with stronger magnetization of paramagnetic materials. By repeatedly tuning and confining the assembly of IONPs through photopolymerization, the microactuator was manufactured wherein all its portions move in different directions at a uniform magnetic field. They demonstrated a polymer nanocomposite actuator capable of 2D and 3D complex actuations (e.g., caterpillar movement) in which conventional microactuators could not achieve. By selecting the appropriate magnetic field direction and strength for the desired configuration, the actuator could obtain the accurate movement of a microlooper.
Huang, Sakar, and colleagues  developed a rapid prototyping process inspired by origami to construct a self-powered micromachine with complex body planning, reconfigurable shape, and controllable mobility (Fig. 3e). The research was focused on bio-inspired corkscrew movement and was not limited to the propulsive force generated by the rotating passive flagellum. Highly complex swimming strategy could be realized by engineering with different magnetic axes and applying a time-varying magnetic field. The operation of a flat flagellum micromachine remarkably differs from that of a helical flagellum micromachine. Although the helical flagellum generated propulsion force by breaking the time reversal symmetry, the planar flagellum acted similar to a flexible oar that transformed the entire body, thereby causing the forward movement.
2.1.3 Other nanoparticles
Han et al.  incorporated PDA-NPs, possessing NIR-responsive ability, to the PNIPAAm hydrogel to develop the photo-thermal responsive actuators (Fig. 4c). They synthesized a bilayered hydrogel with PNIPAAm/PDA-NPs layer and a pure PNIPAAm layer to demonstrate the actuation under NIR irradiation (808 nm). After the 30 s irradiation of NIR light, the PNIPAAm/PDA-NPs layer distinctively collapsed, causing bending motion of the bilayered hydrogel (Fig. 4d) .
2.2 1D nanocomposite hydrogel actuators
1D nanomaterials refer to a material that has only one dimension at a nanometric scale and their two other dimensions are larger than the nanometric scale. Unlike bulk materials, their physical and chemical properties have elicited attention academically and practically. Presently, various manufacturing methods for synthesizing diverse structures of these 1D nanomaterials have been proposed. This section will describe the nanocomposite hydrogel actuators containing nanofibers [49–51] and CNTs [52, 53].
Nanofibers can be defined as 1D flexible solid nanomaterials with a diameter of 100 nm and aspect ratio of 100:1 or higher. With the recent rapid development of nanomaterial-related technologies, thin nanofibers have been developed. Hence, many attempts of aligning the nanofibers have been executed to achieve sophisticated structures and anisotropic motions of the hydrogels. Interestingly, this phenomenon could be found in nature. For example, many plants are known to possess operational performance based on the local expansion behavior, resulting from directionally oriented cellulosic fibers. Inspired by this finding, hydrogel actuators that use fibrillated nanofillers have been investigated. By applying a shear force to the dispersion that contains nanofibers and monomers, the nanofibers could align in parallel direction to the shear direction. Furthermore, the anisotropic hydrogel can be obtained via in situ polymerization.
Through electrospinning techniques, nanofiber-, nanorod-, and nanotube-shaped materials can be easily and cheaply produced. Liu et al.  addressed a fibrous bilayer system using thermoplastic polyurethane (TPU) and cross-linked PNIPAAm fibers. The TPU and PNIPAAm fibers were oriented at various angles as passive and active layers, respectively. Then, these fibers can display a pre-programmed rolling movement (Fig. 5c) with temperature change. It was demonstrated that reversible coiling, rolling, bending, and twisting motions in distinct directions for many cycles (at least 50).
CNTs are cylinders of hexagons comprising six carbon atoms connected in a tubular shape. The thermal conductivity of CNTs is the same as that of diamonds, and the tensile strength exceeds that of diamonds. When these characteristics are applied to the hydrogel actuators, it is expected to improve of their current actuation performance or initiate new functions.
For the multi-walled carbon nanotubes (MWCNTs)-based actuator, Shi et al.  first reported a new type of actuator based on PVA/Na-MWCNTs hydrogels (Fig. 6b). They suggested sodium functionalized to MWCNT-COO− as polyelectrolytes additives. Generally, polymer electrolyte gels can reveal a partially swollen and shrunken region under the electric field, resulting in bending motion. In the PVA/Na-MWCNTs composite hydrogels, MWCNT-COO− was regarded as a negatively charged polyanion. When the hydrogel strip was subjected to direct current (DC) electric field, sodium ions moved toward the cathode, but the polyanion MWCNT-COO− did not move. Accordingly, the expansion of the hydrogel on the anode side was accompanied with that of contraction on the cathode side, assisted by the high electric conductivity of the MWCNT. These electroactive hydrogel actuators could be used in various applications, such as microswitches, artificial muscles, robotics, optical displays, and micro-pumps.
