RU175272U1 - ELECTRICALLY CONTROLLED ELEMENT OF THE EXECUTIVE DEVICE BASED ON HYDROGEL - Google Patents

Abstract

The proposal relates to the field of electroactive products from high molecular weight compounds with the effect of converting electrical energy into mechanical energy. The electrically controlled element of the actuator can be used in robotic, biomedical systems and in automation. The electrically controlled element is made in the form of an elongated ring, the small diameters of which correspond to the diameters of the electrodes between which the hydrogel is stretched, which reduces the contact deformation of the hydrogel when attaching the electrodes and reduces contact resistance, and when it is triggered, the forces are transmitted to the movable electrodes through the hydrogel, between which it is stretched. This allows you to adjust the time constant of its mechanical response when applying electric current by appropriate choice of the diameter of the cross section of the hydrogel ring without changing the remaining dimensions. Technological features consist in the fact that the initial compositions are poured into a releasable fluoroplastic form with a high degree of surface purity in the subsequent thermostating and extraction of the obtained ring, which is then aged in an electrolyte solution (sodium chloride, sodium sulfate, etc.) until equilibrium swelling is achieved. Rings are prepared from a mixture of one of the sets of chemicals: a) an aqueous solution of polyvinyl alcohol with acrylic acid methylene bisacrylamide as a crosslinker and ammonium persulfate as an initiator, b) an aqueous solution of polyvinyl alcohol with methacrylic acid, methylene bisacrylamide as a crosslinker and ammonium persulfate, c) an aqueous solution of polyvinyl alcohol with acrylamide methyl propanesulfonic acid, methylene bisacrylamide as a crosslinker and ammonium persulfate as an initiato a. These approaches provide the possibility of using not only electric current, but also a change in the pH of the medium in which it is immersed as an external influence causing compression of the ring element.

Description

The proposal relates to the field of electroactive products from macromolecular compounds, which demonstrate a pronounced effect of converting electrical energy into mechanical energy. The electrically controlled element of the actuator can be used in robotic, biomedical systems and in automation systems. The development of automation, robotics, including in mini-micro and nano-execution, urgently requires the creation of actuators of the appropriate level.

To date, electromechanical actuators that carry out both rotational and translational motion have been developed and introduced into production. As a rule, they consist of an electric motor (asynchronous, alternating and constant currents), the stator of which is located around a circle or deployed in a line. Accordingly, in the first case, the anchor carries out rotational motion (rotation electric motors), in the second - linear movement relative to the stator (linear electric motors). Piston-type pneumatic and hydraulic actuators are often used for linear motion, axial-piston or radial-piston machines, various types of turbines for rotary motion, operating from a pressure accumulator or (through a receiver) from compressor units driven by piston, turbine or electric machines of various types. All of the above devices realize traction (with translational motion) in a fairly wide range from 10 to 1,000,000 n. One of the promising and priority areas of development of modern robotics is the creation of robots with geometric dimensions of mini, micro and nanoscale. The development and production of such robots is fraught with a number of difficulties. These include a limited range of actuators in the specified range of geometric dimensions and technical characteristics. In addition to organizational and production aspects, this problem has certain physical and chemical limitations. With a decrease in geometric dimensions, the energy efficiency of actuators in a traditional design inevitably falls. This is due to the fact that molecular cohesion forces are becoming increasingly important for low-dimensional systems, and at a certain level of device size they exceed the friction forces, and with a further decrease in the size of the molecular cohesion forces, they begin to destroy the material (metal). We also note that with a decrease in the geometric dimensions of actuators in the traditional design, a number of physical limitations cannot be overcome. For example, in order to obtain the necessary electromotive force of the winding of electric motors, relays, etc., it is necessary to carry out an increasingly thin wire. However, in this case, its ohmic resistance sharply increases and with a further decrease in size, this leads to melting of the wire. In addition, for the manufacture of mechanical parts of micro- and nanoscale, there is practically no machinery stock. Therefore, when creating actuators in micro- and nano-execution, one has to resort to non-traditional methods for their synthesis due to the rejection of the traditional construction of actuators and the transition to actuators on other principles, including through the use of polymeric materials that can change their shape, geometric dimensions , physical and mechanical characteristics under external influence ("artificial muscles"). Note that due to the relative novelty of research in this area, the terminology of “artificial muscles” has not yet been established. In GOST, actuators on electroactive polymers are not registered, there is no corresponding terminology [GOST 14691-69. Executive devices for automatic regulation systems. Terms]. New materials for creating actuators include polymer hydrogels. Work in the field of creating polymer hydrogels with a pronounced electrochemomechanical response is a promising bionic direction for research in the field of “soft” organic electronics, the development of compact and miniature capture and positioning devices, android technology, various kinds of intelligent biomedical materials (for example, for exoskeletons, displays Braille), combined actuator sensors, etc. By such parameters as energy efficiency and mass dimensions, artificial muscles and actuators based on polymer hydrogel materials are approaching human muscles. Currently, only a few devices are known that use electrically controlled hydrogels of this type.

