Light-Responsive Hydrogel Microcrawlers,Powered and Steered with Spatially Homogeneous Illumination

Abstract

Sub-millimeter untethered locomoting robots hold promise to radically change multiple areas of human activity such as microfabrication/assembly or health care. To overcome the associated hurdles of such a degree of robot miniaturization, radically new approaches are being adopted, often relying on soft actuating polymeric materials. Here, we present light-driven, crawling microrobots that locomote by a single degree of freedom actuation of their light-responsive tail section. The direction of locomotion is dictated by the robot body design and independent of the spatial modulation of the light stimuli, allowing simultaneous multidirectional motion of multiple robots. Moreover, we present a method for steering such robots by reversibly deforming their front section, using ultraviolet (UV) light as a trigger. The deformation dictates the robot locomotion, performing right- or left-hand turning when the UV is turned on or off respectively. The robots' motion and navigation are not coupled to the position of the light sources, which enables simultaneous locomotion of multiple robots, steering of robots and brings about flexibility with the methods to deliver the light to the place of robot operation.

Introduction

Micrometer-scale structures that can be moved and controlled by a user (microrobots) have been proposed for applications such as micromanipulation, non-invasive surgery, drug delivery, and lab-on-chip technologies.^1–6 In many of these applications, simultaneous operation of multiple robots is required. An ultimate goal is a group of active robots operating parallelly but independently so that they can efficiently complete the desired task in an analogy of a macroscopic robotic assembly line, or such robots could combine their forces to manipulate objects significantly bigger than themselves.^7,8

In addition, top–down controlled microrobots can be used as a complementary method in high-throughput self-assembly methods for correction of inevitable mismatches.^9,10 Hence, methods of microrobot control that enable their parallel independent operation are getting to the center of research interest.

Microrobotic systems can be divided into two categories by how the forces that drive their motion are applied. Forces can be applied externally by means of either magnetic^11,12 or acoustic fields.¹³ Alternatively, they can arise locally from interactions between the microrobots body and their environment, as in the case of shape-changing (actuating) microrobots^14,15 (crawling,^16–21 swimming,^22–26 jumping²⁷), or propulsion-based microrobots.^28,29

Locomotion by external fields allows for control of large samples of microrobotic swarms; however, each microrobot within the swarm experiences the same forces and so generally moves in the same direction or with the same behavior. Advancements in magnetic field-driven microrobotics have demonstrated their ability to exhibit independent behaviors, but require the use of heterogeneous swarms or complex external hardware.^{11,30,31–33} For locomotion by actuation or propulsion, the behavior of the microrobot is governed by its current state and the state of its local environment; hence, it offers potential for each microrobot to act independently within a group.

In many cases of propulsion-driven microrobots, the source of energy is typically dispersed within the environment as chemical fuel and individual microrobots move independently within a large group (neglecting fluidic interactions between microrobots). Similarly, the dispersion of nutrients (chemical fuel) enables independent locomotion within a group of biohybrid microrobots relying on actuating cells as a source of motion.^24,26,34,35

However, in these cases, the microrobots behavior is driven by a supply of energy that cannot easily be switched on and off, making such mechanisms for robot control limited to predefined arrangements of fuel gradients and, thus, cumbersome. An energy source that can be easily controlled both spatially and temporally is light. Used in conjunction with photo-responsive materials, light has been used to induce the locomotion of microrobots that respond to the absorption of light energy by shape changes.^{17,19,20,23,36–38}

These shape changes result in net forces being applied locally to the surroundings by the microrobot, and, hence, if the process can be induced simultaneously in large groups of microrobots, the microrobots will act independently within the group.³ A noteworthy subclass of light-fueled robots are self-oscillating soft actuators, where a steady illumination induces cyclic actuation response.^39–42 Despite promising evidence on macroscale, miniaturization of such systems into microscale is yet to be realized.