2.3 2D nanocomposite hydrogel actuators
2D nanosheets with nanosized thickness and infinite length on a plane emerge as novel materials owing to their special properties [54, 55]. Apart from graphene being a single layer of carbon atoms arranged in a 2D honeycomb pattern, other inorganic analogs, such as TMDs, metal oxides, and 2D compound sheets, have received a considerable attention. Particularly, oxide nanosheets have abundant structural diversity and electronic properties. Thus, they can be utilized in applications ranging from catalysis to electronics. One of the most important and attractive aspects of exfoliated nanosheets is their ability to develop a variety of nanostructures with 2D structural blocks. Currently, studies applying 2D materials to hydrogel actuators, such as GOs [56–59], TiNSs , TMD NSs [61, 62], FHT CL NSs [63, 64], and alumina platelets , are conducted actively. In this section, we will discuss distinctive features and roles of these 2D materials being integrated with hydrogel actuators.
GOs are one of the most extensively used 2D nanomaterials for fabrication of hydrogel actuators owing to their excellent mechanical, electrical, and optical properties [56–59]. Reduced GO nanosheets (rGOs) are obtained from the oxidation, exfoliation, and reduction of graphite. rGOs absorb NIR light and efficiently generates heat. Their photo-thermal efficiency is better than the unreduced GOs. The challenge to consider when using rGOs in an aqueous environment is its aggregation vulnerability due to its hydrophobicity. Therefore, functionalization of the rGOs surface is required.
rGOs can be utilized for improvement of mechanical property, and also rapid electro-responsiveness of the hydrogels. Yang et al.  proposed poly(2-acrylamido-2-methyl-propanesulfonic acid) hydrogels comprising rGOs (poly(AMPS-co-AAm)/rGOs) (Fig. 7c). The rGOs uniformly dispersed in the hydrogel can provide an excellent conductive platform to promote ion transport within the hydrogel, thereby generating a significant osmotic pressure between the exterior and interior of the nanocomposite hydrogel. Therefore, the speed of electrical response and volume change of the hydrogels become rapid and remarkable. Additionally, the mechanical properties, including tensile strength and compressive strength, of the poly(AMPS-co-AAm)/rGOs hydrogels are enhanced by the hydrogen bonding interaction between the rGOs and polymer chains. Figure 7d shows that the poly(AMPS-co-AAm)/rGOs hydrogels exhibit reversible bending behaviors in electrolyte solutions. When applying the electric field for 2 min, the hydrogels shrank to 58–68% of their original weight. After removing the electric field, they recovered their initial state within 6 min. These results indicate the rapid and reversible electro-induced swelling–deswelling properties of these nanocomposite hydrogels. Moreover, the responsive rate and degree can be controlled by the contents of the rGOs.
The anisotropic internal structures are well known to induce large and directional deformation. Hence, Xu et al.  developed a new type of PNIPAAm/GOs hydrogel by applying DC electric field to induce gradually oriented GOs into the thermoresponsive hydrogel. A one-pot synthesis strategy was performed to achieve the desired form of the PNIPAAm/GOs hydrogel, as shown in Fig. 7e. When DC field was applied, negatively charged GOs moved to the anode site during electrophoresis. Simultaneously, PNIPAAm chains were chemically cross-linked by N,N′-methylene bis(acrylamide) (BIS), whereas oxygen-containing GOs and amide groups of PNIPAAm chains were physically cross-linked by hydrogen bonding. Consequently, in the hydrogel, the gradually oriented GOs were obtained along the direction from the cathode to anode side based on the DC direction. Figure 7f illustrates the hydrogel lifted loading cargo successfully after 30 s of NIR irradiation (808 nm).
