Rapid self-positioning and anti-peristaltic adhesion for colon drug delivery using a microneedle robot

Micro-needles containing micro-robots have the ability of transmucosal drug delivery and controlled drug release, providing a promising approach for gastrointestinal drug delivery. However, there are still many challenges, such as complex control patterns, lack of theories of motility and adhesion behavior, failure caused by peristalsis and fluid flow, and the risk of intestinal obstruction. Here, Professor Ren Lei/Assistant Professor Wang Miao of Xiamen University and Associate Chief Physician Cai Shuntian of Xiamen University Zhongshan Hospital designed a non-embolized micro-needle robot for specific colonic drug delivery, which can be dispensed, achieve rapid self-orientation and adherence to the mucosa, counteract physiological peristalsis, and reduce the risk of obstruction. In addition, the detachable layer between the micro-needle and the robot can be degraded within 6 min, which ensures safe excretion under a small excretion force of 20 mN caused by fluid flow. In vivo experiments also demonstrated the effectiveness and feasibility of the robot. These robots can be used as a universal platform to treat diseases such as chronic inflammation and colon cancer to reduce invasive surgical interventions and patient suffering.

Design of robot containing micro-needles

This study constructed a millisecond self-guided micro-needle containing robot for colonic drug delivery. The application mechanism of the robot containing micro-needles is shown in Figure 1. The robot containing microneedles is placed into the colon at a random angle, and can quickly orient itself perpendicular to the mucosa due to the robot's low center of gravity. The microneedle array gradually penetrates the mucosa under the combined force of the robot's gravity and the peristaltic force of the colon. As the detachable layer is gradually degraded by the colonic fluid, the robot detaches from the microneedle array embedded in the mucosa and is expelled from the colon. The inserted microneedles are retained in the mucosa to release the loaded drug as it slowly dissolves.

Figure 1 Design and mechanism of a fast self-orienting microneedle robot for colon drug delivery

Parameter optimization of the robot

The shape and thickness of the robot containing microneedles were optimized to achieve rapid self-positioning and the ability to resist external forces. In order to study the effect of shape on orientation time and stability, three shapes, hemisphere, sphere and ellipsoid, were designed in this study (Figure 2A). Since the initial angle of the robot affects their self-positioning time, the robots of these three shapes were studied at initial angles of 90° and 180°, respectively (Figure 2B). These self-positioning times are shown in Figure 2B (IV). The results show that the orientation time of the hemisphere shape is shorter than that of the sphere and ellipsoid shapes when the initial angles are 90° and 180°. Under motion conditions, the self-positioning of the hemisphere shape occurred within a few seconds (Figure 2C). The

effect of the robot's mass distribution on the orientation time was studied by designing robots with different solid heights of 2 mm, 3 mm and 4 mm, respectively (Figure 2A (III-V)). When the solid heights were 3 mm and 4 mm, respectively, the self-orientation probability was as high as 95%, and the self-orientation time was slightly different (Figure 2D). As shown in Figure 2E , the self-orientation time of the optimized microneedle robot containing random initial angles was observed within 0.6 s.

Figure 2 Robot parameter optimization

Self-orientation properties

Since the colon presents an elastic surface filled with mucus, the self-orientation properties of the microrobots were further investigated on artificial platforms of agarose gel (Figure 3A (I-II)) and mucus (Figure 3B (I-II)). As shown in Figure 3A (III), the orientation time was prolonged with the increase of elastic modulus. The robot with a viscosity of 100 mPa·s could not achieve self-orientation on the mucus surface, while the orientation time was minimal at a viscosity of 1.0 mPa·s. As the slope increased, the self-orientation time of the robot decreased, and the final state of the microneedle-containing robot was that the microneedle tip faced the mucosa (Figure 3C (I-III)).

Figure 3. The robot's self-orientation performance on different surfaces

Assembly of detachable layer and parameter optimization of microneedle array

As shown in Figure 4A, the pointed microneedle array was neatly attached to the robot through the polyvinyl alcohol/polyvinyl pyrrolidone (PVA/PVP) detachable layer, and there were no obvious bubbles on the surface of the microneedles. The robot containing the microneedles was immersed in PBS buffer and shaken on a shaker at 100 rpm (Figure 4B). The detachable layer after 3 min was significantly thinner than the initial time.