The idea of using hydrogels as the active elements of actuators is based on their ability to greatly change the degree of swelling under small changes in external conditions (electric field, pH of the medium, ionic strength, light and heat). The deformation (compression, tension, bending) of a polyelectrolyte hydrogel in an electric field has an electrokinetic and electroosmotic nature due to a pronounced change in its degree of swelling when the ion equilibrium shifts in the hydrogel volume, ion transport through the hydrogel, and electrochemical reactions on the electrodes.

Despite the apparent simplicity of producing polymer hydrogels, the researchers are faced with a serious difficulty associated with the task of integrating an electrically controlled element based on a hydrogel into the mechanism of the actuator.

The main difficulties in using hydrogels in actuators are associated with: 1) the low physical and mechanical characteristics of hydrogels, 2) the difficulty of forming products from hydrogels, 3) the difficulty of supplying electric current to the hydrogel (the problem of electrodes).

There is an approach to designing an actuator with an electrically controlled element based on a hydrogel, in which a cylindrical sample of the hydrogel is spring-loaded between two flat ends of the electrodes [US 5,389,222, 02/14/1993, Shahinpoor]. The inclusion of an electric field causes compression of the hydrogel element; when the field is off, the hydrogel restores size under the action of the spring. In another embodiment of said patent, the annular hydrogel is spring loaded and attached to the spindle. The supply of an alternating field on two opposite sides of the ring causes alternate contraction and compression of the opposite sides of the ring, due to which the spindle rotates in one direction or the other. A significant drawback of this design is that the task of restoring the original size of the electrically controlled element based on a hydrogel is solved using a return spring, as well as the fact that this invention does not solve the problem of the transfer of force from the hydrogel in the actuator.

A known method in which a cylindrical sample of a hydrogel is used as an electrically controlled element when the electrode is located inside the gel itself [US 20070190150 A1, 08.16.2007, Ito and others]. The electrode is a wire twisted into a spiral, which is expanded and compressed inside the hydrogel volume when its degree of swelling changes. In this case, an electrochemically controlled process of hydrogen evolution at the electrode is used for activation. To this end, a special electrode material with an occlusion effect is used. The disadvantage of this approach is the high cost of the electrode, which is proposed to be made from palladium. In addition, this method has not proposed a solution to the problem of attaching a hydrogel to an actuator and transmitting the generated force and movement.

The closest invention to the claimed one - the prototype - is a system consisting of a spherical sample of a hydrogel in which the electrodes treated with plasma are placed, and the hydrogel itself is immersed in an elastomer capsule [US 008088453 B1, 03.01.2012, Rasmussen]. The action of the electric field causes volumetric deformation of the hydrogel, which manifests itself in the flattening of the ball. The deformation of the hydrogel ball is converted into motion by a system of levers that are attached to an elastic spherical capsule. Thus, a change in the volume of the gel is converted into translational movement of the levers. In addition to the above, the author suggests using the developed system in the form of a torus as a peristaltic valve or pump.

The disadvantage of this solution is its excessive complexity associated with the need to form an elastomeric coating on the surface of the hydrogel element and the transfer of force through this coating to the lever system. Another disadvantage is that the generation of force and displacement is provided using a lever system, which imposes certain restrictions associated with the golden rule of mechanics. The authors do not provide for the use of hydrogel as a traction element, which provides direct transmission of traction to the actuator. A significant disadvantage of the prototype is the difficulty of achieving a given level of speed of the electromechanical response and movement without scaling lever devices.

Thus, the analysis of well-known analogues showed that the problem of creating electrically controlled elements of actuators for robotics of the micro- and nanoscale based on polymer materials, primarily hydrogels, remains relevant.

The objective of the proposed proposal is the creation of an actuator in which the electrically-controlled element is made entirely of a swelling hydrogel attached directly to the electrodes, and which is capable of electromechanical response and movement without additional forced lever devices (as in the prototype device).

This problem is solved by the claimed utility model - an electrically-controlled element of the actuator based on a polymer hydrogel.

The inventive utility model is characterized by the following set of essential features, which are simultaneously distinctive from the prototype device:

1) to reduce the contact stresses of deformation that occur when the hydrogel is attached to the electrode, the element is made in the form of an elongated ring 1 (see Fig.), The small diameters of which correspond to the diameters of the electrodes 2, between which the hydrogel is stretched;

2) to reduce contact electrical resistances, the element is made in the form of an elongated ring 1, stretched over the movable electrodes 2, and the action of an external current pulse supplied through the electrodes causes a change in the center distance due to compression of the ring;

3) the forces when it is triggered due to compression of the ring are transferred to the moving electrodes through the hydrogel, between which it is stretched;

4) the time constant of its mechanical response when applying electric current is regulated by the appropriate choice of the diameter of the hydrogel ring;

5) as an external influence causing compression of the ring, not an electric current can be used, but a change in the pH of the medium in which it is immersed;

