Mg-Based Micromotors with Motion Responsive to Dual Stimuli

Mg-based micromotors have emerged as an extremely attractive artificial micro/nanodevice, but suffered from uncontrollable propulsion and limited motion lifetime, restricting the fulfillment of complex tasks. Here, we have demonstrated Mg-based micromotors composed of Mg microspheres asymmetrically coated with Pt and temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogel layers in sequence. They can implement different motion behaviors stemming from the driving mechanism transformation when encountering catalyzed substrates such as H2O2 and respond to both H2O2 concentration and temperature in aqueous environment. The as-constructed Mg-based micromotors are self-propelled by Pt-catalyzed H2O2 decomposition following the self-consuming Mg-H2O reaction. In this case, they could further generate bilateral bubbles and thus demonstrate unique self-limitation motion like hovering when the phase transformation of PNIPAM is triggered by decreasing temperature or when the H2O2 concentration after permeating across the PNIPAM hydrogel layer is high enough to facilitate bubble nucleation. Our work for the first time provides a stimuli-induced “hovering” strategy for self-propelled micromotors, which endows Mg-based micromotors with an intelligent response to the surroundings besides the significant extension of their motion lifetime.

Mg-based micromotors, as a typical class of selfconsuming artificial micromotors, not only could be powerfully driven by the Mg-H 2 O reaction in physiological condition but also produce a byproduct of Mg 2+ ions, an additional nutritional ingredient that benefits the human bodies [14,15]. Additionally, the vast quantities of therapeutic hydrogen generated in the propulsion can further be used to eliminate excessive hydroxyl radicals which are harmful to living bodies [16]. They displayed superior compatibility for biological systems and have already been applied for drug delivery in the gastrointestinal tract of mice with enhanced therapeutic efficacies [17][18][19][20][21][22]. However, they generally suffer from poor motion controllability and relatively short motion lifetime stemming from the single driving reaction and selfconsuming feature and cannot stop their movement unless the solid "fuel" Mg cores are almost consumed [23,24]. To prolong the lifetime and improve the motion controllability of Mg-based micromotors, several strategies have so far been developed including tuning the opening size, controlling the pitting anions, and being functionalized with photocatalysts [21,25], soft hydrogels [26], or biological materials [19,25,27,28]. Yet they still face challenges in stimuliresponding propulsion and desirable "on/off" characteristics, and further efforts are necessary toward intelligence and controllability, which may lay the foundation for Mg-based micromotors to fulfill tasks in complex environments.
Here, we present a Mg-based micromotor with motion responsive to both H 2 O 2 concentration and temperature in the surroundings. It consists of a Mg microsphere asymmetrically coated with Pt and temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogel layers in sequence. It can demonstrate the propulsion mechanism transformation between Mg-H 2 O reaction and Pt-catalyzed H 2 O 2 decomposition and unique concentration-or temperature-induced self-limitation behaviors like hovering. The temperature-induced volume phase transformation (swelling/shrinking) of PNIPAM and the H 2 O 2 concentration both affect the permeability of H 2 O 2 into the hydrogel layer and thus make the generation of O 2 bubbles in either single or bilateral sides, resulting in self-limitation motion. This work presents a stimuli-induced "hovering" strategy for the manipulation of Mg-based micromotors as well as a method for prolonging their lifetime by taking use of fuels from the surroundings. Given the possible changes of H 2 O 2 concentration and temperature in human bodies, especially in diseased sites, the herein reported dual stimuli-responsiveness of Mg-based micromotors with extended motion lifetime may to some extent promise potential advanced biomedical applications.