Additionally, Sun et al.  reported an anisotropic hydrogel actuator that generated earthworm-like peristaltic crawling. Figure 8c illustrates the synthesized hydrogel with cylindrical shape containing AuNPs as the photo-thermal convertors, thermoresponsive polymer network (PNIPAAm) for permittivity switching of the gel interior, and cofacially oriented 2D material TiNSs to adjust their anisotropic–electrostatic repulsion. The researchers monitored the temperature change of the PNIPAAm/TiNSs/AuNPs hydrogel using a thermal camera during irradiation of 445 nm laser light (power density of 5.6 W/cm2). The result showed that the irradiation for only 30 s could increase the hydrogel temperature up to 85 °C. Meanwhile, the irradiation of the NIR light lower than 1 s was sufficient to improve the temperature above the LCST. This photo-thermal conversion was also repeatable without any loss of its efficiency by turning on and off the laser light. In contrast, the reference hydrogel without AuNPs did not exhibit any critical temperature change. Within this tremendous change material, they demonstrated that the PNIPAAm/TiNSs/AuNPs hydrogels performed earthworm-like peristaltic crawling. As the NIR-irradiated region of the hydrogels spatiotemporally became long and thin, the friction on the capillary wall was thereby reduced (Fig. 8d). For the crawling movement, the irradiation spot was moved toward the left end with a velocity of 3.4 mm/s. Additionally, the hydrogel actuator containing TiNSs/Au nanorods (AuNRs) was synthesized, where AuNRs can convert the 1064 nm laser light into thermal energy. They demonstrated that penetrated NIR laser light through a ~ 2-mm-thick piece of meat could cause the deformation of the PNIPAAm/TiNSs/AuNRs hydrogel within 6 s (Fig. 8e). Therefore, by means of the anisotropic structure, a rapid, large, repeatable, spatiotemporal, and anisotropic photo-thermal deformation of the hydrogel was possible.
Recently, TMDs have elicited increasing interest because of their tremendous properties, such as high surface area, large band gap, and unique optical characteristics. Thus, TMDs have a remarkable potential in biological applications, electrocatalysis, and optoelectronic devices. Although they have been utilized in outstanding studies, specific cooperation with functional polymers is still required.
In the same research group, Lei et al.  reported flexible anisotropic actuators that are Molybdenium disulfide nanosheets (MoS2 NSs) based and dual responsive. MoS2 NSs acted as the photo-thermal transduction agents during the NIR irradiation (808 nm), the same as the previously described MoSe2 NSs. A smart flexible actuator was developed by utilizing biomacromolecule-functionalized MoS2 NSs, tough hydrogel matrix with tunable volume phase transition temperature (VPTT), and well-designed anisotropic architecture. Chitosan was used for the exfoliation of the MoS2 NSs, rendering MoS2 NSs to be hydrophilic. Thus, the as-prepared nanosheets became remarkably stable in water (Fig. 9d). Owing to the good dispersibility of MoS2 NSSs in water, inorganic–organic hybrid hydrogels could be easily obtained, resulting in the poly(N-isopropylacrylamide)-co-dimethylacrylamide (PNIPAAm-co-DMA) composite hydrogel. The PNIPAAm-co-DMA/MoS2 NSs hydrogel was synthesized with bilayer system. First layer was composed of PNIPAAM and MoS2, and the second layer was formed by chitosan, DMA and LAPONITE. After the NIR light was irradiated, the hydrogel temperature reached the VPTT in the first layer at 10 s, causing the bending of the entire hydrogel (Fig. 9e). Therefore, the bilayered structure rendered the hydrogels produce a smart behavior in shape deformation and self-wrapping with remotely controlled light or heat.
2.3.4 FHT LC NSs
2.3.5 Alumina platelets
Erb et al.  proposed a hydrogel actuator with programmable bio-inspired microstructures. The presented complex hydrogel actuators were inspired by the bending and twisting mechanisms represented by pinecones. In these particular structures, bending and twisting can be performed, assisted by the structurally oriented rigid cellulose microfibers (CMFs). To mimic this structure, gelatin-based hydrogels comprising magnetically oriented alumina platelets were synthesized (Fig. 10d). The alumina platelets can be oriented through magnetic fields because they had been coated with superparamagnetic IONPs. Such platelets have a distinct geometry relative to CMFs, offering 2D reinforcement. The researchers demonstrated an artificially replicated bilayer structure exhibited by pinecone that bent during dehydration/hydration (Fig. 10e). The orientations of platelets in the upper and lower layer of bilayered hydrogel were different from each other, so their deformation exhibited in different ways with hydration or dehydration. By systematically adjusting the platelets orientation angles in the upper and lower layers, deformation modes such as curling and clock- and anticlockwise twisting were programmed.
Classification of additives, polymers, and required stimuli of the hydrogel actuators and actuation comparisons in terms of their maximum deformability, response speed, and response efficiency
Zero dimension (0D)
Gold nanoparticle and nanoshell (AuNP and AuNS)
Near Infrared light (NIR)
4.28 cm−1 
Iron oxide nanoparticle (IONP)
Alternating magnetic field (AMF)
Ytterbium and neodium
808 and 980 nm light irradiation
< 10 s
One dimension (ID)
Thermoplastic urethane (TPU)
Carbon nanotube (CNT)
~ 2.7 s
Multiwall carbon nanotube (MWCNT)
Two dimension (2D)
Reduced graphene oxide (rGO)
Elastin-like polypeptide (ELP)
Graphene oxide (GO)
Titanate nanosheet (TINS)
In a second
~ 70% s−1
TiNS + AuNP
< 0.5 s
Transition metal dicalchogenide (TMD)
0.22 mm−1 
70 s 
In this review paper, we have briefly introduced various nanocomposite hydrogel actuators sorted by dimensions of additive functional organic/inorganic materials. Each of these nanocomposite hydrogel actuators induces distinct behaviors based on several stimuli, such as pH, heat, light, electric field, and magnetic field.
