A Magnetic Millirobot Walks on Slippery Biological Surfaces for Targeted Cargo Delivery

Small-scale robots hold great potential for targeted cargo delivery in minimally invasive medicine. However, current robots often face challenges in locomoting efficiently on slippery biological tissue surfaces, especially when loaded with heavy cargo. Here, we report a magnetic millirobot that can walk on rough and slippery biological tissues by anchoring itself on the soft tissue surface alternatingly with two feet and reciprocally rotating the body to move forward. We experimentally studied the locomotion, validated it with numerical simulations, and optimized the actuation parameters to fit various terrains and loading conditions. Furthermore, we developed a permanent magnet set-up to enable wireless actuation within a human-scale volume that allows precise control of the millirobot to follow complex trajectories, climb vertical walls, and carry cargo up to four times its own weight. Upon reaching the target location, it performs a deployment sequence to release the liquid drug into tissues. The robust gait of our millirobot on rough biological terrains, combined with its heavy load capacity, makes it a versatile and effective miniaturized vehicle for targeted cargo delivery.


Introduction
Wirelessly actuated small-scale robots show great potential for minimally-invasive medical pr ocedures [1][2][3].Besides their tiny footprints, small-scale robots exhibit high dexterity and pre cise controllability, as they are actuated by physical or chemical fields in a wireless way.Vari ous physical fields have been reported to power robots, for example, magnetic fields [4][5][6][7][8][9][10], ultrasound fields [11][12][13] and light fields [14].Among all actuation schemes, magnetism is on e of the most common modalities used for biomedical applications due to its low attenuation i n biological tissues, strong actuation force and precise controllability.For example, helical mi cropropellers can swim in biological fluids [4][5][6], soft robots use multimodal locomotion for ca rgo transportation [7,8], and biped robot can walk on a substrate surface [9].A capsule robot at centimeter scale was demonstrated to controllably move on the surface of a porcine stom ach for endoscopy and also perform a needle biopsy [10].Despite these advances, there are still many major challenges for small-scale robots to be applied in real medical scenarios in t he human body.
First, real biological environments pose a challenge for a small-scale robot to locomote as th ey are mostly viscoelastic and difficult to penetrate.The tissue surfaces often exhibit complic ated three-dimensional (3D) shapes and features, and are coated with slippery biological flui ds.Most current small-scale robots are designed and tested under idealized lab conditions, s uch as propulsion in water and crawling on flat and dry surfaces.Hence, these results often do not readily transfer to useful schemes in real medical applications.Additionally, they usual ly lack sufficient grip to the slippery biological surface, which inhibits efficient crawling of the r obot.Sharp tips that penetrate the superficial layer of tissues have been used in medical imp lants and larger scale crawling robots to anchor to soft tissues [15,16], however, the anchorin g mechanism has not been widely considered for millirobots due to its difficulties in micro-fab rication and characterization.Here, we tackle this challenge by fabricating 3D sharp metallic tips as robot's feet for efficient anchoring to slippery biological surfaces.
Second, it is difficult to generate a sufficient magnetic field strength or gradient to actuate a r obot while maintaining a large enough space to accommodate a human as the magnetic fiel d decays significantly over distance [17].Mainly two types of magnetic actuation systems ha ve been used for the actuation of small robots.On the one hand, electromagnetic systems e xhibit high controllability and can be completely turned off, however, they have limitations in t he working volume and field strength, and also require huge amounts of electric power and c ooling system.In contrast, permanent magnet systems offer a higher magnetic field strength and a bigger working volume without cooling issue, but are limited in the driving frequency a nd suffer from mechanical noise and vibration [18].Here, we develop a permanent magnet s et-up to wirelessly actuate the walking robot, which can incorporate a human body aiming at real medical applications in the future.
In this paper, we report a millirobot that can walk on a slippery biological surface and inject d rugs into the tissue at a target location.The millirobot is actuated wirelessly by an oscillating magnetic field, generated by a set of permanent magnets, comprising a static pair of magnet s and a rotating magnet.The robot has specially designed feet, which can alternatingly anch or themselves to soft biological tissues.By alternating 3D rotations around the two front feet, the robot exhibits a bipedal gait.The locomotion was experimentally measured, geometricall y validated by numerical simulations, and an optimization of the walking speed on rough and slippery terrain was carried out.The robot is capable of climbing on vertical walls, carrying fo ur times of its own weight.Changing the actuation mode of the permanent magnet, the robot is capable of performing a second motion sequence, which deploys the carried cargo, i.e., to inject liquid drug into the soft tissues.The motion of the millirobot is controlled to follow arbitr arily-designed trajectories on a 3D surface.The reported method brings new insights for desi gning small-scale robots crawling on rough and slippery biological surfaces and may lead to many potential applications in minimally-invasive medical procedures.

