3D printing for soft robotics – a review

Figures & data

Figure 1. Examples of printed soft robots and soft devices. (a) Pre-strained polystyrene substrate with inkjet-printed hinges made of carbon black ink. (b) 3D-printed jumping soft robot. (c) 3D stereolithography-printed bat with curvature time lapse. (d) 4D-printed composite with swellableable hinges. (e) 4D-printed unfolded box composed of shape memory polymers. (f) A jumping soft robot with 3D-printed mould. (g) 4D printing of hydrogel composites for soft robotic applications. (h) A snake inspired soft robot with 3D-printed mould. (i) Multi-step 3D-printed octobot. (j) Pneumatic actuator for spinal compression and flextion with 3D-printed mould. (k) Embedded 3D printing of soft strain sensor for soft robots. (l) Multicore print head shell capacitive sensor.

Figure 2. (a) Multi-material 3D printing system by Advanced Micro Mechatronics (AMM) Research Lab, Jeju National University, South Korea. (b) Photograph of the AMM’s multi-material 3D printing system. (c) Soft omnidirectional actuator by AMM Lab. (d) (i) Fabricated soft-bot actuation of each leg at different time intervals. (ii) Model of actuation to generate movement at different time intervals. (iii) Finite-element displacement simulation results of one complete actuation cycle. (iv) Finite element strain simulation of one complete actuation cycle.

Table 1. Summary of 3D printing technology with respect to soft robots.

Mechanism	Robot part	State of starting material	Layer creation technique	Materials	Size	Advantage	Disadvantage	Application	Sub-Heading number	Reference
Stereolithography (SLA)	Bio-bot arrays		Layer by layer	Poly(ethylene glycol)		Less resin material, large build volume, Precise control, Rapid polymerization	Post-curing, warping, brittle parts with a tacky surface, support required, little choice of material, unused material is toxic	Miniaturized Walking	I	[59]
				Diacrylate (PEGDA)
								Biological Machines
	Multi-material cantilevers		Layer by layer	Poly(ethylene glycol) diacrylate (PEGDA) and acrylic-PEG-collagen (PC) mixtures	2 × 2 × 4 mm					[60]
	Actuator		Bottom up approach	Elastomeric precursor; Spot-E resin, Spot-A	40.0 × 71.1 mm^2			Octopus Tentacles		[61]
Inkjet Printing	Bellows actuators, gear pumps, soft grippers and a hexapod Robot,	Liquid	Multi-material layer by layer printing	Tangoblack+	14 × 9 × 7 cm			Hydraulically actuated robots	Ii	[62]
	Complete robot		Multi-material UV-curable printing	Urethane and epoxy	80 × 5 × 5 mm			Tri-legged soft bot with spider Mimicry		[63]
	Mould			ABS and silicon rubber				Caterpillar-inspired soft-bodied rolling robot		[64]
Selective laser sintering (SLS)	Bellow actuators		Top-Down	Elastic silicone material		No support required		Soft robotic hand	Iii	[65]
	Flexure hinges			Polyamide (PA 12, Nylon)	0.1 mm × 0.5 mm			Soft Robot Kinematics of snake		[66]
DIW	Extensible sensing skin			Silicone elastomer, hydrogel elastomers, polyacrylamide etc.	3.75 × 3.75 cm		Unable to fabricate continuous fibre-reinforced composites	Tactile Machines with kinesthetic sense	Iv	[69]
SDM	Cockroach Limbs			Viscoelastic polyurethane	120 mm × 120 mm × 50 mm			Biomimetic Components	V	[70]
	Small robot limbs			Viscoelastic polyurethane	–			Biomimetic Robotic Mechanisms		[71]
	Fingers of robot			Two-part industrial polyurethanes				Robust compliant grasper		[73]
	Hexapedal Robots			Viscoelastic polyurethane, polyester fibres and low melting temperature wax	16 cm			Performance and locomotion Dynamics of insects		[72]
	Fingers of Grasper			Polyurethane elastomer	116 cm^3			Soft, Atraumatic and Deployable surgical grasper		[74]
Fused deposition modelling (FDM)	Actuator		Layer by layer	Silicone elastomer	50 × 20 × 40 mm			Soft robot prototypes	Vi	[75]
	3D structures		Layer by layer	Nafion	5 mm × 10 mm × 0.5 mm			Macro-scale soft robotic systems		[76]
	Actuator modules			Silicon rubber elastomer	–			Soft snake		[77]
	Flexible Fingers			Poly(vinyl chloride) (PVC) sheets	–			Soft prosthetic Finger		[78]
	Soft pneumatic actuators			Thermoplastic elastomer filament ninjaflex (ninjatek, PA)	150 mm × 25 mm × 11 mm			Soft Robotic applications		[79]
3D Printing				Fugitive (Pluronic F127) and catalytic inks				Entirely soft octobot with embedded electronics		[47]
	Moulds			Elastomeric silicon				Pneumatic networks for soft robotics		[48]
	Moulds			ABS	15 cm			Robotic Tentacles		[49]
	Soft Actuators, main frame		Layer by layer	ABS Plastic	30 × 10 mm			Rehabilitation of spinalized rodents		[50]
	Whole body		Multi-material printing	Tangoplus and veroclear	8 cm			Mimicking of caterpillar motion		[51]
	Soft Skin		Multi-material printing	Tangoplus	173 cm^3			Safe human-robot interaction		[52]
	Robot Body		Multi-material printing					Combustion-powered robot		[53]
	Outer mould, lid, model core		Layer by layer	Silicon rubber	0.45 m × 0.19 m × 0.13 m			Hydraulic autonomous soft robotic fish		[54]
	Mould, passive wheel, valve holders, tail enclosure			Silicon rubber				Dynamics of a fluidic soft robot		[55]
	Mould			Silicon rubber	2.5 × 2.5 × 11 cm			Soft Robotic Gripper		[56]
	Softworms			Deformable rubber-like Polymer				Bio-inspiration soft robots		[57]

