Investigation into output force performance of an ionic polymer artificial muscle based on freeze-drying process
Graphical abstract
Introduction
With the continuous developments of electric actuators and smart polymer materials, an Ionic Polymer Artificial Muscle (IPAM) has become the research focus in bionic engineering field, which is the key to soft robots, biomedical and intelligent mechanical systems [[1], [2], [3]]. However, it is found that the IPAM still has many problems that greatly limits its practical applications, such as the small output force and its “pathological tremor” like human muscle [4,5], which significantly affects the working stability. Thus it is urgent to conduct in-depth research on output force performance of the IPAM prepared by Sodium Alginate (SA), which is a breakthrough attempt and full of great values in the artificial muscle and green actuator fields [[6], [7], [8]]. Also, the SA is a natural and linear copolymer extracted from sargassum or kelp, and it is irregularly connected by α-1,4-l-guluronic acid chains (G) and β-1,4-d-mannuronic acid links (M), whose M/G ratio is 1:1 and the average molecular mass is about 222.00. In recent years, there are few researches on the IPAM about its preparation process and output force performance, and then a lot of work needs to be carried out in depth [[9], [10], [11], [12], [13]]. For instance, Bekin and Jiang et al. [14,15] have studied dielectric properties and electrical response of the SA/polyacrylic acid interpenetrating polymer cross-linked by glutaraldehyde, which founds that the dielectric constant and electrical conductivity of SA are reduced after the cross-linking reaction, and its total bending degree reaches the highest in 0.15 mol/L NaCl solution. Yang et al. [16,17] have adopted the double-side casting and free-radical grafting technologies to fabricate the IPAM, which shows that it is capable of appearing a good reversible bending behavior with big equilibrium strains. Sun and Kim et al. [18,19] have reported a kind of cellulose electroactive paper that can produce large displacement at a low humidity level or driving voltage. Liu et al. [20] has utilized a simple and effective preparation method of the SA and polyvinyl butyral to manufacture a hydrogen peroxide biosensor, which has high sensitivity and reproducibility.
However, traditional heat-drying technology should take a long time to prepare the IPAM [21,22], and it is also limited by the material properties. So it is urgent to improve the preparation technology of IPAM; in detail, a freeze-drying process belongs to the physical foaming method that is to remove water from the IPAM with its initial foaming state [23]. Through this approach, the IPAM has little damage to characteristics of its solution, which is because cold air will rapidly lock internal structure of the IPAM solution, and then the ice crystals formed are all sublimated leaving a porous structure of the IPAM. Serving as pore-forming agent, its inner water crystallizes during the freeze-drying process and sizes of the crystal grains formed directly determines pore shapes of the IPAM fabricated by the freeze-drying technology. Further, formation characteristics of the crystal grains are directly related to the freeze-drying temperature in freeze-drying process of the IPAM.
Therefore, in this work, a freeze-drying process was provided and its effects of freeze-drying temperature on output force and tremor behaviors of the IPAM were investigated systematically. Based on an experimental platform, the specific capacitance, ion channel and water retention rate of the IPAM at different freeze-drying temperatures were precisely tested, which obtained its electric conductivity and internal microstructure. Through macroscopic morphology of the IPAM, the relationship between crystal grains growth and its mechanical properties was analyzed. Combined with thickness and elastic modulus of the IPAM, its optimal freeze-drying temperature and improvement mechanism on the output-force tremor behaviors were acquired, which was of great significance for studying output force performance of the IPAM and its working stability.
Section snippets
Preparation process
The materials and instruments for preparation of the IPAM are mainly SA (analytical reagent, 90 %), Sodium Dodecyl Sulfate (SDS, analytical reagent, ≥97 %), Multi-walled Carbon Nanotube aqueous dispersion (MWCNT, 10.0 wt%), distilled water, magnetic stirrer (SG-5411), ultrasonic cleaning machine (FRQ-1001 T), freeze-dryer (SJ-200) and glass mold (self made). Preparation process of the IPAM included four steps; the solution preparation, ultrasonic defoaming, casting and freeze drying.
Output force and response speed of the IPAM
Electric stimulation experiments were conducted on the IPAM samples at different freeze-drying temperatures to obtain the output force performance, as shown in Fig. 4(a). It was indicated that force-time curves of the samples would reach equilibrium values presented in Fig. 4(b) after a period of time, and values of their linear parts could be linearly fitted for acquiring the response speeds, which were shown in Fig. 4(c). Therein, with the freeze-drying temperatures of -30°C, 40°C, 50°C,
Thickness of the IPAM
Fig. 7 presented the macro-morphology in thickness direction of the IPAM prepared by high and low freeze-drying temperatures, in which its thickness formed at high freeze-drying temperature was obviously smaller than that of under a low freeze-drying temperature. Because when the freeze-drying temperature was high, crystal structure of the IPAM solution was unstable in its freeze-drying process, and then a serious shrinkage phenomenon would occur due to generating poor physical properties and
Conclusions
In summary, the IPAM was prepared by freeze-drying technology, and then its output force performances were systematically studied at different freeze-drying temperatures. Experimental results demonstrated that the best freeze-drying temperature ranged from -50°C to -70°C, then IPAM presented the optimal output force and its response speed of 6.76 mN and 0.166 mN/s respectively, which were 1.7 times and 4.9 times larger than the minimum values. Additionally, with the freeze-drying temperature
CRediT authorship contribution statement
Junjie Yang: Conceptualization, Writing - original draft, Writing - review & editing, Supervision, Methodology. Zhijie Wang: Investigation, Software, Validation, Formal analysis. Gang Zhao: Project administration, Data curation, Resources.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (NSFC) [grant number 51675112].
Junjie Yang is a teacher in the School of Mechanical Engineering at Northeast Electric Power University, China. He received his Doctor’s degree in mechanical engineering major from the Harbin Engineering University in 2019. His current research interest is smart materials and structures, biological artificial muscle, chemical actuators and intelligent manufacturing. He is a member of the International Society of Bionic Engineering. He has published 6 SCI research paper, 1 EI journal article and
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Junjie Yang is a teacher in the School of Mechanical Engineering at Northeast Electric Power University, China. He received his Doctor’s degree in mechanical engineering major from the Harbin Engineering University in 2019. His current research interest is smart materials and structures, biological artificial muscle, chemical actuators and intelligent manufacturing. He is a member of the International Society of Bionic Engineering. He has published 6 SCI research paper, 1 EI journal article and has 5 patents, which in the refereed international journals and conference.
Zhijie Wang is an engineer in the Materials Application Department of R&D Center at Weichai Power Co., Ltd, China. He received his Doctor’s degree in mechanical engineering major from the Harbin Engineering University in 2019. His current research interest is nanoscale functional materials and devices, and bionic artificial muscle and robotics. He has published 5 SCI research paper, 2 EI journal article and has 4 patents, which in the journals of international repute.
Gang Zhao is a professor, doctoral supervisor in the College of Mechanical and Electrical Engineering at Harbin Engineering University since March 1993. His current research activities involve: smart materials and structures, marine engineering bionics and intelligent manufacturing system technology. Prof. Zhao has published over 40 SCI journal papers and has over 28 patents. He has presided over two research projects that were supported by the National Natural Science Foundation of China [grant number 51,675,112 and 51,275,102].