​Uses of Nanomaterials in Bio-Inspired Soft Robotics

​Uses of Nanomaterials in Bio-Inspired Soft Robotics

Nano-sized particles exist in nature and they can be created with a variety of materials and products, like silver or carbon.

Nanomaterials are characterized by their small size and are measured in nanometers. Engineered nanomaterials (ENMs) are materials that are engineered on small scale and they have exceptional electrical, mechanical, and optical properties, which exhibit great impact in electronics, medicine, and many other fields. ENMs are also used in environmental remediations, imaging, and drug delivery.


A synthetic method for nanomaterials is to produce a material that has properties that can be a result of their characteristic length scale being in the nanometer range of 1- 100nm. There are two types of synthetic methods, which are bottom-up and top-down. Bio-inspired soft robots have the potential to enhance and match the multifunctionality and versatility of natural organisms. The bio-inspired design needs passive mechanics, tight integration of sensing, active moment, and also control. Most of the initial efforts in soft robotics were encouraged by the demand of co-robots (cobots) that are human-friendly and could be used for human-machine physical interaction. Researchers have put extensive efforts into the development of strategies for soft robotic actuation with built-in biomimetic intelligence, depending on a variety of responsive soft materials like liquid crystal networks (LCNs), hydrogels, and shape memory polymers (SMPs).

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An Overview of Nanomaterials

There is no precise definition for nanomaterials according to scientists, they settled on this, nanomaterials are characterized by their small size and are measured in nanometers (one-millionth of a millimeter is approximately 100,000 times smaller than the human hair diameter).

Existence of Nanoparticles

Nano-sized particles exist in nature and they can be created with a variety of materials and products, like carbon or silver, by the definition of nanomaterials, they should have at least one dimension and that is approximately less than 100 nanometers. Nanoscale materials cannot be seen with the naked eye or even not with a conventional lab microscope.

Engineered Nanomaterials

Those materials, which are engineered on such a small scale are known as engineered nanomaterials (ENMs), which have unique magnetic, electrical, optical, and many other properties. Such properties have the potential for huge impacts in medicine, electronics, and other fields.

For example,

1. Nanotechnology has a role in designing pharmaceuticals that can be used in targeting specific organs or cells of the body such as cancerous cells and enhance therapeutic effects.

2. Nanomaterials are used in cloth, cement, and other materials to enhance their strength and also make them lighter.

3. Nanomaterials' small size makes them very useful in electronics and can also be used in the neutralization of toxins, clean-up to bind with, and in environmental remediations.

Where are Nanomaterials Found?

Some naturally occurring nanomaterials such as blood-borne proteins which are essential for life and lipids which are found in body fat and blood. Scientists have been more inclined towards engineered nanomaterials (ENMs), which are made to be used in many devices, structures, and commercial materials. Using engineered nanomaterials (ENMs), many products such as cosmetics, sporting goods, sunscreen, stain-resistant clothing, electronics, and tires are manufactured. ENMs are also used in environmental remediations, imaging, drug delivery, and in medical diagnosis.

Synthesis of Nanomaterials

The purpose of any synthetic method for nanomaterials is to produce a material that has properties that can be a result of their characteristic length scale being in the nanometer range of 1- 100nm. The synthetic method should have control of size in this range so one property can be attainable.

There are two types of methods, that are bottom-up and top-down.

Bottom-up Methods

In bottom-up methods, atoms, and molecules are assembled into nanostructured arrays. The raw material sources for these methods can be in the form of solids, liquids, or gases. The former required some kind of disassembly before they incorporated the nanostructure. Bottom-up methods are classified into two categories: controlled and chaotic.

Controlled Delivery of the Constituent Atoms

In controlled processes, the controlled delivery of constituent particles to that field of nanoparticle formation in such a way that it can grow in a controlled manner to a particular size. Normally, the state of constituent particles is never far from the required for nanoparticle formation. The control state of the reactants controls the formation of nanoparticles. Controlled processes examples are self-limiting growth solution, shaped pulse femtosecond laser method, self-limited chemical vapor deposition, and molecular beam epitaxy.

Chaotic Processes

In these processes, constituent atoms and molecules are elevated to a chaotic state, and then to make the state unstable, some conditions get changed. By manipulating some parameters, products are formed due to insuring kinetics. When a chaotic state is collapsed, it becomes difficult to control. The manipulation of the end state of the product controls the formation of nanoparticles. Some examples of chaotic processes are combustion, flame pyrolysis, arc, exploding wire, laser ablation, and precipitation synthesis methods.

