Chitosan in soft actuators for artificial muscles

Imitating natural drive systems is a major challenge in robotics. One approach to this is polymer hydrogels that contain chitosan, for example, and can be spun into fibers using electrospinning.

Chitosan/PVA Nanofibers as Potential Material for the Development of Soft Actuators

Olvera Bernal RA, Olekhnovich RO, Uspenskaya MV. Chitosan/PVA Nanofibers as Potential Material for the Development of Soft Actuators. Polymers (Basel). 2023 Apr 25;15(9):2037. doi: 10.3390/polym15092037. PMID: 37177184; PMCID: PMC10181017.

Biomimetics deals with the research and imitation of methods, designs and processes of nature in technical contexts, such as robotics. One of the greatest challenges in robotics is the imitation of motion systems that enable living creatures to move. The materials used for this are often referred to as actuators or artificial muscles and are defined by the fact that they can change their shape or size in response to physical stimuli such as light, moisture, pH value or electricity. Electroactive polymers (EAPs) are of particular interest. They have interesting properties for this application, such as good formability, flexibility, low density, adaptability and are often easy to produce. Depending on the drive mechanism, EAPs can be divided into two different groups. Electronic EAPs are driven by electric fields or Coulomb forces. Examples are ferroelectric polymers, dielectric elastomers and liquid crystal elastomers. In contrast, ionic EAPs, such as ionic polymer gels, ionic polymer-metal composites or conductive polymers, are moved by changing their shape through the diffusion of ions and their conjugated substances. Ideal materials for soft actuators are dielectric elastomer actuators (DEAs) due to their high energy density, high elongation and good electromechanical actuation performance. However, they have disadvantages, such as high activation voltages, which can lead to dielectric breakdowns.
For this reason, hydrogel materials are increasingly being investigated for their suitability as soft actuators. Their ability to react to external stimuli such as pH, light, magnetic and electric fields makes them promising candidates. Biopolymers such as chitosan, cellulose or gelatine are of particular interest. Due to the presence of polar groups, such as the amino, hydroxyl or carboxyl group, they have good electromechanical properties. Chitosan has a backbone of amino and hydroxyl groups, which gives it good polycationic properties.
In the study presented here, chitosan in various concentrations (2.5; 3; 3.5 and 4 wt.%) is to be electrospun with PVA (wt. 5 %) to form a chitosan/PVA nanofiber hydrogel as a potential material for soft actuators. A chitosan with a molecular weight of 260 kDa was used. The nanofibre samples were characterized using Fourier transform infrared analysis, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), optical microscopy and tensile tests. Subsequently, the electroactive behavior of the nanofiber hydrogels was tested under different HCl pH values (2-6) at a constant voltage (10 V).

RESULTS

• Chitosan/PVA nanofibers with different chitosan content were successfully fabricated using the electrospinning method
• The nanofiber actuators showed fast tip displacement at low voltage and reached a maximum speed of 1.86 mm/s at pH 3 and voltage of 10 V (4 wt. % chitosan)
• The electroactive response test showed a dependence between the chitosan content of the nanofiber and pH with the bending velocity shift
• Deconvolution results showed that the proportion of free amine groups increased with increasing chitosan content and was 3.6% and 4.59% for nanofibers with a chitosan content of 2.5 and 4 wt. %, respectively
• The results of the electroactive reaction were further supported by determining the proportion of free amine groups by deconvolution of FTIR spectra in the 3000-3700 cm-1 range

Conclusions: The study showed that chitosan/PVA nanofibers are a potential material for the development of soft actuators using biopolymers as a base material. Due to the good biocompatibility of chitosan, this also opens up possibilities in the biomedical field.

Link to article: https://www.mdpi.com/2073-4360/15/9/2037

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