6) the structure and chemical composition of the hydrogel-based ring is a double interpenetrating network, one of the components of which is crosslinked polyacrylic acid, and the second is polyvinyl alcohol, while the concentration of all reagents used to obtain the hydrogel corresponds to the optimum in terms of achieving a pronounced electromechanical response and acceptable level of mechanical properties: the ring is prepared by radical polymerization of a 25% aqueous solution of acrylic acid using meth enbisakrilamida as a crosslinking agent, and ammonium persulfate as the initiator in a 10% aqueous solution of polyvinyl alcohol; the specified reaction mixture is injected into a releasable PTFE mold with a high degree of surface cleanliness and pre-defatted, thermostated at 50 ° C for 30 minutes, after which the resulting ring is removed and immersed in an electrolyte solution (for example, sodium chloride, sodium sulfate, etc.) until equilibrium swelling is achieved,

7) as a mixture for the manufacture of the ring, you can use an aqueous solution of polyvinyl alcohol (concentration 10%) with methacrylic acid (concentration 25%) with a crosslinker methylene bisacrylamide and an initiator of ammonium persulfate; this mixture is poured into a detachable PTFE mold with a high degree of surface cleanliness, thermostatically controlled at 50 ° C for 30 minutes, after which the resulting ring is removed and immersed in an electrolyte solution (for example, sodium chloride, sodium sulfate, etc.) until equilibrium swelling is achieved. ;

8) as a mixture for the manufacture of the ring, you can use an aqueous solution of polyvinyl alcohol (concentration 10%) with acrylamide methylpropanesulfonic acid (concentration 25%) with a crosslinker methylene bisacrylamide and an initiator of ammonium persulfate; this mixture is poured into a detachable PTFE mold with a high degree of surface cleanliness, thermostatically controlled at 50 ° C for 30 minutes, after which the resulting ring is removed and immersed in an electrolyte solution (for example, sodium chloride, sodium sulfate, etc.) until equilibrium swelling is achieved. .

The set of essential features of the claimed device provides a technical result - the creation of an electrically controlled device made entirely of specially selected polymer hydrogel, which provides the necessary level of mechanical and electrophysical properties, in the original form - in the form of a ring. The ring structurally provides reliable fastening of the hydrogel to the electrode, the transfer of force and deformation from the hydrogel to the actuator with the necessary level of movement and speed of the electromechanical response without the use of scaling auxiliary (lever) elements. This allows to reduce the heat emission of the controlled element and the value of the control electric potential. In addition, the claimed device allows you to use the ability of the claimed hydrogel to perform mechanical work not only under the influence of an electric field, but also when varying the parameters of the external environment, such as the pH of the liquid into which the controlled elements of the device are immersed, which saves energy.

An example of a specific implementation.

A hydrogel based on an interpenetrating network of polyvinyl alcohol and polyacrylic acid was used. A 10% aqueous solution of polyvinyl alcohol with a molecular weight of 70,000 was mixed with a 25% solution of acrylic acid, methylene bisacrylamide (0.36%) and ammonium persulfate (0.001%) and filled into a closed fluoroplastic mold using a syringe. The molding cavity of the mold was an elongated ring round in cross section with external dimensions L × H = 47 × l3 mm (see Fig.) And a cross-sectional diameter of 2 mm. The forming cavity was previously thoroughly degreased. A closed mold filled with a polymerization mixture was placed in a thermostat at a temperature of 50 ° C and held for 30 minutes. After this, the mold was removed, the hydrogel in the form of a ring was removed from the mold and immersed in a 5% aqueous solution of sodium sulfate for a day. The equilibrium swollen ring had a degree of swelling of 3.5 g / g, an elastic modulus of 20 kPa, a tensile strength of 70 kPa and a relative elongation of 375%.

Evaluation of the effectiveness of the inventive electrically-driven element of the actuator based on a polymer hydrogel was carried out on a preliminary layout of the actuator. An equilibrium swollen ring was suspended between two graphite rods with a diameter of 8 mm, one of which was rigidly fixed, and the second is movable. A small load (about 0.05 H) was applied to the movable shaft, sufficient to prevent sagging of the ring. A source of constant electrical voltage was connected to the graphite rods. The stress caused compression of the ring, which manifested itself in a decrease in the distance between the graphite rods. It was found that on an elongated hydrogel ring with dimensions L × H = 35 × 10 mm and a cross section diameter of 1.8 mm, a maximum displacement of up to 7 mm can be obtained (without using scaling lever devices) at an average displacement speed of 1-1.5 mm / s. The operating voltage on the device varied from 30 to 100 V, the current was in the range of 9-12 mA.

Thus, the electrically controlled element of the actuator based on the hydrogel ensures the creation of effective robotics and automation devices on its basis.

Claims (3)

1. An electrically controlled element of a hydrogel-based actuator, characterized in that said element is made in the form of an elongated ring, small diameters of which correspond to the diameters of the electrodes between which it is stretched, the elongated ring consists of an equilibrated hydrogel of crosslinked polyacrylic acid and polyvinyl alcohol in an electrolyte solution . 2. An electrically controlled element according to claim 1, characterized in that the elongated ring consists of an equilibrium swollen hydrogel of crosslinked polymethacrylic acid and polyvinyl alcohol in an electrolyte solution. 3. An electrically controlled element according to claim 1, characterized in that the elongated ring consists of an equilibrium swollen hydrogel of crosslinked polyacrylamide methyl propanesulfonic acid and polyvinyl alcohol.

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