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Jaksa, et al.
developed by Spectroplast AG (Zürich, Switzerland), a also work with having both the soft matrix and a harder
spinoff company of ETH Zürich. This method uses layer- reinforcement being deposited through DIW . In any
[51]
wise photopolymerization in a liquid silicone bath . case, this strategy would allow the hardening, toughening
[43]
Another method called Picsima by Fripp Design Ltd. (further referred to as “up-tuning”) of bulk mechanical
(Rotherham, UK) represents a different bath-based printing properties compared to the original matrix material. Since
approach, namely extruding the catalyst component of a both FFF and extrusion-based DIW can print closed and
two-part silicone into a bath of the base component . empty cavities, the weakening, and softening (further
[41]
SAM may also utilize a non-planar coordinate system. referred to as “down-tuning”) of mechanical properties
Coulter et al. developed a printing method specialized on would also be possible .
[15]
rotating printing surfaces, which offers unique advantages Therefore, the main aim of this research was to
in realizing certain geometries [44,45] . Despite the promising design, build and test a 3D printer based on the concept
development that these technologies represent, almost of combining hard and soft materials for printing more
all focus on single-material printing. Therefore, the realistic anatomic models. As a proof of concept, the
capabilities to tune mechanical properties are limited to printer should be capable of printing at least one soft
realizing porous structures with internal cavities . and one hard material, and thus achieve both up-tuning
[32]
and down-tuning to influence mechanical properties.
1.3. Problems in mechanical realism Moreover, the printer should also realize thin-walled
These AM technologies (IJP, FFF, and DIW) are highly structures and closed internal cavities with the soft
applicable to create personalized anatomic models that material since these are relevant features in anatomic
are geometrically unique . However, geometric or color models. In this study, a 3D printer with these features was
[3]
fidelity alone do not satisfy all possible needs of medical built, and its abilities were evaluated through qualitative
device development, surgical education, or preoperative analysis of various printed proof-of-concept objects,
planning. For more advanced applications, models including a small ribcage model based on a medical
should behave realistically under physical manipulation image. The applicability of the system in the field of
with hands or surgical instruments . To achieve such anatomic models and the future direction of research are
[20]
surgical realism, the materials used to represent various also discussed.
biological tissues need to have similar mechanical
properties to the tissues, such as density, elastic modulus, 2. Materials and methods
hardness, tensile strength, or viscoelasticity [15-17] . While 2.1. Technology definition
matching hard tissues like bone with AM is already a
mature field, there are still many unsolved problems The design process of this novel AM system started
regarding soft tissues . Most biological tissues – unlike with a comparison of various AM technologies and
[20]
technical materials – exhibit multi-level hierarchic their specifications, as clarifying differences is
structures of various functional building blocks, which critical for choosing the right printing concept. The
often results in anisotropic and viscoelastic mechanical fact that IJP, DIW, and FFF can handle different
properties [15,50] . This behavior could be approximated materials in the same print is a required feature to
with soft-hard multi-material structures [14,16,17] , but to produce multi-material structures. Other technologies
date, there are no AM technologies available that can based on material jetting or vat photopolymerization,
approximate a multitude of tissues . Therefore, two such as binder jetting (BJ), stereolithography (SLA),
[15]
major areas for improvement could be printing both and digital light processing (DLP) all use a single-
hard and soft materials simultaneously, and tuning local material bath (or “vat”) of liquid resin or powder [21] .
mechanical properties through multi-material structuring. This prevents multi-material printing, and the
These should happen simultaneously to produce high creation of closed air inclusions. For IJP, DIW, and
quality anatomic models that resemble real tissues from a FFF, changing materials simply requires switching
mechanical standpoint . to a different filament, cartridge, or printhead.
[15]
Mimicking the macroscopic mechanical properties
1.4. Research aims of biological tissues through up- and down-tuning
Combining extrusion-based AM technologies such as requires printing both soft and hard materials.
FFF and DIW may be helpful for making more realistic Extrusion is the preferred method to create closed
anatomic models. While using FFF to produce the whole internal cavities and support structures, if needed. The
model is ineffective regarding mechanical realism, mentioned technologies are compared considering
thermoplastics may be used as fiber reinforcement if our construction preferences in Table 2. Further
printed into a softer matrix material, like a silicone rubber descriptions and schematics of these technologies are
that is deposited by a DIW printhead. Such a concept may available in other literature [21,32] .

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