Results and Discussion
Given the motion environment diversity of micromotors when used, we focus on the changes of chemical reaction propulsion mechanism and dual stimuli-responsive motion behaviors of Mg-based micromotors in the liquid environment system. The aim is to achieve their long-term lifetime and motion manipulation. In view of the popularity of temperature difference and low concentration H 2 O 2 in either biological or environmental system, we have integrated Mg microspheres with catalytically active Pt to H 2 O 2 and temperature-responsive poly(N-isopropylacrylamide) (PNI-PAM) hydrogels to construct Mg-based micromotors. They were fabricated by asymmetrically coating Mg microspheres with Pt and PNIPAM hydrogel layers in sequence via magnetron sputtering and UV polymerization techniques according to our previous literature [29]. The as-fabricated Mg-based micromotors are about 40 μm in size and exhibit "opening" of about 14 μm of the surface (Figures S1 and S2). As shown in Figure 1, the Mg-based micromotor when operated in an aqueous solution (e.g., containing NaHCO 3 and poly(vinylpyrrolidone) (PVP)) is driven by asymmetrical H 2 bubble release generated by Mg-H 2 O reaction due to the effective shielding of Pt layer from water in a gradually increasing confined space. When encountering catalyzed substrates such as H 2 O 2 , the propulsion mechanism of the Mg-based micromotor will transform spontaneously from the self-consuming from the self-consuming mode to the catalytic mode. When the temperature is below the lower critical solution temperature (LCST, 32°C), the PNIPAM hydrogel is in the hydrophilic swelling state and has a large diffusion constant to solution [31]. Consequently, H 2 O 2 aqueous solution can easily penetrate the gel layer to react with the middle Pt layer resulting in a temperature-induced self-limitation motion like hovering due to bubble O 2 recoils from bilateral sides. Here, the self-limitation motion behavior means that a micromotor moves in 20 s only within a circle with the radius less than its twice size. The area of the motion region is characterized as S 20 = πfðx Max − x 0 Þ 2 + ðy Max − y 0 Þ 2 g; here (x Max , y Max ) is the farthest track point and (x 0 , y 0 ) is the initial track point in 20 s. A similar transformation of motion behavior can also be triggered by increasing H 2 O 2 concentration denoted as a concentration-induced selflimitation. A high concentration of H 2 O 2 increases the concentration gradient between the two sides of the gel layer, inducing more H 2 O 2 through the PNIPAM hydrogel layer for bilateral O 2 bubble generation. In a word, the asconstructed Mg-based micromotor can implement different motion behaviors stemming from the propulsion mechanism transformation when encountering Pt-catalyzed substrates such as H 2 O 2 and respond to both the concentration of catalyzed substrates and temperature in aqueous environment with extended motion lifetime. This suggests that the asprepared Mg-based micromotor may be taken as an example to address its intrinsic drawbacks of uncontrollable propulsion and limited motion lifetime arising from its selfconsuming features. Although the H 2 O 2 concentration used here is not in the nontoxic physiological range of less about 50 μM, the "fuel" concentration for the effective motor driving can be reduced via enhancing catalytic reaction activities, regulating motor structures, and so on [32,33]. Moreover, the LCST of the hydrogel can be adjusted to fit the body temperature range by copolymerization [34,35]. Thus, this will to some extent promote the development of Mg-based micromotors toward biomedical application. Figure 2 and Video S1 show the motion behaviors of the Mg-based micromotor in the aqueous solution without or with 0.1 wt% H 2 O 2 at 38°C or 22°C, respectively. All systems contain 0.5 M NaHCO 3 and 5 wt% PVP to peel off the Mg(OH) 2 passivation layer in Mg-H 2 O reaction and to facilitate the suspension of the micromotor [36][37][38]. As shown in Figure 2 [30]. Simultaneously, Pt-catalyzed H 2 O 2 decomposition has a much higher rate constant (about 3:60 × 10 −2 min −1 ) than Mg-H 2 O reaction (about 1:05 × 10 −2 min −1 ) at an approximate pH of 7 [39,40]. Additionally, the lower critical nucleation concentration of O 2 (68 mM) is much lower than that of H 2 (about 250 mM) bubbles [41,42]. Both can induce vast quantities of O 2 bubble generation to powerfully drive the motor.  The corresponding angular velocities of the Mg-based micromotor in the four motion states of (a). The clockwise is specified as the positive direction. The inset is the schematic of force analysis for the Mg-based micromotor. The bubble breaks out of the micromotor and gives it a recoil (F 1 and F 2 ) that contributes forces (F 1⊥ and F 2⊥ ) for the motor rotation and translational motion toward the opposite direction of bubble generation, respectively. α and β are the angles between the directions of the centroid connection line and F ⊥ ; L 1 and L 2 are the vertical distances between the center of mass of the Mg-based micromotor and F 1⊥ and F 2⊥ .