Top-down Methods

In Top-down methods, some forces like a laser, and mechanical forces are used to break heavy materials into nanoparticles. One such famous method involves breaking down materials into nanomaterials, also known as 'ball milling'. Laser ablation can also make up nanoparticles in which short pulse lasers (femtosecond laser) are applied on a solid material (target).

Mechanical Properties of Nanoparticles

Research has shown that mechanical properties in nanomaterials show a significant variation as compared to bulk materials. The mechanical properties of nanoparticles are due to the surface, volume, and quantum effects. It is observed that when nanomaterials are added to bulk materials, these nanomaterials refine the grains and form intragranular and intergranular structures which enhances the grain boundaries and the material’s mechanical characteristics.

Grain Boundary Refinements

Refinement of grain boundary provides strength to form transgranular and intergranular fractures. For example, it is observed that when nano Silica is added to cement, it improves the tensile strength, bending strength, and compressive strength. Such properties of nanoparticles will enhance the use of these particles in various fields, for example, tribology, surface engineering, and nanofabrication/nanomanufacturing.


For private, industrial, and military sectors, the chemical processing and synthesis of high-performance technological components require the use of polymers, purity ceramics, material composites, and glass ceramics. In the Condensed bodies which are formed from fine powders, the irregular shapes and sizes of nanoparticles in a normal powder lead to non-uniform packing morphologies that cause variation in packing density in the powder compact.

Uncontrolled Agglomeration

Due to attractive Van der Waals forces, uncontrolled agglomeration of powders leads to microstructural inhomogeneities. Non-uniform drying shrinkage develops differential stresses and is directly related to the rate at which the solvent is removed and highly dependent on the porosity distribution. These stresses are associated with a plastic-to-brittle transition in consolidated bodies and if not relieved, they can cause crack propagation in the unfired body.

Nanomaterials as Essential Elements of Soft Robotics

Bio-inspired soft robots have the potential to enhance and match the multifunctionality and versatility of natural organisms. To accomplish this, the bio-inspired robot design needs passive mechanics, tight integration of sensing, active moment, and also control. These abilities can be achieved through the clever integration of rigid, biological, and soft materials into structures that show global deformability and compliance. The progress not only depends on the new multifunctional materials development but also robust materials interfacing, novel systems-level design, and 3D manufacturing advancement.

An Early Effort in Soft Robotics

Most of the initial efforts in soft robotics were encouraged by the demand of co robots that are human-friendly and could be used for human-machine physical interaction. It included robotic arms for industrial automation (Rubbertuator) and orthoses which are pneumatically powered for human grasp assistance (McKibben hand orthotics). Today, most industrial robots are hydraulically driven or motorized, despite the history of pneumatic artificial muscles, and rely on solid materials for load bearing and actuation. These robotic devices have been rigid in handling heavy objects and performing precise movements and positioning of objects but in addition, they have posed safety hazards at workplaces, therefore these human robots' interaction should be monitored and controlled properly.

Making Great Strides Over the Past Decade

To bridge the gap between natural organisms and conventional robots, bio-inspired soft robotics provide opportunities to produce engineered components, machines, and devices, while soft robotic advances have been making great strides over the past decade but the area of soft bio-inspired robotics has been of getting attention. Human and robot interaction can be enabled by these new tools and also between robots and the natural world. Soft bio-inspired robots have many promising applications which are biomechanically suitable to humans, including their needs in healthcare, disaster relief, and strategies for locomotion.

Challenges in the Development of Soft Robotics

System integration, design, and materials are some of the few challenges that exist in developing soft bio-inspired robots. Some guidance from engineering principles helps in the development of bio-inspired soft robots, but new issues can emerge. Versatile functionalities of robots can be produced by achieving two objectives.

1. By replacing modular systems (separate hardware for sensors, motors, etc ) with fully integrated materials that improve these functionalities.

2. By replacing hard and rigid mechanisms with soft matter (gels, fluids, elastomer, etc) that can be in contact with other objects physically.

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Nature as a Source of Inspiration in Soft Robotics

Mimicking the biological softness, compliance of the body, and diverse living organisms' vital functions, has been done because of their nature as a source of inspiration. The prime example like octopus, generating active force, sophisticated motions, skeletal muscle, and unstructured environments adaptation. Bio-inspired soft robots from soft responsive materials have attracted the perspective of both technological applications and fundamental discoveries. Bio-inspired soft robots possess several qualities compared to conventional rigid robots for example human-friendly interaction, structural deformability, environmental adaptability and compliance, and also freedom of actuation.