Achieving locomotion by shape changes in the micro-regime is not straightforward. The sub-millimeter robotic systems are mostly operated in fluidic environments to suppress the dominance of adhesive forces, which hinder any motion over the substrate. Such robots, with dimensions < ∼1 mm, typically move in liquids in the low Reynolds number regime. Here, viscous forces dominate over inertial forces and, thus, directional motion is provided only by non-reciprocal, cyclic shape changes.⁴³

Alternatively, other phenomena can be exploited to break symmetry within a single degree of freedom (DOF) actuation cycle to provide directional motion, such as asymmetric friction.^17,19,21,36 Multiple microrobot designs have been reported that mimic non-reciprocal gaits utilized by various organisms, where light was used to control the actuation, being localized to induce shape changes at specific regions of the microrobot.^18,23,38

In these cases, the resulting locomotion direction is dictated by the spatial/spatiotemporal light distribution, such as focused beam localization or light pattern traveling direction. Multiplying such illuminating schemes to simultaneously drive multiple robots in arbitrary directions would require accordingly complex illumination setups, with the level of complexity increasing with every robot added to the system.

Hence, it would be preferred to have microrobots that, when illuminated with light of spatially uniform intensity, undergo shape transformations that result in net displacements. This is not an easy approach, as is witnessed by sparse evidence of its experimental microrobotic realizations.

It has been demonstrated in swimming hydrogels that undergo non-reciprocal shape changes owing to out-of-equilibrium actuation of their helical bodies²² and crawling liquid crystal elastomers with asymmetrical frictional properties.³⁶ Moreover, for successful application of such microrobots, these must not only provide continuous net displacement but also their direction of motion must be controllable.

In this research, we present hydrogel microrobots that display directional motion in response to a homogeneous illumination source. The hydrogels undergo reciprocal actuation, which is coupled to a friction hysteresis between shrinking and expanding phases. The head–tail asymmetry, required for the net displacement of the microrobots, is provided by material heterogeneity of the microrobot's body, with only the tail section of the body being responsive to light.

Introducing this asymmetry removes the need for spatial localization of light and allows multiple microrobots to locomote by a global light source. Further, we demonstrate the ability to steer the microrobots by appending a simple pH-responsive hydrogel block to the microrobot body. This block collapses and re-swells, asymmetrically, on a photoinduced pH change. The change in the robot's left-right symmetry induced by this volumetric response controls its trajectory.

Materials and Methods

N-isopropyl acrylamide (NIPAM), poly(ethylene glycol) diacrylate (PEGDA; average M_n = 700 Da), N,N’-methylenebisacrylamide (MBAAm), poly(ethylene glycol) (PEG 200; average M_n = 200 Da), acrylic acid (AA), lithium phenyl-2,4,6-methyl benzoyl phosphinate (LAP), Darocur 1173, trisodium citrate, poly(ethylene glycol) methyl ether thiol (PEG-thiol; average M_n = 6000 Da), Tween 20, Pluronic F-127, and pyranine were purchased from Sigma Aldrich.

Tetrachloroaurate trihydrate was purchased from Alfa Aesar. Sylgard^® 184 elastomer kit (poly(dimethylsiloxane) base and curing agent) was purchased from Dow Corning. All chemicals were purchased in standard purities as provided and used as received. Reverse osmosis water (MilliQ) was used for all experiments (18.2 MΩ at 25°C). One percent w/w gold nanosphere (diameter 15 nm) colloidal solution was prepared using the citrate method reported in Frens⁴⁴ and subsequently incubated with 0.01% w/w PEG-thiol.⁴⁵ Gold nanorods with longitudinal plasmon resonance around 800 nm were prepared using the seed-mediated method.⁴⁶

Precursor solutions

Standard photoresponsive NIPAM hydrogel precursor was prepared as described previously (Rehor et al.¹⁷ and Vrba et al.⁴⁵) by dissolving 37 mg NIPAM, 2.5 mg LAP, and 20 μL PEGDA in 100 μL of gold nanosphere solution. The more light-sensitive NIPAM hydrogel was prepared by dissolving 48 mg NIPAM, 8 mg MBAAm, and 1.5 mg of LAP in 100 μL PEG 200 and 30 μL of gold nanorods solution.

