A spinning 3D printer nozzle creates soft robots with built-in air channels that bend in programmed directions, turning flat printed structures into grippers and shape-shifting devices.
(Nanowerk Spotlight) Pneumatic soft robots use pressurized air for actuation. Hollow channels embedded within a flexible elastomer body inflate when air is pumped in, causing the surrounding material to expand. If the channel sits off-center within a filament, with a thinner wall on one side and a thicker wall on the other, the thin side stretches more readily. This produces controlled bending rather than uniform ballooning. By varying channel placement, size, and orientation throughout a structure, designers can program specific shape-morphing behaviors: bending in multiple directions, localized hinges, twisting motions, or combinations thereof.
This principle underlies applications from grippers that conform around delicate objects to wearable devices that assist human movement. The more precisely these internal channel features can be controlled during fabrication, the more sophisticated the resulting actuation.
Conventional manufacturing approaches cast elastomers into molds to pattern channels, then laminate layers together to encapsulate them. Each new design demands a new mold. Intricate channel geometries require multiple fabrication steps that introduce alignment errors and limit achievable complexity. Multimaterial 3D printing has begun to help, but most methods cannot produce asymmetrical channel cross-sections with continuously varying orientation along a printed filament.
A study published in Advanced Materials (“Rotational Multimaterial 3D Printing of Soft Robotic Matter With Embedded Asymmetrical Pneumatics”) describes a technique that overcomes these limitations. Researchers at Harvard University and Stanford University developed rotational multimaterial 3D printing, which produces soft robotic structures with embedded asymmetrical pneumatic channels in a single continuous operation. The method allows programmable control over channel orientation, cross-sectional area, and geometry along each printed filament.
Rotational multimaterial 3D printing of soft robotic filaments with embedded pneumatics. (a) Schematic view of a simple fluidic actuator showing the effect of an asymmetric pneumatic channel on bending. (b) Rendered image of rotational 3D printing printhead that enables co-printing of elastomeric and fugitive inks with customized nozzles at controlled ink flow rates. (c) Illustration of the printing nozzle outlet showing the fugitive ink-filled sector defined by φ and (d) optical image of the nozzle outlet with quarter-circular φ = 45° (left) and semicircular sectors φ = 0° (right). (e) Side profile of filament printing at a dimensionless rotation rate of ω = 0 and (f) ω = −1. (g) Cross-section images of printed filaments with θf = 0° (top) and θf = 180° (bottom) configurations. (h) Continuous filament printed in a serpentine pattern with varying ω and Δθf. (i) Effect of varying dimensionless fugitive flow rate Qf = 0.83 (top) and Qf = 0.27 (bottom) on the cross-section of the fugitive ink-filled channel. (j) Storage (G’) and loss (G”) moduli as a function of shear rate for fugitive and elastomeric inks at printing temperature (22°C) and for fugitive ink at cooled evacuation temperature (0°C). (k) Optical images showing evacuation of fugitive ink by pumping cold water (0°C) through the filament. (Scale bar = 1 mm unless otherwise denoted). [Note: Printed filaments with Qf = 0.83 (and Qtotal = 0.86) have a diameter of 2.6 ± 0.06 mm (n=9). All filaments are printed at a velocity v = 3 mm/s and print height of h = 3.1 mm (dimensionless printing height, h = h/(2R) = 1.1)]. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
The technique co-extrudes two inks through a custom nozzle capable of controlled rotation during printing. The structural ink is a photocurable urethane acrylate elastomer. The fugitive ink is 30 wt% Pluronic F-127 in water, which behaves as a gel at printing temperature (22 °C) but liquefies at 0 °C, allowing it to be flushed from the cured structure to leave hollow channels behind.
Each nozzle is fabricated via direct light projection printing at roughly 50 µm resolution and contains three flow channels that position the fugitive ink asymmetrically within the elastomeric shell. The asymmetry geometry is defined by an angle φ from the nozzle bisecting line. The researchers tested φ = 0° (semicircular) and φ = 45° (quarter-circular) configurations. The quarter-circular geometry worked better for 2D architectures because it reduces lateral expansion during inflation, keeping bending primarily along the filament axis.