By incorporating several organic or inorganic additives to hydrogels, the limitations of stimuli-responsive hydrogel actuators, such as slow response, small deformation, and low mechanical property, were generally addressed. Multiple hybridization of different materials including 0D-, 1D-, 2D additives and functional polymers would give rise to synergic effects, allowing much improved performance of actuators and also generation of new functions.
Additive materials are not only useful in various stimuli-responsive mediators but also aid in mechanical property improvement of hydrogel actuators. For the development of flexible and mechanically durable hydrogel actuators, physical or mathematical modeling of the polymers and additives, in terms of complementarity, should be implemented. Furthermore, the 3D printing technique could offer a new avenue to make the innovative 3D hydrogel actuators.
In order to realize hydrogel actuators to be applied to future artificial muscles or soft robots, new functions of nanocomposite materials must be derived and manufacturing techniques also should be further developed. We hope this review article can be used as guide in selecting nanocomposites for synthesizing novel hydrogel actuators.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019R1C1C1002836), the POSTECH Basic Science Research Institute Grant, and POSCO Green Science Program.
IKH and YSK wrote the manuscript. All authors contributed to this work in the manuscript preparation. YSK supervised the overall progress of this manuscript preparation. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- H. Li, G. Go, S.Y. Ko, J.-O. Park, S. Park, Magnetic actuated pH-responsive hydrogel-based soft micro-robot for targeted drug delivery. Smart Mater. Struct. 25(2), 027001 (2016)View ArticleGoogle Scholar
- G.W. Ashley, J. Henise, R. Reid, D.V. Santi, Hydrogel drug delivery system with predictable and tunable drug release and degradation rates. Proc. Natl. Acad. Sci. USA 110(6), 2318–2323 (2013)View ArticleGoogle Scholar
- X. Cheng, Y. Jin, T. Sun, R. Qi, B. Fan, H. Li, Oxidation- and thermo-responsive poly(N-isopropylacrylamide-co-2-hydroxyethyl acrylate) hydrogels cross-linked via diselenides for controlled drug delivery. RSC Adv. 5(6), 4162–4170 (2015)View ArticleGoogle Scholar
- X. Dong, C. Wei, J. Liang, T. Liu, D. Kong, F. Lv, Thermosensitive hydrogel loaded with chitosan-carbon nanotubes for near infrared light triggered drug delivery. Colloids Surf. B Biointerfaces 154, 253–262 (2017)View ArticleGoogle Scholar
- J. Li, L. Ma, G. Chen, Z. Zhou, Q. Li, A high water-content and high elastic dual-responsive polyurethane hydrogel for drug delivery. J. Mater. Chem. B 3(42), 8401–8409 (2015)View ArticleGoogle Scholar
- S. Senapati, A.K. Mahanta, S. Kumar, P. Maiti, Controlled drug delivery vehicles for cancer treatment and their performance. Signal Transduct. Target Ther. 3, 7 (2018)View ArticleGoogle Scholar
- S. Kiruthika, G.U. Kulkarni, Energy efficient hydrogel based smart windows with low cost transparent conducting electrodes. Sol. Energy Mater. Sol. Cells 163, 231–236 (2017)View ArticleGoogle Scholar
- Y. Zhou, Y. Cai, X. Hu, Y. Long, Temperature-responsive hydrogel with ultra-large solar modulation and high luminous transmission for “smart window” applications. J. Mater. Chem. A 2(33), 13550–13555 (2014)View ArticleGoogle Scholar
- Y. Cheng, K. Ren, D. Yang, J. Wei, Bilayer-type fluorescence hydrogels with intelligent response serve as temperature/pH driven soft actuators. Sens. Actuat. B 255, 3117–3126 (2018)View ArticleGoogle Scholar
