TECHNICAL FIELD

The present invention relates to the technical field of soft robot, and more particularly, to a method for making hydrogel-based soft robots with stretchable coating.

BACKGROUND

It is inevitable that medical apparatus and human tissues come into contact with one another during surgery, and it is necessary to minimize adverse stimulation or physical damage to human body caused by such contact or heated instruments. For example, the size, heat, burning, and rigidity of medical apparatus should be reduced, and incision or wound should be minimized. As a basic device during surgery, the endoscopic instrument is generally used to enter into the body of a patient and feedback image information on the operation site in real time. Therefore, such device usually needs to have a spindly shape, which is easy to be accepted by the patient and can provide multiple degrees of freedom.

Soft robot is a new type of flexible robot. As a new and rapidly growing field of robot engineering, the soft robot has broad application prospect in the fields of human-computer compliant interaction and interaction between a robot and an unknown environment. Compared to a traditional, rigid and inflexible robot, the soft robot can output a smaller pressure load to an external environment and is less likely to cause injury or damage to an applied object in the external environment. Therefore, the soft robot can be applied to many human-computer interaction applications such as rehabilitation robots and minimally invasive surgery robots. In the case of minimally invasive surgery inside a living body, the soft robot with heat-resistive coating will have great potential in minimally invasive surgery, which allows flexible and controllable actuators to minimize the damage to surrounding tissues as much as possible while providing necessary flexibility and strength. For example, a pneumatic actuator of the soft robot is generally designed to be used inside the living body and made of a flexible biocompatible material that can withstand the pressure in a particular biological internal environment. The main actuating mechanism of the soft robot with stretchable coating is generally made of a non-metallic flexible material, such as silica gel, to increase flexibility, compressibility and safety in human-computer interaction.

The material forming the main actuating mechanism of the soft robot is generally soft and elastic, and has specific hardness and flexibility. Silica gel is considered as a viable material for making the soft robot, since the silica gel has flexibility and is ease to manufacture. However, although it is easy to manufacture silicone components, this technology is currently limited by that the silica gel cannot be adapted to configurations of small/sophisticated soft robots. This is because the silica gel has high viscosity and it cannot be cast and solidified in a small mold. Moreover, the use of silica gel in medical applications also presents a problem that the silica gel poses a threat to biocompatibility. In the internal environment of human body, silica gel has a tendency to release toxins, which causes an immune reaction and leads to infection. In addition, it is worth noting that silica-gel-typed soft robots fabricated by using pre-prepared silica gel mixtures generally have standard mechanical properties, so that they are impossible to be customized and applied according to specific scenes.

The hydrogel is one of the most commonly used materials in bioengineering for a long time. With the high water content, the hydrogel could be an ideal biocompatible material for tissue engineering and cell culture. The cross-linked hydrophilic porous structure of the hydrogel can also absorb and release water easily and maintain structural regularity. In some cases, the hydrogel has been used to construct a millimeter-scale gel gait device which can make response to electrical stimulation. With the emergence and continuous development of multi-dimensional 3D bio-printer, the fabricated hydrogel can be scaled down to micron scales comparable to cell interactions. Although this is a promising technique, there are limitations in the fabrication of hydrogel, for example, due to the complexity of swelling, diffusion processes and soft recovery properties, it is difficult to achieve troublesome multi-step polymerization processes. Due to the high water content and the cross-linked structure, the commonly used hydrogel, such as agarose and polyethylene glycol (PEG), generally has very low stretchability and compressibility.

Additionally, stretchable flame-retardant coating would be a desirable feature in the applications of protective fabrics, wearable technologies with high battery safety, and soft robotics that can work in a fire scene. In general, it is difficult to combine effective flame retardancy (requiring dense, continuous layers to delay the oxygen permeation and prevent the escape of pyrolysis products) with high stretchability (achieved by reversible molecular reorganization with randomly distributed elastomeric networks) due to the conflicting requirements on the desired molecular structures. Although some organohalogen flame-retardants are stretchable, the products have been banned recently by U.S. Consumer Product Safety Commission due to serious health problems. Therefore, major challenges remain for the development of flame-retardant coatings that are stretchable, low cost, eco-friendly, and capable of being integrated as protective skins for mechanically dynamic devices such as soft robotic machines or wearable technologies.

Among many inorganic retardants, montmorillonite (MMT) continues to be one of the frequently used nanomaterials to enhance the flame retardancy of targeted objectives. These MMT nanocoatings are strong under tension or compression, but rather undergo brittle failure under stretching. Another issue is the low heat resistance of MMT: although the MMT can retard the oxygen penetration and prevent the fire from beginning, the inorganic nature makes MMT an inefficient heat insulator.