Design and fabrication of the millirobot
As shown in Fig. 1a, the millirobot consists of three parts: a chassis with four feet, a magneti c actuator, and a cargo, for example a capillary loaded with drugs.The chassis is made of M olybdenum (>99% purity, 50 μm thick) with four feet, i.e. sharp tips (~10 μm at the apex), fab ricated by femtosecond laser cutting (MPS FLEXIBLE, ROFIN-BAASEL Lasertech, Germany ) and folded towards the bottom by pressing against a metallic rod (OD 1.2 mm).The actuat or is a cylindrical permanent magnet magnetized along the longitudinal direction (NdFeB-N4 5, OD 1 mm, Height 1 mm, Supermagnete, Germany).It was assembled on the chassis usin g an adhesive (Loctite 401, Henkel, Germany).The cargo was assembled to the actuator.Fo r instance, a capillary (OD 1 mm, ID 0.8 mm, CM Scientific Ryefield Ltd.) was diced to a shar p tip by a diamond cutter and affixed to the upper edge of the robot, positioned to pierce the t issue surface when the robot is tilted over 90 degrees.This enables the robot to inject liquid drugs effectively into soft tissues at the target location.

The permanent magnetic actuation system
Fig. 1b and 1c show the magnetic actuation system.The system comprises three permanent magnets, which generate a superimposed oscillating magnetic field in the working volume at the center of the set-up.A static magnetic field Bh is generated by a pair of magnets M1 and M2 (NdFeB-N45, 110.6 × 89 × 19.5 mm3, Supermagnete).And an oscillating magnetic field Br is generated by a third magnet M3 (NdFeB-N40, 50.8 × 50.8 × 50.8 mm3, Supermagnete) .The system has in total two rotational degrees of freedom as control inputs, labeled as α an d β respectively.It is possible to achieve controlled rotation of M3 around the X-axis at an an gle α using a stepper motor (23HS30-2804Sm, OMC Co. Ltd., China).The robot's walking di rection can be controlled by rotating the magnetic actuation system around the Y-axis at a co ntrolled angle β.The resulting magnetic field of the system, represented by the vector B, rot ates on the lateral surface of a virtual cone as shown in Fig. 1d.The vector B is determined by the distances of the permanent magnets W and D (Fig. 1c), and the angle α, which define s the two rotational degrees of freedom θ and φ of the millirobot.
The robot aligns its magnetic moment to the applied external magnetic field B, thus the two a ngles mentioned above determine the gait of the millirobot.The actuation system was design ed to exhibit a large accessible space that is suitable for future medical applications on a hu man patient.As shown in Fig. 1c, W (the working width between M1 and M2) and D (the dist ance from the surface of M3 to the center of the working volume) are larger than 50 cm and 15 cm, respectively.

Numerical simulation of the magnetic actuation system
3D numerical simulations of the magnetic actuation system were performed using the AC/DC Module in COMSOL Multiphysics (COMSOL Multiphysics, Burlington, MA, USA).The centerto-center distance from M1 to M2 and to M3 were defined as 484 mm and 136 mm, respectiv ely.To simplify the model, the surrounding air (1000×1000×1000 〖mm〗^3) was defined as a magnetic insulation boundary condition.The simulation was conducted as steady-state ma gnetic field at the angle α from -75˚ to 75˚ with a step size of 1˚ for the validation of a magnet ic flux density in the permanent magnetic actuation system.An integrated physics-controlled mesh was used for extremely fine element size.