- S. Ishii, H. Kokubo, K. Hashimoto, S. Imaizumi, M. Watanabe, Tetra-PEG network containing ionic liquid synthesized via michael addition reaction and its application to polymer actuator. Macromolecules 50(7), 2906–2915 (2017)View ArticleGoogle Scholar
- D. Kim, H.S. Lee, J. Yoon, Highly bendable bilayer-type photo-actuators comprising of reduced graphene oxide dispersed in hydrogels. Sci. Rep. 6, 20921 (2016)View ArticleGoogle Scholar
- J. Shang, P. Theato, Smart composite hydrogel with pH-, ionic strength- and temperature-induced actuation. Soft Matter 14(41), 8401–8407 (2018)View ArticleGoogle Scholar
- X. Sui, X. Feng, M.A. Hempenius, G.J. Vancso, Redox active gels: synthesis, structures and applications. J. Mater. Chem. B 1(12), 1658–1672 (2013)View ArticleGoogle Scholar
- D. Suzuki, T. Kobayashi, R. Yoshida, T. Hirai, Soft actuators of organized self-oscillating microgels. Soft Matter 8(45), 11447–11449 (2012)View ArticleGoogle Scholar
- L.W. Xia, R. Xie, X.J. Ju, W. Wang, Q. Chen, L.Y. Chu, Nano-structured smart hydrogels with rapid response and high elasticity. Nat. Commun. 4, 2226 (2013)View ArticleGoogle Scholar
- X. Zou, X. Zhao, L. Ye, Synthesis of cationic chitosan hydrogel with long chain alkyl and its controlled glucose-responsive drug delivery behavior. RSC Adv. 5(116), 96230–96241 (2015)View ArticleGoogle Scholar
- N. Bassik, B.T. Abebe, K.E. Laflin, D.H. Gracias, Photolithographically patterned smart hydrogel based bilayer actuators. Polymer 51(26), 6093–6098 (2010)View ArticleGoogle Scholar
- T. Satoh, K. Sumaru, T. Takagi, T. Kanamori, Fast-reversible light-driven hydrogels consisting of spirobenzopyran-functionalized poly(N-isopropylacrylamide). Soft Matter 7(18), 8030–8034 (2011)View ArticleGoogle Scholar
- W. Jiang, D. Niu, H. Liu, C. Wang, T. Zhao, L. Yin, Y. Shi, B. Chen, Y. Ding, B. Lu, Photoresponsive soft-robotic platform: biomimetic fabrication and remote actuation. Adv. Func. Mater. 24(48), 7598–7604 (2014)View ArticleGoogle Scholar
- W.J. Zheng, N. An, J.H. Yang, J. Zhou, Y.M. Chen, Tough Al-alginate/poly(N-isopropylacrylamide) hydrogel with tunable LCST for soft robotics. ACS Appl. Mater. Interfaces. 7(3), 1758–1764 (2015)View ArticleGoogle Scholar
- Z. Xiong, C. Zheng, F. Jin, R. Wei, Y. Zhao, X. Gao, Y. Xia, X. Dong, M. Zheng, X. Duan, Magnetic-field-driven ultra-small 3D hydrogel microstructures: preparation of gel photoresist and two-photon polymerization microfabrication. Sens. Actuat. B 274, 541–550 (2018)View ArticleGoogle Scholar
- J. Tang, Z. Tong, Y. Xia, M. Liu, Z. Lv, Y. Gao, T. Lu, S. Xie, Y. Pei, D. Fang, T.J. Wang, Super tough magnetic hydrogels for remotely triggered shape morphing. J. Mater. Chem. B 6(18), 2713–2722 (2018)View ArticleGoogle Scholar
- E. Palleau, D. Morales, M.D. Dickey, O.D. Velev, Reversible patterning and actuation of hydrogels by electrically assisted ionoprinting. Nat. Commun. 4, 2257 (2013)View ArticleGoogle Scholar
- D. Morales, E. Palleau, M.D. Dickey, O.D. Velev, Electro-actuated hydrogel walkers with dual responsive legs. Soft Matter 10(9), 1337–1348 (2014)View ArticleGoogle Scholar
- Y. Kaneko, K. Sakai, A. Kikuchi, R. Yoshida, Y. Sakurai, T. Okano, Influence of freely mobile grafted chain length on dynamic properties of comb-type grafted poly(N-isopropylacrylamide) hydrogels. Macromolecules 28(23), 7717–7723 (1995)View ArticleGoogle Scholar