Non-responsive PEGDA hydrogel precursor was prepared by dissolving 2.5 mg LAP in a mixture of 60 mg PEGDA and 140 μL water. pH-responsive AA hydrogel precursor was prepared by dissolving 55 mg NIPAM, 25 mg MBAAm, and 1 mg LAP in a mixture of 182 μL of 25% w/w Sodium Acrylate solution in water (pH adjusted to 7 with sodium hydroxide) and 138 mg PEG 200. All components of a particular precursor solution were mixed together, sonicated for 5 min in a bath, and subsequently centrifuged at 1000 rcf for 1 min to remove dust contamination.

Microrobot synthesis

Microrobots were produced using the multi-stream stop-flow lithography technique described in Ref.⁴⁷ Briefly: precursor solutions were introduced in 2 or 3 separate inlets and pumped into a microfluidic channel (400 μm wide, 30 μm deep). With the pressure removed, and flow stopped, ultraviolet (UV) light was focused through a photomask inducing polymerization of the precursor in a region defined by the photomask. The position and orientation of the photomask with respect to the laminar flows of the precursors was set to achieve the desired microrobot constitutions.

After production, the channel was purged with 0.5% w/w Tween 20 solution and the microrobots were collected in PCR tubes. The sample was washed four times with fresh Tween 20 solution by sedimentation of the microrobots, removal of the supernatant, and replacement with fresh Tween 20 solution. “T”-shaped microrobots were subsequently transferred into mercaptoethanol (10% v/v), Darocur 1173 (1% v/v), and ethanol (89% v/v) solution, kept under UV lamp for 2 h to remove remaining acrylates (the reason of which is discussed in Supplementary Text S3 of Supplementary Data), and subsequently washed.

Microrobot actuation

Polystyrene lids of 96-well plates (purchased from Thermo Fisher Scientific) were used as wells for crawling experiments. Produced microrobots were sedimented in these circular, 0.3 mm-high wells that were 4 mm in diameter. Wells were then filled with aqueous surfactant solutions and covered with a glass slide. The surfactant solution for bimaterial crawlers was Tween 20 (0.5% w/w), as in Rehor et al.¹⁷ For the pH-responsive steering, Pluronic (5% w/w) was used.

All experiments were performed with a Nikon Ti-U inverted microscope. For photo-induced pH change, 1% w/w of pyranine in 5% Pluronic solution was used. For the high area global illumination experiments, a 3 W near infrared (NR) 808 nm laser diode was aimed directly at the sample with microrobots under the inline 45°, to avoid the transmitted light entering the objective.

In the pH-steering experiments, a 1 W green 532 nm laser diode was aimed to the sample using a custom-made setup comprising a 420 nm shortpass dichroic mirror and a shutter (Supplementary Fig. S5). When the shutter was opened, the laser beam was reflected by the dichroic mirror to the condenser lens (Supplementary Fig. S5). By changing the height of the condenser lens, the laser beam focus was adjusted to provide sufficient intensity for full contraction of crawlers (3 × 10³ W/cm² for the focused green 532 nm laser and 3 × 10² W/cm² for the NIR 808 nm laser), and with the spot size exceeding the dimensions of the crawler.

A color filter was placed directly at the top of the objective to absorb the laser beam and prevent it from entering the objective. With “T”-shaped microrobots; the setup described earlier was used for their actuation, whereas episcopic UV illumination was used for pyranine excitation and the associated pH decrease and microrobot shape change.

Results

Head-tail symmetry breaking

In our previous research, we induced directional motion with prismatic¹⁷ or disk-shaped⁴⁶ microgels by light-induced 1 DOF actuation comprising isotropic shrinkage and expansion of a section of their body. The microrobots are composed of thermo-responsive poly-N-isopropyl acrylamide (PNIPAM), made photo-responsive by the incorporation of gold nanoparticles.

When a section of the robot body is illuminated by focused laser light, this section heats up, resulting in local shrinkage of the illuminated gel area. On cyclic pulse illumination, the rapid, out-of-equilibrium, shrinking and expansion processes yield a hysteresis in the properties of the surface of the gel which is exhibited as non-reciprocally varying friction coefficients between the gel and the surface. The result is a net displacement of the microrobot after each shrinking-expanding cycle, along the symmetry axis of the deformed body in the direction from the center of irradiation to the geometrical center of the shape.