Here we demonstrate a facile fabrication process for MMT-hydrogel two-layer architectures that are with high stretchability, effective flame retardancy as well as high heat insulation, and the bilayer devices can be applied as flame-retardant protective skins for soft robotic grippers and nitrile gloves. We first prepared the two-dimensional (2D) MMT nanosheets and deposited them on thermally-responsive substrates followed by a substrate shrinkage to deform the planar MMT nanocoatings into higher dimensional textures with wrinkled or crumpled features. The textured MMT nanocoatings demonstrated low oxygen permeability and can be transferred on hydrogel substrates to achieve the MMT/hydrogel composites with high mechanical stability. The bilayer devices are washfast and can be reversibly stretched/relaxed. The MMT hybrid composites were further evaluated as stretchable flame-retardant barriers under extreme deformation and can survive after repeated combustibility tests. Also, with incorporation of hydrogel, the heat transfer can be successfully retarded, so the whole stretchable barriers can remain at close to room temperature after combustion testing. The stretchable MMT protection was further utilized as flame-retardant skin for both soft robotic actuators and daily-use nitrile gloves. With the mechanically-stable MMT flame barriers, the soft pneumatic actuator was capable of continuous inflation/deflation under combustion and can act as compliant grippers for manipulating and rescuing the irregular objects from the flame. The conformal MMT nanocoatings can be further applied on inflammable clothing that can endure direct flame contact and retard heat transfer. Overall, we provide a generalized fabrication route that can create mechanically-stable, stretchable nanomaterial/elastomer devices by harnessing the surface instability to pattern the nanomaterial coatings followed by infiltrating hydrogel as soft backings. It is also of great interest for further exploitation for the applications including wearable technology, lightweight personal protective equipment, heatproof and fireproof coatings, and functional soft robotics.

SUMMARY

In view of the above, in order to address the above disadvantages of the prior art, it is an object of the present invention to provide a new method for making hydrogel-based soft robots with stretchable coating.

To achieve the above object of the present invention, a method for making a soft robot with stretchable coating is provided, including the following steps:

providing a mold for an actuator member of the soft robot with stretchable coating;

preparing a hydrogel mixture including agarose, acrylamide, N, N'-methylene-bisacrylamide, photoinitiator and deionized water;

heating the hydrogel mixture to dissolve the agarose completely, and maintaining the hydrogel mixture to be in a liquid form;

introducing the hydrogel mixture in the liquid form into the mold;

cooling the mold into which the hydrogel mixture has been introduced, to form agarose gel;

exposing the mold into which the hydrogel mixture has been introduced to ultraviolet radiation, and rotating or inverting the mold continuously, so that the ultraviolet radiation is evenly distributed on the hydrogel mixture, the acrylamide is polymerized to form polyacrylamide, and the hydrogel mixture forms double-network hydrogel; cooling the mold with the double-network hydrogel therein at a room temperature;

demolding the formed double-network hydrogel from the mold to obtain a double-network hydrogel-based actuator member, wherein the actuator member includes three pneumatic cavities; and

connecting the three pneumatic cavities of the actuator member to three gas injection pumps through a flexible pipe respectively, each of the three gas injection pumps being driven by a motor.

In one embodiment, the mold includes: a cylindrical housing; a cylindrical pin, disposed at a central position inside the housing along a central axis of the housing; and three rectangular pins, disposed inside the housing and equally spaced around the cylindrical pin; wherein the housing is provided with a through hole at bottom, and the through hole is adapted to introduce the hydrogel mixture into an interior of the housing.

In one embodiment, the mold is made of acrylic acid.

In one embodiment, the hydrogel mixture includes 126.74 mg agarose, 2.85 ml acrylamide, 0.15 ml 5 mg/ml N, N'-methylene-bisacrylamide, 35.99 mg photoinitiator and 2.00 ml deionized water.

In one embodiment, the photoinitiator is 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone.

In one embodiment, introducing the hydrogel mixture in the liquid form into the mold includes immersing the mold into a container filled with the hydrogel mixture and sucking the hydrogel mixture into the mold by suction.

In one embodiment, the mold into which the hydrogel mixture has been introduced is exposed to the ultraviolet radiation for 120 minutes.

In one embodiment, after demolding the double-network hydrogel from the mold, a winding machine is used to form threads around an outer surface of the obtained double-network hydrogel -based actuator member.