As the PNIPAM hydrogel is a temperature-responsive polymer with LCST, it usually exhibits a sharp volume phase transition at LCST, which is slowed by further increasing/ decreasing temperature [43]. On the basis of the swellingshrinking kinetics of the PNIPAM hydrogel, the collective diffusion coefficient for swelling (D 0 = 2:0 × 10 −7 cm 2 s −1 ) is about twice than that for shrinking (D 0 = 1:1 × 10 −7 cm 2 s −1 ) [44,45]. Although at the temperature below the LCST (32°C) the PNIPAM has a volume swelling with a large cooperative (collective) diffusion of the gel network [31,44], the micromotor can only be propelled by asymmetrical bubble recoils during the Mg-H 2 O reaction because of the effective blocking from water by the middle Pt layer (Figure 2(a), i and iii). This will change when H 2 O 2 appears in the system. As shown in Figure 2(a), iv, the micromotor shows an enhanced rotation with a decreased speed of about 15.8 μm/s and a decreased S 20 , which looks like hovering because of bilateral bubble generation and release. This can be explained by the assumption that H 2 O 2 aqueous solution may diffuse across the swelling hydrogel layer, reacting with the middle Pt layer to induce a self-limitation motion behavior of the Mg-based micromotor.
In order to explicate the temperature-induced selflimitation behavior, the corresponding angular velocity curves of Figure 2(a) are given in Figure 2(b). The singleside bubble-propelled micromotors (i, ii, and iii) show similar angular velocity curves with placid angular changes, while the bilateral-side bubble-propelled micromotor shows a relatively large angular change (iv). This can well explain the originality of the observed self-limitation state of the micromotor shown in Figure 2(a), iv. Under low Reynolds number conditions, the inertial effect can be neglected. The motion of the as-prepared Mg-based micromotor can be described by a set of differential equations for translation and rotation [46][47][48]: where m represents the mass of the motor, J is the inertia tensor, and v and ω denote the translation velocity and angular velocity. The terms that distinguish these Langevin equations from standard Langevin equations for inactive Brownian particles are the driving force and torque contributions arising from bubble detachment. Thereinto, F Brownian and T Brownian represent the diffusion arising from the Brownian force and torque with zero mean. The purely Brownian force for a passive particle can be described by a theoretical translational diffusion coefficient (D T = k B T/6πηR) [49] and a rotational diffusion coefficient [50]. In this system, as a spherical particle with a radius about 40 μm, the characteristic time scale for rotational diffusion τ R and translational diffusion coefficient D T is calculated to be 728.5 min and 1:22 × 10 −10 cm 2 /s, respectively. This means the effect of Brownian force on the motion of the as-prepared micromotor is limited. F drive and T drive are the reaction force and torque caused by the detachment of the bubbles, which will be balanced by the Stokes viscous drag force generated in the opposite direction during motion. Because of the spherical structure of the as-prepared micromotor, F Stokes = 6πηRv (for translation) and T Stokes = 8πηR 3 ω, where R is the particle (hydrodynamic) radius and η is the fluid viscosity. For the motion of the as-prepared Mg-based micromotor, the contributions of F Stokes and T Stokes are passive depending on v and ω and the rotation of bubble recoil-propelled micromotors mainly arises from the bubble detachment deviating from their symmetry [26,[51][52][53]. Therefore, a simplified force analysis of a micromotor based on the driving force is used to understand the motion states. F 1 and F 2 represent the instant propulsion forces provided by the bubble detachment from the opening and the coated side, respectively. For the cases shown in Figure 2(a), i, ii, and iii, F 2 = 0 and the micromotor is only driven by single-side bubble recoil on the opening. Their rotational motions are all caused by the rotational component of F 1 contributing an instant torque expressed as where F 1⊥ represents the vertical component of F 1 for motor rotation, L 1 is the effective lever arm, and α is the angle between the directions of the centroid connection line and F 1⊥ . The differences in their motion states including translational and rotational velocities as well as rotational directions mainly depend on α and the bubble detachment positions. For the micromotor moving in 0.1 wt% H 2 O 2 aqueous solution at 22°C, neither F 1 nor F 2 is 0 due to bilateral bubble generation and ejection. As a result, the instantaneous torque that contributed to the micromotor's rotation is the vector sum of the two torques: T 1 ðT 1 = F 1 L 1 sin αÞ generated by bubble escape from the open end and T 2 ðT 2 = F 2 L 2 sin βÞ generated by bubble release from the coating side. When αβ > 0, the micromotor exhibits an enhanced rotation corresponding to a large angular velocity (Figure 2(b), iv), while it shows a weakened rotation as αβ < 0 ( Figure S3, Video S2). The self-limitation behavior of the Mg-based micromotor in the H 2 O 2 aqueous solution below the LCST of PNIPAM is attributed to the enhanced rotation caused by the bilateral-side bubble detachments.