Extensive Efforts in the Development of Soft Robotic Strategies

A huge amount of effort has been put by researchers into the development of strategies for soft robotic actuation with built-in biomimetic intelligence, depending on a variety of responsive soft materials like liquid crystal networks (LCNs), hydrogels, and shape memory polymers (SMPs). Due to the reversible, anisotropic, and programmable shape-morphing features of LCNs, they have been very promising for soft stimulus-driven actuators with robotic motions, like walking, swimming, gripping, and oscillation, and also intelligent functions such as self-regulation, reconfigurability, and associative learning. The soft robotic system which is based on LCN is mostly hindered by their inefficient intrinsic electrical or thermal conductivity and their mechanical incompatibility with the nanomaterials.

Gallium-Based Liquid Metals

Gallium-based liquid metals in terms of their properties like superior fluidity, high electrical and thermal conductivity, extraordinary shape-deformability, excellent biocompatibility, and photothermal characteristics, are prominent in soft robotics research. However, adaptive locomotion and shape transformation of liquid metal (LM)-based soft robots are often driven by chemical stimuli or electric fields, and in an acidic aqueous environment to prevent oxidation, which can limit their potential application and functionality.


The result shows that hybrid system which combines Liquid metals and stimuli-responsive smart materials have been attracting important attention. For example promotion of improved photothermal conversion efficiency, LM-based shape transformers which are light-driven were accomplished by encapsulating LM microdroplets in graphene-quantum-dot-containing polydopamine. There have been reports of the LM microdroplets in soft elastomer matrices, stretchable composite films with increased dielectric constant, electrically self-healing ability, and thermal conductivity.

Natural Cellulose Nanofibrils

Natural cellulose nanofibrils have been used as a structural matrix for liquid metal microdroplets to make free-standing LM soft actuators, which respond to electric fields and humidity. For electrically activated shape morphing actuator with coupled sensing capability by embedding liquid metal microdroplets into a crystalline Liquid matrix has been investigated by Anisotropic liquid metal contained liquid crystalline composite. Ultrasonication for hydrogen doping in the presence of aliphatic polymers has been considered to endow the insulation of oxide skin around viscoplastic liquid metal particles with electrical conductivity.

General Strategy

A general strategy to create a shape programmable liquid metal-liquid crystal network soft actuators that have superior electrical/thermal conductivity of LMs and shape morphing reversible properties of LCNs. By using ultrasonicating eutectic Gallium-Indium (EGaIn) liquid metals and miniature carboxylated gold nanorods (MiniGNRCOOH) in a biological nanocellulose aqueous suspension, a colloidal Liquid metal ink was made. MiniGNR-COOH not only stabilizes the liquid metal nanoparticles but also improves the photothermal properties of colloidal Liquid metal ink, it results in deposited on a variety of substrates which also includes shaping deformable SMP and Liquid Crystal Networks through evaporation-induced self-assembly.

Exhibition of Excellent Electrical Conductivity in Liquid Metal (LM) Liquid Crystal Network LCN Thin Films

Extraordinary electrical conductivity and excellent interfacial adhesion have been exhibited by LM LCN thin films and that is due to the evaporation-induced partial sintering of the liquid metal microdroplets in a continuous conductive film while in the presence of nanocellulose. Using a low direct current (DC) voltage, the LM LCN film can be actuated electrothermally. LM-LCN soft actuators which are near-infrared (NIR) light-driven with temporal programming and robust shape morphing features were achieved by the selective coating of colloidal liquid metal ink and superior photothermal conversion of the embedded MiniGNRCOOH.

Robotic Functionalities

A Near-infrared (NIR) light-fueled self-sustained soft oscillator developed under constant near-infrared irradiation and an inchworm-inspired soft crawler translocating (light-driven) on a ratchet substrate via cyclic NIR irradiation, as proof of concept robotic functionalities. The conceptualization of the NIR light-fueled programmable soft robotic shadow play, known as shadow puppets and an old form of entertainment and storytelling held between a translucent screen and a source of light, by taking the advantage of shape deformable and electrically conductive LM LCN and LM SMP soft actuators. It is crucial to know that NIR light is attractive particularly for the untethered soft robots realization because of its invisibility, spatiotemporal controllability, and ubiquity.