In one embodiment, the method further includes changing elasticity, strength and rigidity of the obtained double-network hydrogel-based actuator member, by changing a concentration of the N, N'-methylene-bisacrylamide or changing a duration in which the mold into which the hydrogel mixture has been introduced is exposed to the ultraviolet radiation.

In one embodiment, the method further includes storing the double-network hydrogel-based actuator member at a temperature below zero degrees Celsius.

According to the method for making the soft robot with stretchable coating of the present invention, double-network hydrogel can be obtained from agarose gel formed by agarose and polyacrylamide polymerized by acrylamide. This kind of soft robot with stretchable coating based on double-network hydrogel is highly stretchable, and has mechanical properties applicable to soft robot applications, so that the soft robot with stretchable coating has a more compliant and flexible human-computer interaction in the human body. Moreover, this type of soft robot with stretchable coating based on double-network hydrogel maintains the original biocompatibility of the hydrogel and is less likely to cause immune responses of the human body.

Compared with the troublesome multi-step polymerization process in a traditional hydrogel manufacture, the present invention adopts a "one-pot" method to prepare double-network hydrogel, which simplifies the manufacturing process. In addition, the mechanical properties of the double-network hydrogel can be changed by changing components and manufacturing processes of the double-network hydrogel, for example, the concentration of N, N'-methylene-bisacrylamide, the duration of exposure to ultraviolet radiation and the standing time of the double-network hydrogel. Therefore, a special double-network hydrogel can be customized according to a specific application of the soft robot with stretchable coating.

The double-network hydrogel with stretchable flame-retardant coating prepared by the present invention can be widely used in manufacturing various robots with stretchable coating (such as surgical robots), since it has biocompatibility, heat-resistive, comparable mechanical properties and customizability.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 A is a schematic structural diagram illustrating an actuator of a soft robot.

FIG. IB is a schematic structural diagram illustrating a main body of the actuator shown in FIG. 1A.

FIG. 1C is a sectional diagram illustrating a distal end portion of the actuator shown in FIG. 1A.

FIG. ID is a sectional diagram illustrating a combination of the distal end portion and the main body of the actuator shown in FIG. 1A.

FIG. 2A is a photo showing a top view of an actuating mechanism of the soft robot.

FIG. 2B is a photo showing a side view of the actuating mechanism shown in FIG. 2A.

FIG. 2C is a photo showing a control mechanism of the actuating mechanism shown in FIG. 2A.

FIG. 2D is a photo showing a pressure sensor panel of the actuating mechanism shown in FIG. 2A.

FIG. 3 is a flow diagram illustrating a method for making a soft robot according to one embodiment of the present invention.

FIG. 4 is a schematic diagram illustrating generating threads on an outer surface of an actuator of a soft robot by using a winding machine. DETAILED DESCRIPTION

Reference will now be made to the drawings to describe embodiments of the present disclosure in detail, so that the above objects, features and advantages of the present invention can be more apparent and understandable. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention can be implemented in many other ways which are different from those described herein, and those skilled in the art can make similar improvements without departing from the essence of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

The following describes the soft robot with stretchable coating targeted by the present invention first. The soft robot with stretchable coating can be used as a new type of medical robot, such as a surgical instrument, which can change with the shape and size of a human tissue or organ and minimize damage to surrounding tissue while providing the necessary flexibility and strength. The soft robot with stretchable coating is mainly composed of an actuator, a stretchable coating layer and an actuating mechanism. The actuator is used to perform corresponding tasks, and the actuating mechanism is used for driving the actuator to move.

As shown in FIGs. lA-lD, the actuator of the soft robot with stretchable coating may include two tubular portions: a main body 11 and a distal end portion 12. The distal end portion 12 is telescopically nested within the main body 11. The main body 11 includes: a first circular cavity 111 extending along the longitudinal direction of the main body 11 and located in the center; and three first rectangular cavities 112 equally spaced around the circumference of the first circular cavity 111. The three first rectangular cavities 112 are at a 120 degree angle from each other with respect to the first circular cavity 111. The distal end portion 12 has a structure similar to that of the main body 11, including: a second circular cavity 121 extending along the longitudinal direction of the distal end portion 12 and located in the center; and the three second rectangular cavities 112 equally spaced around the circumference of the second circular cavity 121. The first circular cavity 111 and the second circular cavity 121 are used to accommodate wires of a micro-camera. The distal end portion 112 and the second rectangular cavity 122 act as a pneumatic gas chamber of the actuator, each of which is rectangular in cross-section, so that the pressure applied on the gas chamber can be maximized (pressure = force / area), when the gas is pumped in a form that the total cross-sectional area is minimized. The diameters of the main body 11 and the distal end portion 12 may be 9 mm and 5 mm, respectively. The main body 11 allows a better and more precise control of the actuation pressure and movement, while the distal end portion 12 allows a better operation in a smaller cavity.

