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Zolfagharian, et al.
polymeric soft actuators make the design more structural optimization (BESO) , and level set
[19]
[20]
challenging . Finite element analysis (FEA) can be used to design a structure with maximum
[9]
could be used as a digital tool in conjunction stiffness density. The structural optimization
with topology optimization (TO) to simulate method permits the normalized density of 1
the behavior of soft actuators before proceeding and , where describes the minimum
[21]
with the fabrication process . Computer-aided material value, to not to create singular matrices .
[11]
design and the performance attributes of the The actuation of bioprinted polyelectrolyte
design are assessed by optimization engines to hydrogel used in this paper is caused by the
save the labor and time in finalizing the design Donnan effect where the applied voltage causes
in an additive manufacturing-oriented design an ionic concentration gradient in the direction of
approach [12] . the applied electric field that initiates an osmotic
There have been a number of studies on the pressure gradient within the hydrogel, causing
dynamic response of hydrogels to external the reversible bending of the actuator. In recent
stimuli where hydrogels were 3D printed into study, the relationship between the actuator
shape memory plastics to produce structures that deflection, applied voltages, ion concentration,
exhibit reversible volumetric strains of up to and reaction parameters to achieve the maximum
10 times the original volumes, thus simulating deflection were determined. Moreover, it was
muscle behavior [13-15] . Reversible bending demonstrated that the actuator deflection was
motion of a hydrogel was demonstrated in an pattern dependent and the lattice patterned
electrochemical cell where the electrochemically specimen exhibited larger bending deflection
induced actuation was achieved by the osmotic compared to that of cast solid actuator [22] .
pressure caused by the Donnan effect [16] . In Although the latticed sample enabled better
another study, electroactive polymers were bending performance, the pattern can be further
used to manifest reversible movement due to optimized to maximize bending using equal
the diffusion of dopant anions through a porous quantity of the material.
membrane within the layered structure of the In this study, we designed a soft actuator with
composite polymer [17] . an optimized lattice pattern using two different
This paper investigates the optimization materials. In this regard, boundary conditions
of the multimaterial printing of electrically were defined based on a cantilever beam with
responsive 3D bioprinted soft actuator with a distributed force caused by osmotic pressure
respect to actuation performance. FEA and TO created in the electrolyte. The objective of this
were used to investigate the effect of material study is to optimize the actuator performance
configuration on bending amplitude of the soft through material configurations in different layers
actuator at constant volume fraction. This study while maintaining the volume fraction of the
demonstrates an approach to optimized design material.
that can be applied to other similar soft robot and The loading and boundary conditions are
actuator systems. illustrated in Figure 1. A stress-based TO would
require several constraints that increase the
2 Two-material TO of soft actuator complexity of computation; moreover, the non-
linearity of the stresses would cause computational
TO aims at optimizing design by arranging overload due to convergence difficulties . The
[23]
material placement. For example, materials can TO modeling for the soft actuator was based on
be removed from low stress areas and applied to the SIMP approach and the design goal set to
the locations of high stress resulting in a porous minimum strain energy or maximum stiffness
structure with variable density. Common TO with optimal structural configuration. To control
methods, namely, solid isotropic material with the actuator stiffness and the optimization
penalization (SIMP) , bidirectional evolutionary convergence, a volume constraint was set .
[18]
[24]
International Journal of Bioprinting (2020)–Volume 6, Issue 2 51