(Nanowerk Spotlight) Due to its excellent electrical and thermal conductivity, high surface area, and high flexibility, graphene has attracted much attention in recent years. Besides applications in flexible electronics, energy storage, and anti-corrosion coatings, just to name a few, graphene-based nanomaterials have been used in the construction of smart materials for instance for soft robotics, which can be actuated by various external stimuli, i.e., stimulated by electrical, electrochemical, and optical energy.
Soft robotics represents an exciting new paradigm in engineering that challenges researchers to re-examine the materials and mechanisms that they use to make conventional hard robots so that they are more versatile, life-like, and compatible for human interaction.
Among the various actuation mechanisms driven by different stimuli, light-driven systems have garnered more and more attention due to their advantages in wireless/remote control, localized rather than whole-field driven capabilities, and electrical/mechanical decoupling.
"While graphene shows decreasing absorption from visible to near-infrared (nIR), there is brilliant photothermal conversion efficiency in the band of nIR, which has caused tremendous interest in biomedical applications, such as drug delivery and photothermal and photo-dynamic therapies," Dr. Weitao Jiang, from the State Key Laboratory for Manufacturing Systems Engineering, Xi'an Jiaotong University in China, explains to Nanowerk. "Graphene can be incorporated into different polymer matrices to improve relevant properties, namely mechanical, thermal and electrical. Because of its photothermal effect and high thermal conductivity, graphene and its composites show promising photoresponsive properties."
Inspired by the photothermal effect of graphene in biomedical applications, Jiang's team proposed an easily fabricated and remote/wireless control light-driven approach to actuation mechanism based on graphene nanocomposites.
"What has motivated us to develop a novel actuation mechanism based on graphene nanocomposites is its potential application in soft robotics applicable in clinical medicine, i.e., implantable surgery robotics, or drug delivery devices," Jiang notes.
a) Fish swims in water through the movement of its caudal fin. b) A designed microfish which can swim to any directions when locally stimulated by near infrared (nIR) irradiation, with a similar mechanism to fish swimming. (Reprinted by permission from Wiley-VCH Verlag)
As the key part in soft robotics platforms, stimuli-responsive materials have drawn enormous attention due to their brilliant intriguing shape or volume recovery properties under different external stimuli, which are helpful for creating mechanical motion rapidly and precisely.
In contrast to the actuation mechanism driven by electrical and electrochemical stimuli based on graphene nanocomposites, this photoresponsive soft platform can work both in air and water. The bilayer design combines soft matter PDMS and graphene. PDMS has already been widely used and studied in numerous fields and its good biocompatibility is a great advantage for applications in in vivo or in vitro.
Meanwhile, graphene has been incorporated into different polymer matrices to improve mechanical, thermal and electrical properties. In previous works, polymer composites consisting of graphene nanoplatelets (GNP) and PDMS have been shown to exhibit a large light-induced reversible and elastic response.
In their work, Jiang and his team demonstrate an effective method for the fabrication of a polymeric bilayer biomimetic platform, which can be light-actuated both in air and water. The bilayer platform is composed of a pure PDMS layer and a PDMS/GNPs composited layer, in which each layer has a different coefficients of thermal expansion (CTE) and Young?s modulus due to the existence of the GNPs.
The polymeric bilayer can be reversibly deflected at millimeter scale in response to near infrared (nIR) irradiation, which can be attributed to the photothermal effect of graphene. The deflection performances, i.e., deflection magnitude and response time, are determined by the light intensity and GNPs concentration.
To demonstrate the capabilities of their bilayer soft robotics platform, the team designed biomimetic microfish which can move forward, backward, and turn around in water under nIR irradiation, to mimic fish swimming in nature (see figure above). The moving directions and velocities can be remotely adjusted by light.
"Soft robotics is a fairly young sub-category of robots, which combine classical principles of robots design with the study of fluids, gels, soft polymers, and other easily deformable matter," notes Jiang. "It is learning from the animals and plants in nature composed primarily of soft, elastic structures which are capable of complex movement as well as adaptation to their environment. It represents an exciting new paradigm in engineering that challenges us to reexamine the materials and mechanisms that we use to make conventional hard robots so that they are more versatile, lifelike, and compatible with human interaction."
As Jiang points out, the results may not only be promising for developing light-driven drug-delivery platforms but also bio-robotic microgripper applications in vivo and in vitro.
"We believe that this soft robotic platform can further be explored for many other applications, such as biomimetic research, microcantilevers, micro/nanorobotics, drug delivery, minimally invasive medicine applications, implanting medical robots, etc." he says. "Due to its excellent penetration ability in biological tissues, near infrared light provides a promising approach to remotely actuate robotic devices within the body."
The researchers are already working on next steps, where they will integrate functional devices, such as a camera and stimuli tips, into the design and explore their applications in novel endoscopy and internal stimuli systems.
"We will also try to develop a new style of implantable flexible device which is either charged with near-infrared light or powered by the photothermal bilayer effect of soft robotics," Jiang concludes.