Nozzle rotation provides another degree of freedom. A dimensionless rotation rate ω* = Rω/v relates angular velocity to print speed. At ω* = 0, the channel maintains constant orientation. At ω* = −1, continuous rotation produces a helical channel. Discrete rotational steps allow abrupt reorientations: a +180° rotation over a short transition length flips the channel position, reversing the bending direction in that segment.
The filament geometry is governed by dimensionless flow rate parameters. Standard conditions produced filaments with a diameter of 2.6 ± 0.06 mm and shell thickness of 300–360 µm. Mechanical characterization of 25 mm filaments showed curvature increasing with applied pressure up to 103 kPa. Computational models using Ogden hyperelastic parameters matched experimental results within 20%. Blocking force measurements yielded 10.3 mN for a single filament, scaling to 52 mN for four parallel filaments at the same pressure.
Varying the fugitive flow rate along a filament creates regions of differential actuation. Reducing the dimensionless flow rate Q*f from 0.83 to 0.27 produces a smaller channel that barely inflates under pressure. The researchers used this to fabricate hinge actuators: a 10 mm active segment flanked by 25 mm “inert” segments produces localized high-angle bending, achieving nearly complete folding at 103 kPa.
Combining orientation control and flow rate modulation allows complex transformations. A single continuous filament containing eight hinges separated by inert struts, with programmed orientation changes, transforms into a cubic wireframe upon inflation to 83 kPa. A filament printed with continuous rotation bends and twists into a coil exhibiting 880° of angular displacement at the free end.
The researchers also showed that filling an actuated filament with photocurable resin and UV-curing it locks in the deformed shape, allowing a 25 mm filament to support a 200 g load. This capability suggests potential for hybrid stiff-soft robotic systems.
Two-dimensional architectures bring additional design considerations. The interface orientation between regions of opposing channel direction affects behavior: when the interface runs parallel to the print path, adjacent channels constrain each other; when perpendicular, bending proceeds freely. The researchers demonstrated checkerboard patterns that yield complex three-dimensional surface deformations upon pressurization.
Perhaps the most notable aspect of the work is the integration of algorithmic print pathing. The team adapted connected Fermat spirals, an algorithm originally developed for thermoplastic printing, to generate continuous toolpaths for arbitrary 2D geometries. The algorithm computes iso-contours from a distance transformation of the input boundary, then connects them through linking functions to produce a single unbroken trajectory. The researchers modified this framework to track printhead orientation automatically, adjusting rotation to maintain consistent channel positioning relative to local filament direction.
This workflow was demonstrated on a vectorized flower image, producing a structure that curls its petals upward when pressurized. The most complex demonstration started from a photograph of a human hand. Each digit was vectorized as a separate fluidic domain, processed through the Fermat algorithm with hinge locations at physiological knuckle positions. The printed gripper actuates individual fingers independently at 83 kPa and successfully grasped and lifted a foam ball through sequential digit pressurization.
The approach has limitations. Cyclic testing revealed fatigue constraints: actuators performed consistently at lower pressures through 100 cycles, but at 86.2 kPa showed progressive drift, and at 103.4 kPa ruptured after fewer than 10 cycles. The researchers suggest more fatigue-resistant silicone elastomers could improve durability in future versions.
By integrating rotational nozzle control, variable flow rate modulation, and algorithmic pathing, the method creates a streamlined route from design to fabrication. Structures that would require extensive manual planning with conventional methods can instead be generated from input images through a largely automated workflow, broadening what is practically achievable in pneumatic soft robot design.
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ORCID information
Natalie M. Larson (Stanford University)
, 0000-0002-2020-2326 corresponding author
Jennifer A. Lewis (John A. Paulson School of Engineering and Applied Sciences, Harvard University)
, 0000-0002-0280-2774 corresponding author
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