The  [55]. Figure 4 shows the Mg core amounts and motion speeds versus time, respectively. After overcoming the initial gravity-induced subsidence ( Figure S4 and Video S4), the micromotor keeps moving by bubble recoils with the Mg core amount continuously decreasing as shown in Figure 4(a) and Video S5, indicating its selfconsumed feature. At this time, the lifetime of the micromotor mainly depends on the opening size ( Figure S5) and is in the range of a few minutes to dozens of minutes. With the Mg-H 2 O reaction proceeding, there forms a confined space inside the micromotor, where the nucleation and growth of H 2 bubbles occur. At the early stages, the ejected bubbles are small, while they become large when the Mg core is almost gone. The size of the formed bubbles is mainly dependent on the growth time in the confined space. At the initial stages, the narrow space between the Mg core and the coating layer is too narrow for the growth of bubbles before they eject from the micromotor due to the capillary effect. Upon the enlarged confined space over time, the growth period is extended resulting in the formation of large bubbles. Figure 4(b) shows that the as-prepared micromotor has a relatively constant motion speed and consumption rate of the Mg core during Mg-H 2 O reaction. This is not in agreement with the previous reported surface area-related reaction rate and can be explained by the effect of the confined space, in which the permeation of water is competitive with the escape of bubbles.
The as-constructed Mg-based micromotors can improve the motion controllability and lifetime by the spontaneous transformation of propulsion mechanisms from the selfconsuming reaction to the catalytic reaction in terms of the surroundings. In H 2 O 2 aqueous solution, the as-prepared Mg-based micromotor can be driven by Pt-catalyzed H 2 O 2 decomposition and keeps constantly moving until the fuel (H 2 O 2 ) runs out. The lifetime of the motor moving in H 2 O 2 aqueous solution is determined by the amount of H 2 O 2 and has been significantly prolonged because of the far excess "fuel" in the system. Figure 5 shows the effect of H 2 O 2 concentration on the motion behaviors of the Mgbased micromotors. Taking a micromotor moving at 38°C for example, the as-prepared micromotor first accelerates and then decelerates the motion with increasing the H 2 O 2 concentration ( Figure 5(a)). This is dramatically different from the previously reported catalytic micromotors, whose speeds generally increase with increasing catalytic substrate concentrations [56][57][58]. The speeds of the Mg-based micromotor before and after the addition of H 2 O 2 with different concentrations are shown in Figure S6. When the micromotor moves in the solution without H 2 O 2 , the motion speed is basically stable with an average value of about 13.2 μm/s depending on the relatively constant H 2 bubble size and generation frequency (Video S5). After adding H 2 O 2 , the motion speeds of the micromotor contributed by O 2 detachment from Pt-catalyzed H 2 O 2 decomposition are closely related to H 2 O 2 concentration. The initial increase of the speed with increasing H 2 O 2 concentration (0~0.4 wt%) is attributed to an increased bubble generation frequency. The large variability of speed in 0.4 wt% H 2 O 2 aqueous solution may be attributed to the rapid changes of α and the bubble escape positions in motion (Video S6). The remarkable decrease of motion speed occurs as the H 2 O 2 concentration is further increased to 0.8 wt%, implying a possible change of motion behavior. As shown in Figure 5(b) and Video S7, the micromotor displays a self-limitation motion like hovering due to the bilateral O 2 bubble generation and release as H 2 O 2 concentration is higher than 0.8 wt%. This indirectly reduces the asymmetry of the micromotor and leads to the reduction of the speed. After diluting 0. 8   PNIPAM hydrogel layer and react with the middle Pt layer. As a result, the generated O 2 accumulates and reaches the heterogeneous nucleation energy on the side of the hydrogel layer for O 2 bubble nucleation and growth, resulting in a concentration-induced self-limitation.