- R. Kishi, H. Kihara, T. Miura, H. Ichijo, Microporous poly(vinyl methyl ether) hydrogels prepared by γ-ray irradiation at different heating rates. Radiat. Phys. Chem. 72(6), 679–685 (2005)View ArticleGoogle Scholar
- Y. Takeoka, M. Watanabe, Polymer gels that memorize structures of mesoscopically sized templates. Dynamic and optical nature of periodic ordered mesoporous chemical gels. Langmuir 18(16), 5977–5980 (2002)View ArticleGoogle Scholar
- A. Matsumoto, T. Kurata, D. Shiino, K. Kataoka, Swelling and shrinking kinetics of totally synthetic, glucose-responsive polymer gel bearing phenylborate derivative as a glucose-sensing moiety. Macromolecules 37(4), 1502–1510 (2004)View ArticleGoogle Scholar
- D.D. Evanoff Jr., G. Chumanov, Synthesis and optical properties of silver nanoparticles and arrays. ChemPhysChem 6(7), 1221–1231 (2005)View ArticleGoogle Scholar
- V.K. Sharma, R.A. Yngard, Y. Lin, Silver nanoparticles: green synthesis and their antimicrobial activities. Adv. Colloid Interface Sci. 145(1–2), 83–96 (2009)View ArticleGoogle Scholar
- A.H. Lu, E.L. Salabas, F. Schuth, Magnetic nanoparticles: synthesis, protection, functionalization, and application. Angew. Chem. Int. Ed. Engl. 46(8), 1222–1244 (2007)View ArticleGoogle Scholar
- J. Park, K. An, Y. Hwang, J.G. Park, H.J. Noh, J.Y. Kim, J.H. Park, N.M. Hwang, T. Hyeon, Ultra-large-scale syntheses of monodisperse nanocrystals. Nat. Mater. 3(12), 891–895 (2004)View ArticleGoogle Scholar
- Z. Sun, H. Xie, S. Tang, X.F. Yu, Z. Guo, J. Shao, H. Zhang, H. Huang, H. Wang, P.K. Chu, Ultrasmall black phosphorus quantum dots: synthesis and use as photothermal agents. Angew. Chem. Int. Ed. Engl. 54(39), 11526–11530 (2015)View ArticleGoogle Scholar
- J.E. Song, E.C. Cho, Dual-responsive and multi-functional plasmonic hydrogel valves and biomimetic architectures formed with hydrogel and gold nanocolloids. Sci. Rep. 6, 34622 (2016)View ArticleGoogle Scholar
- S.R. Sershen, G.A. Mensing, M. Ng, N.J. Halas, D.J. Beebe, J.L. West, Independent optical control of microfluidic valves formed from optomechanically responsive nanocomposite hydrogels. Adv. Mater. 17(11), 1366–1368 (2005)View ArticleGoogle Scholar
- Q. Shi, H. Xia, P. Li, Y.-S. Wang, L. Wang, S.-X. Li, G. Wang, C. Lv, L.-G. Niu, H.-B. Sun, Photothermal surface plasmon resonance and interband transition-enhanced nanocomposite hydrogel actuators with hand-like dynamic manipulation. Adv. Opt. Mater. 5(22), 1700442 (2017)View ArticleGoogle Scholar
- H. Haider, C.H. Yang, W.J. Zheng, J.H. Yang, M.X. Wang, S. Yang, M. Zrinyi, Y. Osada, Z. Suo, Q. Zhang, J. Zhou, Y.M. Chen, Exceptionally tough and notch-insensitive magnetic hydrogels. Soft Matter 11(42), 8253–8261 (2015)View ArticleGoogle Scholar
- J. Kim, S.E. Chung, S.E. Choi, H. Lee, J. Kim, S. Kwon, Programming magnetic anisotropy in polymeric microactuators. Nat. Mater. 10(10), 747–752 (2011)View ArticleGoogle Scholar
- S. Watanabe, H. Era, M. Kunitake, Two-wavelength infrared responsive hydrogel actuators containing rare-earth photothermal conversion particles. Sci. Rep. 8(1), 13528 (2018)View ArticleGoogle Scholar
- L. Han, Y. Zhang, X. Lu, K. Wang, Z. Wang, H. Zhang, Polydopamine nanoparticles modulating stimuli-responsive PNIPAM hydrogels with cell/tissue adhesiveness. ACS Appl. Mater. Interfaces 8(42), 29088–29100 (2016)View ArticleGoogle Scholar
- W. Li, D. Wu, A.D. Schlüter, A. Zhang, Synthesis of an oligo(ethylene glycol)-based third-generation thermoresponsive dendronized polymer. J. Polym. Sci. A Polym. Chem. 47(23), 6630–6640 (2009)View ArticleGoogle Scholar
