ETH Library
Indirect 3D and 4D Printing of Soft
Robotic Microstructures
Journal Article
Author(s):
De Marco, Carmela ; Alcantara, Carlos; Kim, Sangwon; Briatico, Francesco; Kadioglu, Ahmet; de Bernardis, Gaston; Chen,
Xiangzhong; Marano, Claudia; Nelson, Bradley ; Pané, Salvador
Publication date:
2019-09
Permanent link:
https://doi.org/10.3929/ethz-b-000359536
Rights / license:
In Copyright - Non-Commercial Use Permitted
Originally published in:
Advanced Materials and Technologies 4(9), https://doi.org/10.1002/admt.201900332
Funding acknowledgement:
702128 - 3D-printed magnetic microfluidics for applications in life sciences (EC)
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1
DOI: 10.1002/((please add manuscript number))
Article type: Communication
Indirect 3D and 4D Printing of Soft Robotic Microstructures
Carmela De Marco*&
1
, Carlos Alcantara&
1
, Sangwon Kim
1
, Francesco Briatico
3
, Ahmet
Kadioglu
1
, Gaston De Bernardis
2
, Xiangzhong Chen
1
, Claudia Marano
3
, Bradley J. Nelson
1
and Salvador Pané
1
1
Multi-Scale Robotics Laboratory, Institute of Robotics and Intelligent Systems, ETH Zurich,
Tannenstrasse 3, CH-8092 Zürich, Switzerland.
2
Kantonnspital Aarau, Tellstrasse 25, CH-5001 Aarau, Switzerland
3
Department of Chemistry, Materials and Chemical Engineering, Politecnico di Milano
*e-mail: demarcoc@ethz.ch
(& These authors contributed equally)
Keywords: 3D printing, 4D printing, hydrogels, shape memory polymers, microstructures,
soft robotics, direct laser writing
The development of 3D soft-robotic components is currently hindered by material limitations
associated with conventional 3D printing techniques. To overcome this challenge, we propose
an indirect 3D printing approach based on the fabrication of 3D printed sacrificial templates.
High-resolution micromolds produced by direct laser writing were infused with polymers and
then dissolved, leading to the final 3D printed soft microstructures. We used this method to
indirectly print 3D and 4D soft-microrobots. The versatility of our technique is shown through
the fabrication and actuation of gelatin helices filled with magnetic nanoparticles. In addition,
we show that stent-like microstructures with shape memory properties can be manufactured
with minimum features of 5 µm, which is 40 times smaller than those reported to date. In
summary, the utilization of this technique can overcome obstacles associated with the
fabrication of soft microrobots and surgical tools for minimally invasive surgery.
2
Untethered small-scale robots have emerged as promising tools for biomedical applications
such as targeted drug and cell delivery, localized diagnosis, and biopsies
[1–3]
. Over the last
decade, the field of small-scale robotics has explored how these small devices should swim
[4,5]
,
adapt their shapes
[6–8]
, and controllably carry and release therapeutic cargos.
[9–14]
While recent
research efforts have also been made towards the use of these tiny machines in vivo
[15–17]
,
several aspects need to be synergistically addressed in order to ultimately provide relevant
clinical solutions with micro and nanorobotic technologies. As in the field of robotics
[18]
, a
widespread tendency today among small-scale roboticists is to substitute hard components
with softer elements, including parts made from elastomers, hydrogels, macromolecular
systems and biomolecules. This paradigm shift in materials is a logical development from the
locomotion and actuation point of view. Soft materials allow for more sophisticated
movements such as deformation, shrinkage/swelling, and changes in morphology
[19,20]
.
Additionally, soft materials display mechanical properties that are much closer to those of
biological structures such as tissues, and they usually exhibit enhanced biocompatibility
characteristics. Soft matter can also be programmed to biodegrade by the body’s chemistry,
for example, by enzymes or pH
[9,10]
. Consequently, the production of micro- and nanorobotic
platforms from soft building blocks will advance the field of small-scale robotics (in terms of
material constituents) towards medical applications.
An impediment to the miniaturization of small soft components lies in the available
manufacturing methods and their related limitations, which are mainly a result of the
physicochemical nature of the soft material. In order to fabricate micro- and nanodevices with
superior capabilities, the production of components with any conceivable shape is essential.
To date, 3D printing (3DP) techniques have offered a wealth of opportunities for creating 3D
structures with virtually unlimited shapes or materials. 3DP has also strongly impacted the
field of robotics, by enabling the fabrication of soft robotic engines with sophisticated 3D
continuous movements. Although substantial research has been devoted to the production of
3
3D printed small-scale robotic tools, these are typically made of non-responsive, stiff
materials
[21–23]
or, in the case of soft materials, their geometrical features are either
rudimentary, or too large (from ~200 µm to a few mm) to be used as biomedical micro or
nanorobots
[24–26]
.
Here, we propose a method that uses the capabilities of Direct Laser Writing (DLW)
[27]
to
produce complex 3D sacrificial templates for molding polymers that cannot be directly 3D
printed at the microscale by any other technique. One of the first examples of using sacrificial
templates obtained by DLW showed the fabrication of gold helices as photonic metamaterials
as broadband circular polarizer
[28]
. Electrodeposition in DLW templates has also been utilized
for the fabrication of magnetic microrobots
[29,30]
. However, DLW templates have not yet been
used for the 3D shaping of polymers at the microscale. The indirect 3D printing technique we
propose enables the fabrication of 3D and 4D soft robotic structures at the microscale. The
process is schematically depicted in Figure 1. We demonstrated the potential of this approach
with two examples of microrobotic structures made from two different types of polymers: (a)
gelatin, a non-photocurable hydrogel, with and without nanoparticles embedded in the matrix
and (b) a mercapto-ester polyurethane-based shape-memory polymer (SMP). The indirect 3D
printing method used to shape these two different polymers is schematically presented in
Figure 1A and Figure 1B, respectively.
4
Figure 1 (A) Schematic of the indirect 3D-printing process: a silicon substrate (i) is spin-coated with a positive-
tone photoresist (ii) and the 3D template is written into the photoresist by DLW (iii), developed afterwards and
filled with the hydrogel (iv). After gelification (v), the 3D printed microswimmers are obtained by stripping
away the photoresist (vi). (B) Schematic of the indirect 4D-printing process: a glass slide (i) is spin-coated with a
positive-tone photoresist (ii) and the 3D template is written into the photoresist by DLW (iii), developed
afterwards and filled with the curable SMP (iii). After curing under UV light (iv), the 4D printed microstructure
is obtained by stripping away the photoresist (v).
Indirect 3D printing of gelatin microstructures
We have chosen to focus on gelatin as the hydrogel and helix as the shape for two reasons:
gelatin is a hydrogel extensively used in tissue engineering and bioprinting for its high
biocompatibility
[31,32]
, and helical microstructures, also known as artificial bacterial flagella
(ABFs), are widely adopted microrobotic designs as they can propel by corkscrew locomotion
with low rotating magnetic fields
[33,34]
. Although we have recently shown that biodegradable