Motion sequences of the millirobot: locomotion and deployment
The robot exhibits a walking motion, as the B field rotates on the virtual cone surface (Fig. 1  d).The gait is illustrated in Fig. 2a.At the beginning, α = 0˚, the robot stands at the angle of θ (see Fig. 1d).The zero degree of α is defined when the north pole of M3 points upwards al ong with Y-axis.When α increases up to 72˚, the robot turns to the right (orange arrow) while the front right (FR) foot remains anchored to the biological soft tissue (orange circle).Revers ely, when the α decreases from 72˚ to 0˚, the robot turns to the left (green arrow) while the fr ont left (FL) foot anchors to the tissue (green circle).By repeating the motions, the robot mov es forward along the X-axis direction.
The second motion of the scheme is cargo deployment.As illustrated in Fig. 2b, when the m agnetic field rotates to 180˚ or more, the robot flips by following the external magnetic field B. The tip of the capillary attached on the robot touches the tissue surface when α is approxima tely 90˚.As the magnetic field keeps rotating, the robot stands upside down on the apex of th e capillary.The magnetic field gradient pulls the robot towards the tissue surface, and the tip can penetrate soft tissue and inject drug into it.The robot rotates its body back to the walkin g motion (α = 72˚) when the drug delivery is done, and can move by the walking motion to th e next target location.During the motion of deployment, the robot loses its anchoring of the f eet and shortly after, the capillary tip enters the tissue to deploy the cargo, which also acts a s a new anchoring point to fix the robot.Hence, slipping can be avoided in both motions 1 an d 2.

Testing of the millirobot on biological surfaces
The millirobot was tested on a hydrogel phantom as well as on ex vivo animal tissues.A 3D blood vessel structure (a 10 mm vessel separates to two smaller branches with the width of 7 mm and 3.5 mm) was designed in SOLIDWORKS 2021 (Dassault Systemes, France), and printed using a 3D printer (Form 3L with the material Elastic 50A, Formlabs Inc., USA).The s tructure was then molded with 3.4 wt% gelatin hydrogel (gelatin from porcine skin, G1890, Si gma-Aldrich, Germany) mimicking a soft tissue (porcine brain) [19].A fresh calf liver was obt ained from a local butcher for ex vivo experiment.It was stored at 4 °C and used for experim ents within 24 h after sacrifice of the animal.The motion of the robot was recorded by a Can on EOS RP camera with a 60 mm lens (Canon, Japan) and a USB camera (acA2440-75ucM ED, Basler, Germany) with a lens 18-55 mm (Canon).The video was analyzed using ImageJ (1.53t, National Institutes of Health, USA).

Characterization of magnetic field for actuation
Experimental-and simulation results of the magnetic flux density and the magnetic gradient are shown in Fig. 3.The sideview of the actuation system in XY plane is illustrated in Fig. 3a.M1 and M2 generate a magnetic field in the direction of +X and M3 generates a magnetic fiel d that rotates around the X-axis.The center point of the two stationary magnets is defined as (0, 0, 0).At a vertical distance of 20 mm (i.e. the position (0, 20, 0), the robot achieves the fa stest velocity while maintaining its walking stability (see Fig. 5 for detailed motion characteriz ation).The working volume for a homogeneous magnetic field is defined by the angle differe nce of ± 10˚ from the angle at the measurement position.A working volume of 35 × 40 × 35 mm3 for the robot is achieved with the current set-up.
The magnetic flux densities in X-, Y-and Z-axes at the measurement position are presented i n Fig. 3b.The magnet M3 rotates α from -75˚ to 75˚, and the experimental and simulation re sults match perfectly at all angles of the M3.The simulation is performed in a specific range as the robot's walking motion angle of α is limited to ± 72˚ for a stable walking motion of the r obot.Fig. 3c shows the magnetic flux density and the field gradient from the measurement p oint along the Y-axis at α = 0˚.As the measurement point moves away from the rotating mag net, the magnetic flux density and magnitude of the magnetic gradient decrease.Both magn etic flux density and magnetic gradient show good agreement between measurements and s imulation.The gradient is important to consider as it keeps the anchoring foot in firm contact with the surface, especially during the vertical climbing case (see Fig. 6b for details).
The numerical simulation results of the magnetic flux density in the YZ cross-section are sho wn in Fig. 3d.M1 and M3 are marked to show the position, and clockwise rotation of α is defi ned as positive.Initial pose of the robot is defined when α = 0˚.The zero-degree of α is defin ed when the north pole of the magnet M3 pointing upwards along the Y-axis.The millirobot a chieves the walking motion by rotating the magnet M3 with rotating angle α.The simulation r esults of the magnetic field vectors B and the corresponding robot's poses are shown in Fig. 3e.