- Z. Zhu, E. Senses, P. Akcora, S.A. Sukhishvili, Programmable light-controlled shape changes in layered polymer nanocomposites. ACS Nano 6(4), 3152–3162 (2012)View ArticleGoogle Scholar
- J. Huang, X. Zhong, L. Wang, L. Yang, H. Mao, Improving the magnetic resonance imaging contrast and detection methods with engineered magnetic nanoparticles. Theranostics 2(1), 86–102 (2012)View ArticleGoogle Scholar
- S. He, H. Zhang, Y. Liu, F. Sun, X. Yu, X. Li, L. Zhang, L. Wang, K. Mao, G. Wang, Y. Lin, Z. Han, R. Sabirianov, H. Zeng, Maximizing specific loss power for magnetic hyperthermia by hard-soft mixed ferrites. Small 14, e1800135 (2018)View ArticleGoogle Scholar
- T. Caykara, D. Yörük, S. Demirci, Preparation and characterization of poly(N-tert-butylacrylamide-co-acrylamide) ferrogel. J. Appl. Polym. Sci. 112(2), 800–804 (2009)View ArticleGoogle Scholar
- T. Shen, M.G. Font, S. Jung, M.L. Gabriel, M.P. Stoykovich, F.J. Vernerey, Remotely triggered locomotion of hydrogel mag-bots in confined spaces. Sci Rep 7(1), 16178 (2017)View ArticleGoogle Scholar
- H.W. Huang, M.S. Sakar, A.J. Petruska, S. Pane, B.J. Nelson, Soft micromachines with programmable motility and morphology. Nat. Commun. 7, 12263 (2016)View ArticleGoogle Scholar
- E. Zhang, T. Wang, W. Hong, W. Sun, X. Liu, Z. Tong, Infrared-driving actuation based on bilayer graphene oxide-poly(N-isopropylacrylamide) nanocomposite hydrogels. J. Mater. Chem. A 2(37), 15633 (2014)View ArticleGoogle Scholar
- S.R. Shin, B. Migliori, B. Miccoli, Y.C. Li, P. Mostafalu, J. Seo, S. Mandla, A. Enrico, S. Antona, R. Sabarish, T. Zheng, L. Pirrami, K. Zhang, Y.S. Zhang, K.T. Wan, D. Demarchi, M.R. Dokmeci, A. Khademhosseini, Electrically driven microengineered bioinspired soft robots. Adv. Mater. (2018). https://doi.org/10.1002/adma.201704189 View ArticleGoogle Scholar
- A.S. Gladman, E.A. Matsumoto, R.G. Nuzzo, L. Mahadevan, J.A. Lewis, Biomimetic 4D printing. Nat. Mater. 15(4), 413–418 (2016)View ArticleGoogle Scholar
- L. Liu, S. Jiang, Y. Sun, S. Agarwal, Giving direction to motion and surface with ultra-fast speed using oriented hydrogel fibers. Adv. Func. Mater. 26(7), 1021–1027 (2016)View ArticleGoogle Scholar
- X. Zhang, C.L. Pint, M.H. Lee, B.E. Schubert, A. Jamshidi, K. Takei, H. Ko, A. Gillies, R. Bardhan, J.J. Urban, M. Wu, R. Fearing, A. Javey, Optically- and thermally-responsive programmable materials based on carbon nanotube-hydrogel polymer composites. Nano Lett. 11(8), 3239–3244 (2011)View ArticleGoogle Scholar
- J. Shi, Z.-X. Guo, B. Zhan, H. Luo, Y. Li, D. Zhu, Actuator based on MWNT/PVA Hydrogels. J. Phys. Chem. B 109(31), 14789–14791 (2005)View ArticleGoogle Scholar
- A.K. Geim, Graphene: status and prospects. Science 324(5934), 1530 (2009)View ArticleGoogle Scholar
- J.K. Wassei, R.B. Kaner, Oh, the places you’ll go with graphene. Acc. Chem. Res. 46(10), 2244–2253 (2013)View ArticleGoogle Scholar
- K. Shi, Z. Liu, Y.Y. Wei, W. Wang, X.J. Ju, R. Xie, L.Y. Chu, Near-infrared light-responsive poly(N-isopropylacrylamide)/graphene oxide nanocomposite hydrogels with ultrahigh tensibility. ACS Appl. Mater. Interfaces 7(49), 27289–27298 (2015)View ArticleGoogle Scholar
- E. Wang, M.S. Desai, S.W. Lee, Light-controlled graphene-elastin composite hydrogel actuators. Nano Lett. 13(6), 2826–2830 (2013)View ArticleGoogle Scholar
- C. Yang, Z. Liu, C. Chen, K. Shi, L. Zhang, X.J. Ju, W. Wang, R. Xie, L.Y. Chu, Reduced graphene oxide-containing smart hydrogels with excellent electro-response and mechanical properties for soft actuators. ACS Appl. Mater. Interfaces 9(18), 15758–15767 (2017)View ArticleGoogle Scholar