Soft-Matter Engineering Strategy

The soft matter engineering strategy has been expected to pave a new way for efficiently forming the desired qualities of liquid metals combined with nanomaterials or functional polymers, highlighting the development of multifunctional materials for reconfigurable, programmable, and untethered soft robotics with biomimetic intelligence.

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Scanning Electron Microscopy

TEM (Transmission electron microscopy) and SEM (Scanning electron microscopy) were implemented for specifying the colloidal liquid metal ink’s nanostructures. In the scanning electron microscope, a specific concentration fortempo-oxidized bacterial cellulose (TOBC) that is 0.2 wt% is crucial for the formation of uniform and stable liquid metal nanoparticles and the MiniGNRCOOH can play an important role in stabilizing the liquid metal nanoparticles.

Energy-Dispersive X-ray Spectroscopy (EDX)

The energy-dispersive X-ray spectroscopy (EDX), when combined with scanning electron microscopy shows the nanostructured composites resulting in 12.33 wt% (In), and 8.17 wt% (Au), 56.43 wt% (Ga), 15.84 wt% (C), 7.23 wt% (O). TEM shows that liquid metal nanoparticles are contained by MiniGNR-COOH and TOBC nanofibers. Carbon dioxide and oxygen are visible on the liquid metal nanoparticles' surface, which shows the evidence of elemental mapping images, and Au is uniformly distributed all over the individual nanofiber. X-ray photoelectron spectroscopy exhibits the peak characteristic at 18.7, 19.8, and 20.8 eV, which can be assigned to gallium oxide(Ga3+), metallic gallium (Ga0), and gallium suboxide (Ga1+).

Stability of Resultant Suspension

The colloidal liquid metal suspensions stay stable for several days at the pH of 7 with negligible precipitation, which can be from negatively charged surfaces and also strong hydrogen bonding or coordination of Ga3+ among MiniGNR-COOH, liquid metal nanoparticles, and TOBC nanofibers. The colloidal liquid metal suspensions show a high optical absorption over a near and visible infrared range that is 400-1000nm and absorption at 808nm, which enhances as compared to TOBC suspension because of the introduction of MiniGNR-COOH.

The Photothermal Properties

Colloidal liquid metal ink’s photothermal characteristics/quality can be increased by increasing the MiniGNR-COOH concentration. Other wavelengths like sunlight can be used instead of irradiation wavelengths.

Investigation of Evaporation-Induced Colloidal LM Ink

To investigate the evaporation-induced self-assembly of the colloidal liquid metal ink, the colloidal liquid metal suspension can be deposited on different substrates by drop casting and then followed by a drying process in the ambient conditions and solvent evaporation. One can attain a free-standing nanostructured thin film after solvent evaporation.

Indication of the Presence of the Tempo-Oxidized Bacterial Cellulose (TOBC) Nanofibers

The presence of TOBC nanofibers can be indicated by SEM images, which are present on the top surface, and the production of stacked liquid metal nanoparticles at the bottom surface. It was confirmed by the EDX and TEM cross-sectional elemental mapping images that the liquid metal was distributed across the nanostructured film, whereas the MiniGNR-COOH was homogeneously distributed across the whole film. The nanostructured thin film can function as a conductive layer for flexible electronics as they exhibit the property of electrical conductivity, which can result from liquid metal nanoparticles’ deposition in the bacterial cellulose presence.

Maximum Ranges of Nanoparticles

108° observed on PTFE as a maximum value, which is lower on SMP 33°, LCN 30° and glass 11°. A decrease in the contact angle is an advantage for improving the interfacial adhesion strength which is indicated by the adhesion force of deposited colloidal liquid metal coating on substrates.

Flexible LCN Films

Flexible LCN films with a thickness of 23 µm are made by photopolymerization of the liquid crystalline monomers at the temperature of 70°C. With a thickness of 0.3 µm, LM coating can be developed by drop-casting deposition of colloidal liquid metal suspension by solvent evaporation and subsequent drying.

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Nano-sized particles exist in nature and they can be created with a variety of materials and products, like carbon or silver. Nanomaterials are characterized by their small size and are measured in nanometers. These materials or particles when engineered on a small scale, are called engineered nanomaterials (ENM), containing various properties like electrical, mechanical, optical, etc, and can be used and applied in different ways including making human-robotic interactions possible and also safe which can results in many advancements for mankind.


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2nd Dec 2022 Jasmine Dankworth

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