Since illumination localization is responsible for the crawling direction, the use of unfocused light would result in isotropic actuation of an entire crawler and, hence, no net displacement over one actuation cycle. To remove the need for spatial localization of the beam spot, we broke the head-tail symmetry of the crawler's response to the light stimulus. To do that, we fabricated microgels (150 μm × 50 μm), composed of two segments—“tail” (photo-responsive PNIPAM) and “head” (non-responsive PEGDA) (Fig. 1).

FIG. 1.

(A) Schematic representation of friction hysteresis-based locomotion. The friction coefficient of the actuating segment changes non-reciprocally along the actuation cycle, dropping steeply during the collapse phase, and rising steeply during the expansion phase, which results in a net displacement over the course of a cycle. The scheme displays a first cycle of a sequence, where the PNIPAM gel starts contracting from the equilibrium state. For consecutive cycles, the picture 1 is to be replaced by the picture 5 and every following cycle consists of pictures 2–5 until the system is left to reach the equilibrium (more details in Rehor et al.¹⁷). (B) Two crawlers actuated simultaneously and crawled from their original position in the direction of their PEGDA segment (bright), that is, toward each other. Consequently, they meet each other and as a result of their physical interaction, they align their orientations and continue in the same direction. Note: The laser position has not been changed throughout the entire course of the experiment. The scale bar corresponds to 200 μm. PEGDA, poly(ethylene glycol) diacrylate; PNIPAM, poly-N-isopropyl acrylamide.

When these crawlers are subjected to pulse illumination with a laser of a spot size exceeding the crawler size, their responsive segment exhibits actuation whereas the non-responsive segment volume remains unaffected by the illumination. This arrangement results in crawler motion. The friction coefficient between the PEGDA segment and the surface remains constant over the course of the actuation cycle, whereas that of the PNIPAM segment changes non-reciprocally, which results in the net displacement of the crawler.

During collapse, the friction coefficient of the PNIPAM significantly decreases, below the value of PEGDA (Supplementary Fig. S1), and hence, the collapsing PNIPAM segment retracts toward the microrobot center (analogously to our previously described solely PNIPAM crawler). During subsequent expansion, the PNIPAM friction coefficient steeply rises above the value of PEGDA and, hence, the PNIPAM segment acts as an anchoring point, pushing the PEGDA section forward. The result is a net displacement of ∼9 μm during one cycle (Fig. 2A).

FIG. 2.

(A) Displacements of rectangular microrobots of varying aspect ratios during one shrinking-expanding cycle. The active, gold–loaded PNIPAM segment dimensions are always 50 × 50 μm, whereas the passive PEGDA region length varies from 50 to 200 μm. All the crawlers provide directional motion on cyclic illumination. Scale bar = 50 μm. (B) Displacement Δ produced in one cycle rescaled with respect to the length of the active segment L, plotted in terms of the ratio R/L between the lengths R of the PEGDA segment and L of the PNIPAM segment (at room temperature). The points correspond to the experimental data, the solid line our model prediction, and the two dashed lines the changes in our prediction for one standard variation in the maximum contraction of the PNIPAM segment.

To increase the illuminated area in which actuation (and concomitantly crawling) can be achieved, we optimized the previously used composition of the responsive section of the microgels to achieve a sharp volumetric transition at the lower critical solubility temperature. Further, we increased the laser power from 200 mW to 3 W. With these changes we achieved a 25-fold increase of the active area (from ∼0.03 to ∼0.8 mm²; details in Supplementary Text S1).

All microrobots within the illuminated area display the same behavior, simultaneously crawling in the direction given by the orientation of their active and passive sections (Fig. 1B). The crawling direction of the individual crawlers is independent of their position and orientation with respect to the center of illumination; they crawl along straight trajectories and when two crawlers hit each other, they can push each other.

Locomotion of bimaterial crawlers

The embedding of the head-tail asymmetry in the geometry of the hydrogel crawler, instead of relying on spatial localization of the irradiation, allows a more proper analysis of the effectiveness of different designs of crawler.