FIGs.2A-2D are photos showing an actuating mechanism adapted to drive the above actuator. The actuating mechanism includes six injection pumps adapted to inject gas into or exhaust gas from the six rectangular cavities (three first rectangular cavities 112 and three second rectangular cavities 122) of the actuator, respectively. Gas inlets and outlets of the injection pumps are respectively connected to the rectangular cavities through respective flexible gas pipes. Six stepper motors are used to linearly drive the six injection pumps respectively, so that the amount of gas supplied to each rectangular cavity in the actuator of the soft robot can be controlled, to adjust the gas pressure in each rectangular cavity. Each of the main body 11 and the distal end portion 12 of the actuator is controlled by three stepper motors, and thus each of the main body 11 and the distal end portion 12 has three independent degrees of freedom. This gives greater flexibility to the entire soft robot with stretchable coating. The actuating mechanism may also include six pressure sensors connected to the six injection pumps, respectively, and each pressure sensor is used to detect the gas pressure and transmit the detection result to a pressure sensor panel. An Arduino Mega circuit board and a Lab View platform can be used to control the stepper motors to adjust the gas pumping rate of each injection pump and the gas pressure applied to each rectangular cavity of the actuator. The entire actuating mechanism may be placed in a transparent plastic box for ease of use and transportation.

The method for making the above soft robot with stretchable coating will be specifically described below by way of example. As shown in FIG. 3, the method for making the soft robot with stretchable coating may include the following steps.

At SI, a mold for an actuator member of the soft robot with stretchable coating may be provided.

In this embodiment, the actuator member corresponds to the main body 11 and the distal end portion 12 of the above actuator. Since the main body 11 and the distal end portion 12 are basically the same in structure, the molds for making the main body 11 and the distal end portion 12 are also basically the same in structure, with only a difference in size. The making of the main body 11 of the actuator will be described below as an example.

In order to form the main body 11 shown in FIG. IB, it is required to provide a mold corresponding to the main body 11. The mold includes: a housing cylindrical; a cylindrical pin disposed at a central position inside the housing along a central axis of the housing; and three rectangular pins, disposed inside the housing and equally spaced around the cylindrical pin. In this embodiment, the cylindrical pin may have a length of 12 cm and a diameter of 1.5 mm, and each rectangular pin may have a length of 8 cm and a width of 3 mm. Since the mold is required to be exposed to ultraviolet radiation during the subsequent solidification process of the hydrogel mixture, the mold must be transparent. In this embodiment, the mold is made of acrylic material by 3D printing. In addition, the housing is provided with a through hole at bottom, and the through hole is adapted to introduce the hydrogel mixture into an interior of the housing.

At S2, a hydrogel mixture may be prepared.

The hydrogel mixture includes: agarose, acrylamide, N, N'-methylene-bisacrylamide (MBA cross-linking agent), photoinitiator and deionized water. In this embodiment, the hydrogel mixture includes 126.74 mg agarose, 2.85 ml acrylamide, 0.15 ml 5 mg/ml N, N'-methylene-bisacrylamide, 35.99 mg photoinitiator and 2.00 ml deionized water. The photoinitiator may be 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone.

The hydrogel mixture may be prepared as follows. The agarose and photoinitiator may be weighed respectively by an electronic scale, and the agarose and photoinitiator may be transferred to a flat-ended plastic test tube with a cover. Defined amounts of acrylamide, MBA and deionized water may be pipetted by a 100-1000 uL Dragon LAB micro-pipette respectively, and injected into the flat-ended plastic test tube. The hydrogel mixture may be stirred and mixed with an electronic stirrer. The well-mixed hydrogel mixture may be poured into a small-width test tube.

At S3, the hydrogel mixture may be heated to dissolve the agarose completely, and the hydrogel mixture may be maintained in a liquid form.

The test tube containing the hydrogel mixture may be placed into a drying oven and heated at a temperature of 90 degrees Celsius for 10 minutes until the agarose is completely dissolved. At this point, it is required to keep the hydrogel mixture still in a warm and liquid form.

At S4, the hydrogel mixture in the liquid form may be introduced into the mold.