Figure 3. 3D Printing techniques used to fabricate soft robots. (i) A liquid resin is selectively photo-polymerized in the process of SLA by a laser. (ii) Inkjet printing is similar to SLA in many ways with a difference that a movable inkjet head is used in this technique to apply a photopolymer being activated by a UV lamp. (iii) The powder of metal material is rolled across a build platform and a laser is directed into the powder followed by rolling the powder over the top of as-deposited layer and this process keeps on repeating till the desired 3D object is completely fabricated. (iv) DIW is an alternative printing technique to FDM for additive manufacturing of desired objects under ambient conditions in which ink passes through a nozzle in a controlled manner. (v) Shape deposition modelling technology consists of several steps including deposition. The material in heterogeneous deposition is changed between each deposition process. (vi) Soft materials are printed in the form of a continuous filament in FDM method with a single layer being deposited at a time.

Figure 4. 3D-printed bio-medical soft robot. (a) 3D CAD model of the carotid artery. (b) 3D-printed scaled version of the carotid artery. (c) Traveling of omnidirectional robot inside carotid artery without controlled steering. (d) Demonstration of static steering of omnidirectional robot inside carotid artery. (e) Traveling of omnidirectional robot inside carotid artery [4].

Table 2. 3D-printed in vitro and in vivo soft structures.

Category	Type	Function	Soft material	3D printing technology	Ref.
In vitro	Exoskeletal Implant	Rehabilitation	Neoprene	3D-Printed mould	[94]
	Wearable exoskeleton	Therapy and monitoring	Silicone rubber	3D-Printed mould	[96]
	Electronic soft skin	Physical senses in implants	Silicone	Direct ink writing	[100]
	Electronic soft skin	Physical senses in implants	ABS/PDMS/SMA	3D-Printed Scaffold	[104]
	Life-like organs on a chip	Drug discovery and testing	Real organs cultured tissues	Multi-material 3D printing	[105]
In vivo	Tracheobronchial implants	Treatment of bronchial disorder	PCL and Polyester	SLS printing	[112]
	Soft robotic sleeve for heart	Supports and strengthens functionality	Silicone	3D-printed alignment fixture	[115]
	Bio-morph cantilever	Organ repair	Cardiac cell sheet/hydrogel PEGDA	Direct 3D printing	[59]
	Muscle powered biological machines	Muscle repair or replacement	Cultured muscle cells, PEGDA	3D-printed mould and skeleton	[116]

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