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Microscale actuators from carbon nanotube and hydrogel composites



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This thesis develops and tests a novel microactuator system using carbon nanotubes (CNT) and poly(N-isopropylacrylamide) (PNIPAM) hydrogel, which have been combined to make a microactuator system with a wide range of possible designs without needing to modify the fabrication approach. These CNT/PNIPAM hydrogel microactuators have sizes ranging from 10 µm to 1 mm respond to light, can be cycled over 1500 times with no sign of degradation, and have response times as fast as 15 ms. The driving force for the actuators is the volume phase transition of PNIPAM. PNIPAM is a lower critical solution temperature (LCST) polymer with a critical temperature (Tc) of 32°C. PNIPAM switches from hydrophilic to hydrophobic at Tc, and therefore swells with water when below Tc and contracts by expelling water above Tc. PNIPAM can be crosslinked to form a hydrogel with a reversible size change of approximately 50%. However, PNIPAM is limited by its slow actuation and the need to transform its isotropic swelling/contraction into useful anisotropic deformations. Similar problems are faced with other hydrogels; the typical approach to addressing these challenges is introducing an additive material.

CNTs are well known for their remarkable strength, with individual tubes showing a Young's modulus of up to 1 TPa and an inherent tensile strength of over 100 GPa. They are highly durable and resistant to fatigue over millions of strain cycles. Vertically-aligned forests of CNTs (VACNTs) can be grown by chemical vapour deposition, and these forests can be grown in predefined shapes by depositing the growth catalyst in the desired pattern using lithography. VACNTs act as pseudo-blackbodies, absorbing up to 99% of light over a wide wavelength range from UV to far IR. This allows the CNTs in the composite hydrogel to absorb light and convert it to heat, which is transferred to the PNIPAM. With sufficient light, the PNIPAM is heated above Tc and contracts. This light responsivity allows for the selective triggering of actuators from a distance, and it speeds up actuation response times because there is no need to heat the entire environment. This also reduces the energy requirements of the actuator system.

Here, PNIPAM hydrogel is synthesised directly on patterned VACNTs using in situ free-radical polymerisation. In this way, CNTs not only act as an additive but also as a skeleton to define the final shape of the hydrogel. Using CNTs as a skeleton facilitates the moulding of the hydrogel into the desired shape, and it is straightforward to achieve complex designs with a size range over three orders of magnitude, from 10 µm to 1 cm. Furthermore, the ability to make microscale actuators allows for faster response times. The actuation timescales are reduced because of the smaller distance over which the water must be transported to be expelled/absorbed. The anisotropic mechanical properties of CNTs are maintained in the CNT/PNIPAM hydrogel composite material, resulting in anisotropic shape changes during actuation. Two types of anisotropy are observed: anisotropy from the nanoscale vertical alignment of the CNTs, and anisotropy from the microscale patterning of the CNT skeleton. The vertically aligned CNTs pin the hydrogel such that the composite structure does not change height during actuation, but does change size laterally. The lateral size change of the composite is slower than that of bulk PNIPAM, which results in non-equilibrium deformations throughout an actuator when the bulk PNIPAM regions contract more quickly than the composite regions. These deformations can be controlled by the microscale patterning of the CNT skeleton. Various microactuator designs are tested to reveal a range of actuation behaviours and design features, including switching from low to high light absorption states, lattice shape changes, localised actuation, locomotion, and three-dimensional deformations.

The properties and dynamics of the CNT/PNIPAM hydrogel microactuator system are investigated by analysing response times and out-of-equilibrium shape changes. In particular, the kinetics of actuator contraction is investigated by analysing the actuator size over time when optically heated. The response time of the actuators is shown to be inversely proportional to the optical heating power used and it is shown that the CNT/PNIPAM hydrogel composite regions of the actuator respond more slowly than the bulk PNIPAM regions. An empirical exponential model is fit to the experimental contraction curves, and it suggests an initial fast response that is described by a compressed exponential, followed by a slower response that is described by a single exponential. A theoretical model is developed to further understand these data by combining heat transfer timescales and models for polymer network diffusion in a hydrogel. It is determined using this model that the polymer network diffusion rate of the CNT/PNIPAM hydrogel composite actuator decreases during actuation, indicating that the actuator's speed is likely to be limited by phase separation within the hydrogel. The theoretical model is then used to predict actuator response times to recommend approaches for increasing actuation speed, such as shrinking the initial actuator size and modifying the hydrogel synthesis procedure.

Microswimmers made from CNT/PNIPAM hydrogel composites are investigated as a case study for applying this microactuator system. The successful implementation of microswimmers requires overcoming a fundamental challenge: the ‘scallop' problem, which describes the limitations of operating in a low Reynolds number regime, where fluid dynamics requires that the motion of swimmers must be non-reciprocal in time (i.e. the forward and reverse swimming strokes must be different) to achieve translation. The CNT/PNIPAM microactuators here intrinsically have a hysteretic switch, indicating that a suitable swimmer design using this system could demonstrate true microscale swimming. A jellyfish-inspired structure is then fabricated and its key behaviours are investigated. While reliable swimming was not achieved, results are observed that are encouraging for future research, including sporadic locomotion and controlled three-dimensional deformations.





Baumberg, Jeremy
De Volder, Michael


carbon nanotubes, hydrogel, light actuation, microactuator, soft actuator, soft robotics


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
EPSRC (2338185)