Walking motion
The image sequence in Fig. 4 shows the robot's movement for a half cycle.The anchoring of the front feet is identical to the schematics as shown in Fig. 2a.Specifically, the front right (F R) foot is anchored at the start to facilitate a right turn.Then the front left (FL) foot touches th e ground and anchors upon rotating back to α = 0˚, enabling the next left turn.Due to the rot ational motion along with anchoring mechanism, a displacement of the robot is observed eve ry half cycle of actuation.The initial and last position are marked by orange and blue dashed line, respectively.The displacement is clearly visualized in the SI Video S1.

Locomotion characterization of the millirobot
Figure 5 shows the characterization results of the millirobot's locomotion under different con ditions, by varying the oscillating angle, the pitch angle, the oscillating frequency, and the car go weight, respectively.
The stride length of the millirobot was first measured while moving on the hydrogel phantom at different oscillating angles.Fig. 5a shows the experimental-and simulation-results.The av eraged moving speeds range from 1.55 to 1.70 mm/cycle in the experiments, which match th e simulation results very well with an average error of 10%.Experimental results exhibit a sli ghtly lower stride length than the simulation because in the real case, the feet of the robot pe netrate the soft material surface, which leads to a decrease of the real turning radius compar ed to the one used in the simulation.When α = 90˚, the robot moves 1.8 % faster than α = 7 2˚, however, the robot falls on the surface of hydrogel phantoms, when the angle α exceeds 72˚.This is mainly due to surface tension of the fluid layer which was also observed when th e robot moves on the biological tissues.To minimize the risk of being stuck at a location, the angle α is limited to 72˚ for all other experiments.Fig. 5b shows the effect on the moving speed of the robot, when modifying the pitch angle θ from 39˚ to 66˚.The speed increases almost linearly from 0.3 to 2.0 mm/s.The pitch angle is determined by the ratio between the magnitudes of Br and Bh (Fig. 1b).While the positions of the magnets M1 and M2 are kept stationary, as the rotating magnet M3 approaches the robo t, the magnetic field strength Br becomes larger, thus the pitch angle increases.The five pitc h angles presented in Fig. 5b correspond to the measured positions in Y ranging from -20 to 20 mm in 10 mm steps.It was observed that the robot does not move when the pitch angle e xceeds 70˚, mainly due to the strong interaction between the rotating magnet and the robot's body.Therefore, further experiments are performed using a pitch angle of 66˚, which provide s a stable and fast crawling motion.
Experimental results also show that the robot's crawling speed also increases linearly with th e oscillating frequency, as shown in Fig. 5c.It is challenging to maintain a stable walking mot ion over 1.5 Hz and consequently, an oscillating frequency of 1.2 Hz was chosen to ensure a stable walking motion of the millirobot.
As shown in Fig. 5d, the weight of the cargo has a tremendous effect on the walking speed.When the cargo weight is added from 0 to ~100 mg, the moving speed decreases from 2.0 t o 1.4 mm/s.The robot's maximal load capacity is ~100 mg, which is approximately four time s of its own body weight.

The control of the millirobot
The magnetic actuation system shown in Fig. 1b offers the possibility to control the in-plane crawling motion of the robot with two degrees of freedom to realize complex trajectories.The overlaid image of the robot's complex trajectory control videos (Video S2 and S3, playback s peed at three times) is shown in Fig. 6a and Fig. 6b.During one trajectory, the maximum osc illating angle of α was set as 72° and oscillating frequency was fixed at a constant of 1.2 Hz.Complex trajectories were achieved by manually rotating the magnet set-up including M1, M 2 and M3 by the angle β, and the sample table was kept stationary.The rotation of the set-u p induces the rotation of the magnetic field around the Y-axis, which steers the robot to align with the external field.It allows the control of the robot to reach any in-plane target as shown in Fig. 6a.The robot is capable of rotating at the same position to make sharp turns, thus it o ffers the possibility of precise motion control.Fig. 6b illustrates the time sequence of the robot climbing a vertical wall by anchoring to the surface by the magnetic gradient force generated by the magnet M3.The robot can crawl at a speed of 0.9 mm/s, facilitated by a perpendicular magnetic gradient force to the surface of 0.24 mN for anchoring.The gradient force is ~ 3 times larger than the gravitational force on t he millirobot.It was also observed that the robot can walk upside down on a flat surface.The controllable crawling against gravity makes it suitable for biomedical applications in complexshaped biological lumen systems.
3.5.Cargo deployment Fig. 7 demonstrates the deployment of the robot's cargo on a hydrogel phantom and a calf li ver.The robot was actuated forward by the oscillating magnetic field and executed the deplo yment scheme by rotating the angle α to 180˚, as shown in Fig. 2b, to release the drug.Two types of cargos were used for demonstrations, i.e. a cotton pad for medical dressing and a c apillary with a sharp tip for liquid drug injection into the surface of a soft tissue.After applying the cotton pad for 20 s, the red stain is found on the surface of the phantom (Fig. 7a).When the robot reaches a target position (Fig. 7b), it turns its body upside down for delivery.The m agnetic gradient (0.1 T/m) generates a pulling force of ~ 0.3 mN on the robot, which provides enough force for the tip of the capillary to penetrate into the liver tissue.