The mold may be immersed into the test tube filled with the hydrogel mixture slowly, and the hydrogel mixture may be sucked into the mold by suction after air bubbles reaching the surface and popping up. Due to low viscosity, the hydrogel mixture can be easily pumped under suction without any bubbles.

At S5, the mold into which the hydrogel mixture has been introduced may be cooled to form an agarose.

The mold into which the hydrogel mixture has been introduced may stand at a room temperature for about 10 to 15 minutes until the agarose forms agarose gel, so that the hydrogel mixture has a gel-like denseness.

At S6, the mold into which the hydrogel mixture has been introduced may be exposed to ultraviolet radiation, so that the acrylamide is polymerized to form polyacrylamide.

The mold into which the hydrogel mixture has been introduced may be placed into an ultraviolet lamp box driven by a motor. Since the mold is transparent, the hydrogel mixture may be completely exposed to the ultraviolet radiation (UV). In the UV-photopolymerization, the mold into which the hydrogel mixture has been introduced may be placed at a position 10 cm away from the light source and rotated at 6 rpm, so that the UV radiation can be evenly distributed on the hydrogel mixture. Under the ultraviolet radiation, acrylamide is continuously polymerized to form polyacrylamide (PAM). In this embodiment, the hydrogel mixture may be exposed to the ultraviolet radiation for 120 minutes, so that the hydrogel mixture may be solidified to form a double-network (DN) hydrogel-agar PAM hydrogel.

At S7, the mold with the double-network hydrogel may be cooled at room temperature.

At S8, the double-network hydrogel may be demolded from the mold.

The humidity of the double-network hydrogel and the smooth surface of the mold made of the acrylic acid ensure a smooth demolding process. The resulting double-network hydrogel-based actuator member after demolding may include three pneumatic cavities, as shown in FIG. IB.

After the double-network hydrogel is demolded from the mold, as shown in FIG. 4, a winding machine may be used to form threads on an outer surface of the resulting double-network hydrogel-based actuator member, to limit the radial expansion of the actuator member.

At S9, assembling.

The three pneumatic cavities of the actuator may be connected to three motor-driven gas injection pumps through flexible pipes respectively. In addition, when the actuator member made by the above steps is a main body of the actuator, it is also possible to use a mold corresponding to the distal end portion of the actuator and repeat the above steps, to make the distal end portion of the actuator. The main portion and distal end portion may be assembled together to get a complete actuator.

In addition to the above steps, the method may also include changing elasticity, strength and rigidity of the obtained double-network hydrogel, by changing a concentration of the MBA or changing a duration of exposure to ultraviolet radiation, so that a special double-network hydrogel can be customized according to a specific application of the soft robot with stretchable coating.

In order to prevent the dehydration of the obtained double-network hydrogel, the double-network hydrogel-based actuator may be stored at a temperature below zero degrees Celsius. According to a recipe, the concentration of water in the agar PAM hydrogel is about 80%, so the hydrogel can be frozen at a temperature below zero degrees Celsius to prevent water loss.

According to the method for making the soft robot with stretchable coating of the present invention, double-network hydrogel can be obtained by the "one-pot" method that is from agarose gel formed by agarose and polyacrylamide polymerized by acrylamide. Compared with the troublesome multi-step polymerization process in a traditional hydrogel manufacture, the method of the present invention simplifies the manufacturing process of the hydrogel. The double-network hydrogel-based soft robot with stretchable coating according to the present invention is highly extensible, and has mechanical properties applicable to soft robot applications, so that the soft robot with stretchable coating has a more compliant and flexible human-computer interaction in the human body. In tests, the double-network hydrogel of the present invention can withstand 1800% tensile strain and 300 kPa tensile stress, whereas the conventional silica gel may be penetrated at a state of 500% strain.

In addition, the double-network hydrogel-based soft robot with stretchable coating according to the present invention maintains the original biocompatibility of the hydrogel and is less likely to cause immune responses of human body. In addition, the mechanical properties of the double-network hydrogel can be changed by changing components and manufacturing processes of the double-network hydrogel, for example, the concentration of MBA, the duration of exposure to ultraviolet radiation and the standing time of the double-network hydrogel. Therefore, a special double-network hydrogel can be customized according to a specific application of the soft robot with stretchable coating.

The foregoing embodiments are merely exemplary embodiments of the present invention, which are described in detail, but they are not intended to limit the protection scope of the present invention. It should be noted that, for those skilled in the art, several variations and improvements may be made without departing from the concept of the present invention, and these are all within the protection scope of the present invention. Therefore, the scope of the present disclosure shall be defined by the appended claims.