The detachable layer around the robot was completely degraded in about 6 min, and the robot was finally separated from the microneedles. Therefore, PVA/PVP as a detachable layer can achieve rapid separation of the microneedle-containing robot. The drug release time of several designed degradable materials ranged from 10 min to 30 days (Figure 4C). The penetration force of the microneedle array to completely penetrate the mucosa was measured to be 0.058 N per microneedle (Figure 4D). From the H&E-stained tissue images, it can be observed that the CS microneedles successfully penetrated the mucosa (Figure 4E). Confocal images of the mucosa using the fluorescent dye Cyanine5.5 (Cy5.5) showed that the penetration depth of the microneedle could reach ≈240 μm ( Figure 4F ).

Figure 4 Design and optimization of detachable microneedle robot

Resistance to colonic peristalsis

This study developed a model to reasonably evaluate the resistance of the robot containing microneedles to colonic peristalsis without detaching from the mucosa (Figure 5A). Peristalsis pushed the robot-containing microneedles forward, and the friction and adhesion between the robot and the colon wall resisted the movement of the robot (Figure 5B). Figure 5C (I-II) shows that when the tensile strength reaches 1.75 N, the robot-containing microneedle can move directly. The results show that the robot containing microneedles can resist colonic peristalsis after being inserted into the mucosa.

Figure 5 Theoretical model of the microneedle-containing robot resisting colon peristalsis

Discharge of the robot after detachment

Due to the wrinkled anatomical structure of the human colon, the anatomical structure of the colon was replicated by using a life-size soft gel model (Figure 6A). As shown in Figure 6B, all robots were stuck at the liquid inlet when the flow rates were 15 m/ms, 75 m/ms, and 135 m/ms, respectively. When the flow rate reached 210 m/ms, the robot was driven by the fluid and moved to the liquid outlet with the flow rate. The propulsion force exerted by the robot at different flow rates was quantified by finite element analysis, and the stress field was evaluated when the flow height was 4 mm (Figure 6C). The flow rate around the robot (Figure 6D) and the resulting surface stress distribution (Figure 6E) were obtained at a flow rate of 210 m/ms. The numerical results showed that the propulsion force Fpropel generated by the surface stress increased with the increase of the flow rate (Figure 6F).



Figure 6 In vitro residence experiment and simulation of the robot containing microneedles

In vivo experiment

The robot containing microneedles was injected into the colon of zebra pigs by colonoscopy (Figure 7A) to evaluate its in vivo characteristics. As shown in Figure 7B, the robot containing microneedles was placed into the colon at an initial angle of 90°, and self-positioning was achieved within 5 s, with the microneedles inserted into the mucosa due to gravity and colonic peristalsis. The robot containing microneedles could be firmly anchored in the mucosa when colonic peristalsis occurred at 30 s and 57 s, respectively.

As the detachable layer of the microneedles containing the robot gradually degraded in the colonic juice, the separation of the robot from the microneedle array was observed within 80 s. The microneedle array then remained in the mucosa and slowly degraded, while the robot could be expelled from the body within 1 h, avoiding colonic obstruction. These results indicate that the roller cup robot can achieve self-oriented insertion and retention of microneedles in the colon, and the robot can be quickly separated and expelled. In addition, the rapid separation and expulsion of the robot from the body verified the high safety of the robot.

Figure 7 In vivo evaluation of the zebra pig microneedle-containing robot

In summary, this study developed a colonic drug delivery-containing microneedle robot that can self-direct to the mucosa without the need for external triggers. The microneedle-containing robot consists of a robot with a low center of mass, a detachable layer, and a microneedle array. These three parts were optimized to achieve rapid self-orientation and resistance to external forces. After the optimization, the robot had a penetration force of 0.058 N per microneedle, allowing them to be fully inserted into the tissue. The robot can be effectively retained under the resistance of colonic peristalsis, but after detachment, the robot cannot resist colon expulsion. In vivo experiments have demonstrated the effectiveness of the microneedle-containing robot, including self-directed microneedle insertion and expulsion of the robot. The robot has a simple structure, does not require external control, reduces the risk of colonic obstruction, and potentially resists colonic peristalsis, making it different from other systems.

Review editor: Liu Qing