To further illustrate the temperature/concentrationdependent self-limitation motion mechanisms of the Mgbased micromotor driven by Pt-catalyzed H 2 O 2 decomposition, we have carried out numerical simulations over the Mg-based micromotor based on a 2D model ( Figure S7) by chemical reaction engineering and dilute chemical species transport modules of COMSOL Multiphysics. Figure 6 shows the numerical simulation results of O 2 concentration distribution around the Mg-based micromotor with different temperatures and H 2 O 2 concentrations. It should be noted that the simplified model only considers the O 2 concentration distribution for nucleation and ignores the fluid disturbance deriving from micromotor motion. In this model, the PNIPAM coating is treated as a porous media, because the swelling/shrinking of the hydrogel network can result in different porosities for O 2 diffusion, which is temperature-dependent. In a swelling state, a large cooperative diffusion coefficient benefits the permeation of H 2 O 2 through the hydrogel layer resulting in a network with large porosity, which is approximately twice than that in a shrinking state [44,59]. When the micromotor is moving below the LCST at a low-concentration H 2 O 2 aqueous solution of 0.1 wt%, the O 2 generated inside the coating layer will reach the critical nucleation concentration of 68 mM within 1 ms due to the accumulative effect of the inner confined void (Figure 6(a)). Simultaneously, in view of the water-swelling feature of the PNIPAM hydrogel layer at this temperature, there are more H 2 O 2 permeating across the hydrogel layer to react with the middle Pt layer resulting in O 2 nucleation on the side of the hydrogel layer. As a result, the micromotor shows a self-limitation behavior caused by bilateral O 2 bubble generation ( Figure 2(a), iv). In contrast, by increasing temperature to 38°C, the access of H 2 O 2 is slowed down by the contracted PNIPAM hydrogel layer. Consequently, the concentration of O 2 generated on the side of the hydrogel layer cannot reach its critical nucleation concentration under continuous outward diffusion (Figure 6(b)). Therefore, the inside generated O 2 bubble detachment from the opening is the origin of the sufficient propulsion of the as-prepared Mgbased micromotor (Figure 2(a) (Figure 6(a)). This enables the O 2 generation on both sides of the middle Pt layer to reach the critical nucleation concentrations and thus induce selflimitation motion of the micromotor.