- Y. Yang, Y. Tan, X. Wang, W. An, S. Xu, W. Liao, Y. Wang, Photothermal nanocomposite hydrogel actuator with electric-field-induced gradient and oriented structure. ACS Appl. Mater. Interfaces 10(9), 7688–7692 (2018)View ArticleGoogle Scholar
- Y.S. Kim, M. Liu, Y. Ishida, Y. Ebina, M. Osada, T. Sasaki, T. Hikima, M. Takata, T. Aida, Thermoresponsive actuation enabled by permittivity switching in an electrostatically anisotropic hydrogel. Nat. Mater. 14(10), 1002–1007 (2015)View ArticleGoogle Scholar
- Z. Lei, Y. Zhou, P. Wu, Simultaneous exfoliation and functionalization of MoSe2 nanosheets to prepare “smart” nanocomposite hydrogels with tunable dual stimuli-responsive behavior. Small 12(23), 3112–3118 (2016)View ArticleGoogle Scholar
- Z. Lei, W. Zhu, S. Sun, P. Wu, MoS2-based dual-responsive flexible anisotropic actuators. Nanoscale 8(44), 18800–18807 (2016)View ArticleGoogle Scholar
- T. Inadomi, S. Ikeda, Y. Okumura, H. Kikuchi, N. Miyamoto, Photo-induced anomalous deformation of poly(N-Isopropylacrylamide) gel hybridized with an inorganic nanosheet liquid crystal aligned by electric field. Macromol. Rapid Commun. 35, 1741–1746 (2014)View ArticleGoogle Scholar
- N. Miyamoto, M. Shintate, S. Ikeda, Y. Hoshida, Y. Yamauchi, R. Motokawa, M. Annaka, Liquid crystalline inorganic nanosheets for facile synthesis of polymer hydrogels with anisotropies in structure, optical property, swelling/deswelling, and ion transport/fixation. Chem. Commun. (Camb.) 49(11), 1082–1084 (2013)View ArticleGoogle Scholar
- R.M. Erb, J.S. Sander, R. Grisch, A.R. Studart, Self-shaping composites with programmable bioinspired microstructures. Nat. Commun. 4, 1712 (2013)View ArticleGoogle Scholar
- D.W. Urry, Physical chemistry of biological free energy transduction as demonstrated by elastic protein-based polymers. J. Phys. Chem. B 101(51), 11007–11028 (1997)View ArticleGoogle Scholar
- K. Trabbic-Carlson, L.A. Setton, A. Chilkoti, Swelling and mechanical behaviors of chemically cross-linked hydrogels of elastin-like polypeptides. Biomacromol 4(3), 572–580 (2003)View ArticleGoogle Scholar
- M.B. van Eldijk, C.L. McGann, K.L. Kiick, J.C.M. van Hest, Elastomeric polypeptides. Top. Curr. Chem. 310, 71–116 (2012)View ArticleGoogle Scholar
- S. Maeda, Y. Hara, T. Sakai, R. Yoshida, S. Hashimoto, Self-walking gel. Adv. Mater. 19(21), 3480–3484 (2007)View ArticleGoogle Scholar
- M. Liu, Y. Ishida, Y. Ebina, T. Sasaki, T. Hikima, M. Takata, T. Aida, An anisotropic hydrogel with electrostatic repulsion between cofacially aligned nanosheets. Nature 517(7532), 68–72 (2015)View ArticleGoogle Scholar
- M. Fullbrandt, E. Ermilova, A. Asadujjaman, R. Holzel, F.F. Bier, R. von Klitzing, A. Schonhals, Dynamics of linear poly(N-isopropylacrylamide) in water around the phase transition investigated by dielectric relaxation spectroscopy. J. Phys. Chem. B 118(13), 3750–3759 (2014)View ArticleGoogle Scholar
- Z. Sun, Y. Yamauchi, F. Araoka, Y.S. Kim, J. Bergueiro, Y. Ishida, Y. Ebina, T. Sasaki, T. Hikima, T. Aida, An anisotropic hydrogel actuator enabling earthworm-like directed peristaltic crawling. Angew. Chem. Int. Ed. 57(48), 15772–15776 (2018)View ArticleGoogle Scholar
- G. Guan, S. Zhang, S. Liu, Y. Cai, M. Low, C.P. Teng, I.Y. Phang, Y. Cheng, K.L. Duei, B.M. Srinivasan, Y. Zheng, Y.-W. Zhang, M.-Y. Han, Protein induces layer-by-layer exfoliation of transition metal dichalcogenides. J. Am. Chem. Soc. 137(19), 6152–6155 (2015)View ArticleGoogle Scholar