Our investigation focused on the effects of the ratio R/L between the length of the passive, PEGDA segment, R, and the length of the active, PNIPAM one, L. We developed a theoretical model of the crawler, discussed in detail in Supplementary Text S2, and fabricated a series of crawlers with different active-passive ratios.

The results are summarized in Figure 2, where we represent the displacement per irradiation cycle as a fraction of the length of the active segment. The displacement is clearly null for R/L = 0 (only PNIPAM), but then increases steeply, reaching a plateau for ratios greater than R/L = 2/3, corresponding to a displacement per actuation cycle of the order of 0.15 times the length L of the active segment.

The average step size achieved during the data collection for this ratio is on average 7.5 μm; time for contraction of the active segment is 0.5 s and for re-expansion 1 s. These times are comparable to those described in our previous publication, and the peak velocity is therefore around 5 μm/s.¹⁷

A comparable performance is then sustained for increasingly larger values of the ratio R/L, up to R/L = 5 and, theoretically, above. We investigated this experimentally only up to R/L = 5 because we could produce maximally 300 μm long robots, being limited by the maximum achievable width of a microfluidic channel during the synthesis.

The ability to push large passive front segments via relatively small active segment is particularly relevant, since the passive section of a robot does not need to serve just as an “anchor” to simply break the head-tail symmetry of the crawler, but it can accommodate further robot functions. Indeed, our design is suitable for integration with additional, bulky functional regions that can be appended to the front without losing, nor significantly penalizing, the robot crawling ability.

We observed inconsistencies in the displacement per irradiation cycle for every crawler, reflected by the large standard deviation error bars. We attribute this to microscopic defects on the polystyrene surface, as well as subtle inhomogeneities in the illumination conditions (point-spread function of the beam, flickering of the laser).

Despite such variability and the complexity of the phenomenon, we are able, with a basic model, to capture the main effect inducing the locomotion of the microrobot and to qualitatively predict the trend for increasing ratios R/L. As we discuss partially in Figure 2 and further in Supplementary Text S2, the observed variability in deformation and friction, although affecting quantitatively our prediction, maintains nonetheless a similar overall predicted behavior compatible with experimental data.

The most interesting variation occurs in the value of the ratio R/L maximizing the rescaled displacement per cycle: the optimum is quite sensitive to the maximum friction coefficient μ_max obtained by PNIPAM in the cooling phase, whose value could be only partially estimated by sliding data since it corresponds to excessively slow speeds.

Although the performance plateau is present for all admissible values of μ_max, the performance peak observed around R/L = 2 is compatible only with a smaller range, thus improving our previous estimate on the coefficient (Supplementary Text S2; Supplementary Fig. S2).

Robot steering

To enable steering of the microrobots without the need for localization of the stimulus, we appended a pH-responsive polyacrylic acid (PAA) segment to the robot head through simple modification of the stop-flow lithography fabrication procedure. Thus, steerable microrobots are composed of three gel segments—PNIPAM, PEGDA, and PAA—distributed as parallel stripes within the robot “T”-shape (Fig. 3).

FIG. 3.

Crawler trajectory can be controlled by UV illumination of the experimental area. The rear end of the robot is composed of gold-loaded PNIPAM and is responsible for its locomotion. The front side of the robot contains a segment of a pH-responsive gel, positioned slightly asymmetrically with respect to the robot's left-right symmetry. The greater friction of the pH-responsive section means the robot's center of friction is shifted to the right-hand side from the central left-right axis and the robot crawls along a curved trajectory. UV illumination of the solution around the robot causes a drop in the solution pH, which results in the deformation of the pH-responsive segment away from the surface. The deformation moves the center of friction to the left-hand side from the robot's left-right symmetry axis, reversing the direction of the robots curved path. The turning direction can be switched repeatedly by changing the pH of surrounding solution. The microscopy image is a montage of several images, displaying the robot's consecutive positions in time. The scale bar corresponds to 200 μm. UV, ultraviolet.

The stripe orientation is slightly tilted with respect to the front edge of the “T”-shape. Therefore, one arm of the “T”-shape consists predominantly of the PEGDA gel, whereas the other consists of the PAA gel. When swollen, the PAA gel exhibits greater friction than the PEGDA, when sheared along the substrate (Supplementary Fig. S1).