Discussion
In this study, we present a new approach for a millimeter-scale robot to locomote on slippery biological surfaces to deliver drug using a customized magnetic actuation system.The robot shows the capability to walk and climb on various surfaces and deliver drugs using two types of cargo.The experimental results of the magnetic field and the motion of the millirobot are i n good agreement with theoretical simulation.
We develop a permanent magnetic set-up for the wireless actuation of millirobots at the scal e relevant to the human body.The set-up achieves an area of 50 cm x 15 cm and generates a flux density of ~7.5 mT at the center point.The average shoulder widths are 41 cm and 37 cm for male and female, respectively [20], which fit into the current system.The set-up can b e extended to even larger distances by using larger permanent magnets.In a future surgical procedure, a human patient could be in a prone position, where the shoulders fit in between t he two static magnets, and the anterior side is close to the rotating magnet.It shows the pot ential to use a similar set-up to actuate millirobots in the biological lumens in internal organs, such as the gastrointestinal tract and the biliary duct in the liver.
The walking motion of the millirobot is characterized under different conditions.Experimental results show that increasing the pitch angle or oscillating frequency can increase the robot's speed but may also lead to instability.The walking motion allows the millirobot to carry four ti mes the body-weight, and also to crawl on a vertical surface of a hydrogel phantom.To allow the robot to climb on a vertical surface of real biological tissues, future work will be carried o ut to optimize the magnetic field, which enables the magnetic torque to be high enough to lift the robot's foot and the magnetic gradient force to be high enough to attract the robot to anc hor to the vertical surface.By balancing these factors, we can design a millirobot that is opti mized for a specific application.
The experimental result of the stride length during full fits the model of the robot's dynamics with one foot anchoring firmly to the surface.This result clearly shows that there is no slip du ring the 3D rotation of the millirobot, owing to the anchoring mechanism of the feet.One of th e key strengths of our approach is the ability of the millirobot to move on a slippery biological surface in a controlled manner, which is in general a challenge for traditional small-scale rob otic systems [21].It is reported that the frictional coefficient of wet hydrogel is 95 times small er than dry surfaces of a paper [22], which poses extra challenge for crawling robots based o n the friction to move on wet biological surfaces.Here, our millirobot utilizes sharp apices to penetrate soft tissues, which offers strong anchoring point for the robot's forward motion.Ex perimental results show that the anchoring is strong enough to hold its position on both hydr ogel phantoms and calf liver tissues.
The anchoring mechanism helps the robot to hold its position on a slippery surface with the h elp of the magnetic gradient.It is observed from the experiment that only a robot with sharp t ip achieve locomotion on a slipper surface not like the tumbling robots which generally requir e a rough surface such as poly(methyl methacrylate) [7] or dry paper [23].The anchoring me chanism is based on the penetration of the sharp metallic tips into the soft tissue, which is pr otected by a wet mucus layer.In the experiments, permanent grooves on the order of 100 mi crometers deep were observed on the surface layer.However, typical mucus layers, for exa mple in the stomach, are around 1 mm thick [24], indicating the potential harmlessness.Futu re studies need to be carried out on real animal tissues to investigate the penetration depth o f the feet to ensure safety in medical applications.Furthermore, to get closer to real biomedic al applications, a consideration of biocompatibility is mandatory.Molybdenum, the material of robot's feet, has been reported as highly biocompatible, and other materials such as medical -grade stainless steel can also be applied to build the robot's feet.The magnetic materials N dFeB on the robots are not biocompatible, but they can be covered with biocompatible coatin gs [25].
The millirobot can readily switch between two types of motion, walking and cargo deploymen t.It can be controlled to navigate to an arbitrary in-plane target location, then deploy the carri ed drug, and it may move further to a second location for drug release at multiple locations.I t is reported that small-scale robots can transport and release cargos [2,26].However, to our knowledge, the cargos on a millirobot were normally released in the surrounding fluids but n ot directly into the soft tissues.Our deployment mechanism allows the injection directly into s oft tissues which will be beneficial for many medical applications, especially for the retaining of drug at the target location.