Conclusion
In this work, we present a kind of dual mechanism-propelled Mg-based micromotor with motion responsive to both temperature and H 2 O 2 concentration.  [29] shown in Figure S1. In detail, Mg microspheres with an average size of about 40 μm were completely scattered on the surface of a glass slide precoated with a thin poly(vinylpyrrolidone) (PVP, K30) film by dripping 100 μL 0.5 wt% PVP ethanol solution on the glass slide and then drying at 60°C for 10 min. Then, the PVP-glass slide with dispersed Mg microspheres was placed in humid air with a relative humidity of 50% for 20 s. During this process, Mg microspheres were partially immersed in the PVP film under the gravitational field to obtain partially covered carriers. After drying in an oven at 60°C for 10 min, the Mg microspheres with partially covered bottom surface were fixed in the PVP film. Then, the exposed Mg surfaces were coated with a Pt layer via magnetron sputtering for 240 s to obtain the Mg/Pt microspheres fixed. The poly(N-isopropylacrylamide) (PNIPAM) precursor solution was prepared by dissolving 0.35g NIPAM in 3.5mL EG followed by adding 0.2g bis(N,N-methylene bis(acrylamide)) and 20 μL 2,2-DEAP into the solution. Subsequently, 100 μL PNIPAM-EG precursor solution was dropped onto the obtained glass slide to form a liquid film on the surface of the Mg/Pt microspheres fixed on the PVP-glass slide followed by spin-coating by a spin coater (speed of 2000 rpm, coating time of 50 s, EZ4 spin coater, Schwan Technology) and then covered by a slip. The obtained glass slide was exposed to the UV light to polymerize NIPAM for 240 s followed with removing the thin cover slips and drying in vacuum at 60°C for 12 h. Finally, the Mg-based micromotors were separated from the substrate by a bladescratching process and washed with acetone twice to remove the PVP layer. images were taken with a Hitachi S-4800 (Japan) at an acceleration voltage of 10.0 kV. The energy dispersive spectroscopy (EDS) analyses were carried out on a JEOL JEM-7500F (Tokyo, Japan) operated at an accelerating voltage of 15 kV. Magnetron Sputtering System (JCP500, Beijing Technol Science Co., Ltd., China) was used to cover the middle Pt layer. The microscopy images and the motion videos of micromotors were recorded with optical microscope using a high-resolution CCD digital camera (Leica DM 3000B, Inc. Germany) coupled with ×5, ×10, ×20, and ×40 objective. The temperature was heated by heating accessories (Temp-Controller 2000-1, Inc. Germany) of the microscope. All videos were analyzed by Video Spot Tracker V08.01 and Matlab R2018a software.

Motion Tests of the
4.6. Numerical Simulation. In order to further confirm the role of temperature and H 2 O 2 concentration in the transformation of the limitation behavior, a two-dimensional numerical simulation of the process is performed using a commercial COMSOL finite element analysis software. The chemical reaction engineering and dilute chemical species transport module are employed. Porous media transfer is used based on the effect of temperature on the PNIPAM hydrogel to simulate the response. The two-dimensional model is created according to the state of the motor at 360 s in Figure 4(a) with an approximately 50 nm Pt layer and 2 μm PNIPAM layer as shown in Figure S7. The reaction engineering module assumes a batch type reactor process with constant volume. The decomposition reaction is The reaction rate constant k f changing with temperature according to Arrhenius expression is The values for activation energy E a are given as 40 kJ/mol, frequency factor of collision A f as 4:32 × 10 5 min −1 , and reaction rate constant k f as 3:60 × 10 −2 min −1 (for 22°C) and 8:32 × 10 −2 min −1 (for 38°C). The surface species (catalyst) is kept with locked concentration. The governing equations in reaction engineering are Here, for a species i, c i is the concentration, R i is the rate of the species, R ads,i is the rate effect of surface species on i, A r is the surface reaction area, and V r is the reactor volume. The study is carried out with time steps of 1 to 20 ms. Transport of diluted species is used to compute the concentration field. The governing physics involves the use of Fick's law for transport by diffusion and adsorption as the adsorption happens on the catalytic surface. The products are formed when they are desorbed from the catalyst surface. And simplify the PNIPAM hydrogel into a porous medium. The governing equation for this problem is the conservative form of the transport of diluted species in porous media interface: Here, for a species i, J i is the flux of the species, ε p is the porosity of the hydrogel, τ F,i is the tortuosity factor based on the Millington-Quirk model [60], D F,i is the diffusion coefficient of the species in fluid, and D e,i is the effective diffusion coefficient of the species. The typical simulation parameters are shown in Table S1.

Conflicts of Interest
The authors declare no competing financial interest.