Thus, the center of friction (see Supplementary Text S2 and Vakkipurath Kodakkadan et al.⁴⁸ for detailed definition) of this front side is offset from the main left-right axis of the crawler toward the PAA arm. The forward force (F_CR in Fig. 3)—realized by the expanding PNIPAM segment—acts along the main left-right axis of the crawler, whereas the counteracting friction force (F_FR in Fig. 3) is localized at the offset center of friction. As a result, a torque emerges between these two forces, and this torque is responsible for the forward motion of the crawler along a circular path in the direction of the PAA arm.

The PAA shows pH-dependent volumetric change, collapsing to ∼40% of its original linear dimensions at a pH of 3.2 (Supplementary Fig. S3). In the collapsed state, only the PAA segment of the crawler is shrunk. Since its collapse is isotropic in x, y and, crucially, z direction, the PAA segment retracts from the surface and only the PEGDA segment remains in contact (Supplementary Fig. S4).

Hence, the center of friction gets shifted to the PEGDA arm side and causes the robot to follow a curved path in the opposite direction, that is, toward the PEGDA arm side (Fig. 3). The inverse volumetric change can be achieved by incorporating a basic monomer into the gel structure (e.g., 2-dimethylaminoethyl acrylate) instead of AA. Similarly, the pH range in which the volumetric change occurs does not need to be the same as mentioned earlier, but it can be shifted by compositional changes of gel´s precursor solution.

To change the pH remotely, without disturbing the environment, a photoacid pyranine was dissolved in the solution. Under illumination with UV light, pyranine is excited, which decreases its pK_a, causing a decrease in pH⁴⁹ (Supplementary Text S3). In this experiment, the robot was steered and powered by two global illumination sources—the defocused, 532 nm laser served as a power source for its actuation whereas 400 nm UV light was used to excite the pyranine, collapse the PAA segment, and change the direction of the robot's locomotion.

When left in the dark, the pyranine gradually relaxes back to its ground state, which is reflected in the pH increase. To deal with slow kinetics of this relaxation as well as partial photodegradation of pyranine (Supplementary Text S3), we replaced part of the solution during this step with a fresh one. This returned the robot to its original shape and its original trajectory (in the direction of the PAA arm) was re-established. Alternating the respective low and high pH periods allowed us to navigate the robot over the substrate (Fig. 3).

We consider the presented single DOF steering system, with alternating left/right turning, a promising steering strategy, since accumulation of more DOFs in soft microrobots is inherently complicated. A similar 1 DOF strategy, relying on alternating left-right turning to achieve the desired direction, has been used to steer early remote controlled solid wing flying models, due to the cost of radio-controlled servo motors.

Conclusion

We present a soft light-powered and steered microrobot that does not require spatial modulation of light for operation. The direction of robot motion is not coupled to the illumination position and proceeds along a straight line following the vector active segment–passive segment. This enables omnidirectional parallel motion of multiple robots found in the illuminated area.

The experiments as well as theoretical analysis showed that even a small active segment is sufficient for moving a large front passive segment (R/L = 5). This enables the incorporation of further functionalities into the robot. We demonstrate a steering functionality, where a microrobot turning direction can be switched between left- and right-handed just by on/off UV illumination. As in the case of the actuation light source, the UV light does not need to be focused.

This together provides a system, with no constraints on the spatial distribution of the light sources, which first enables simultaneous operation of multiple robots and, second, may provide flexibility to the way light is brought to the robot. We believe that our approach may allow the operation of light-controlled robots outside strictly defined microscopic setups, where the light will be delivered to the robot by, for example, optical fibers.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

P.G. acknowledges the GAČR Junior Star Grant 21-09732M, and I.R. acknowledges the UCT Dagmar Prochazkova starting grant.

I.R. acknowledges GACR Grant no. 23-05908K. The work of P.C. was supported by Ministry of Education, Youth, and Sports of the Czech Republic (project no. CZ.02.01.01/00/22_008/0004558, co-funded by the European Union).

Supplementary Material

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