Conclusions
In conclusion, our study shows the capabilities of a human-scale wireless magnetic actuation system for powering a millirobot to walk on slippery biological surfaces.The robot's walking a nd deployment motions are achieved through precise control of the magnetic field, enabling i t to walk on various terrains and deliver cargo to desired positions.The versatility and adapta bility of this magnetic actuation system makes it a promising platform for biomedical applicati ons such as drug delivery and minimally-invasive medical procedures.Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1,Video S1: Walking motion, Video S2: Complex trajectory, Video S3: Vertica l climbing, Video S4: Deployment on phantom, Video S5: Deployment on Liver.

Funding:
This work was partially funded by the Vector Foundation (Cyber Valley research gr oup), the MWK-BW (Az.33-7542.2-9-47.10/42/2)and the European Union (ERC, VIBEBOT, 101041975).Data Availability Statement: All data needed to evaluate the conclusions in the paper are pre sent in the paper.

Figure 1 .
Figure 1.Design of the walking millirobot and the magnetic actuation set-up.(a) Schematic a nd image of the millirobot.The scale bar is 1 mm.(b) Schematic of the actuation system bas ed on a pair of stationary magnets M1 and M2 and a rotating magnet M3.(c) Picture of the magnetic actuation system.(d) Diagram showing the superimposed magnetic field vector at t he center of the working volume.

Figure 2 .
Figure 2. Motion schemes of the millirobot.(a) Motion 1 shows a half cycle of the gait of the r obot to move forward.The oscillating angle α for walking is limited to maximally 72˚ to preve nt the body from touching the ground and to stabilize the gait of the robot.(b) Motion 2 show s the cargo deployment scheme, e.g.drug injection into the tissue by the capillary.FL: front l eft, FR: front right.

Figure 3 .
Figure 3. Experimental and simulation results of the magnetic actuating system.(a) Schemat ic of the system.The working volume (35 x 40 x 35 mm 3 ) and the measurement position (0, 20, 0) are marked in the yellow box and the black cross, respectively.(b) Magnetic flux densi ty of the experimental (dots) and simulation results (Solid lines) at the measurement position (-75˚ ≤ α ≤ 75˚).(c) Magnetic flux density and magnetic gradient along the Y-axis for an actu ation angle of α = 0˚.Solid lines and dots represent simulation and measured data, respectiv ely.(d) Simulation result of the magnetic flux density at different angles of α.The north pole of M3 points upwards along with Y-axis when α = 0 ˚.The colormap indicates the magnitude of the magnetic flux density B. (e) Millirobot's poses and corresponding magnetic flux density at the measurement position.The green arrow indicates the direction of the external magneti c field.

Figure 4 .
Figure 4. Snapshots of the walking motion during a half cycle.The yellow dashed line labels the start position, and the blue dashed line labels the end position after a half cycle.The scal e bar is 1 mm.

Figure 5 .
Figure 5. Characterization of the locomotion of the millirobot.(a) The stride length at different oscillating angles.(b-d) The moving velocity at a constant oscillating angle of 72° but varying the pitch angle θ (b); the oscillating frequency (c); and the cargo weight (d).All scale bars ar e 1 mm.

Figure 6 .
Figure 6.Control of the millirobot to achieve complex trajectories on hydrogel phantoms.(a) The robot follows a complex trajectory.Blue dashed lines depict the moving trajectory, and y ellow dashed line highlights the boundary of the phantom.(b) The robot climbs up a vertical wall.The arrow G indicates the direction of gravity.All scale bars are 1 mm.

Figure 7 .
Figure 7. Targeted drug delivery by a millirobot.(a) Application of a drug using a cotton pad o n a gel phantom (Video S4, playback speed at three times).(b) Locomotion of the robot on a liver tissue to a target position and injection of a drug into the liver tissue by capillary (Video S5).The target location is labeled by a cross and the dashed circle highlights the applied dru g on the surface.All scale bars are 1 mm.