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3D Printer for Anatomic Models
often based on population-averaged geometries [3,13] . deposited simultaneously . While changing or mixing
[26]
The materials used in such models are usually different materials in a single droplet generator unit is difficult,
hard and soft polymers, such as thermoplastics, waxes, IJP can easily achieve anisotropic properties by creating
or rubbers. With casting and molding techniques, the inclusions of various materials [16,17,27] , or even seemingly
mechanical properties of various represented tissues can gradient composition change . However, IJP is limited
[28]
be matched mainly through material selection as these in creating hollow and completely closed cavities because
traditional technologies produce fully dense parts. This droplets need support underneath them. Therefore, internal
level of matching is often sufficient for certain mass- structuring is only possible if the support material can be
produced educational models , but the requirements washed or cut out after printing without damaging the
[6]
of medical product development and testing, as well printed object. From the standpoint of anatomic models,
as preoperative planning may benefit from a better a relevant IJP printer on the market is the J750 Digital
mechanical fidelity [3,14-17] . Anatomy Printer by Stratasys Ltd. (Eden Pairie, MI)
[24,29] . This offers an outstanding performance concerning
1.2. Additive manufacturing (AM) of anatomic geometric representation and the number of materials and
models colors used, including soft materials [30,31] . However, the

AM, also called three-dimensional (3D) printing, has mechanical realism of soft tissue representing materials is
[15]
become an increasingly influential group of technologies still criticized .
in the field of anatomic models and other medically Using soft materials is an intensely researched
[32]
relevant areas in recent years [12,18,19] . Achieving better direction of AM . Besides IJP, thermoset, photoreactive
geometric and mechanical fidelity is possible with the or chemically cured materials like certain silicones, resins
combination of medical imaging technologies and AM . or hydrogels may be deposited through extrusion as well,
[20]
For polymeric materials, the two dominant groups of which is also called direct ink writing (DIW) [3,33-35] . This
AM techniques are based on photopolymerization and is used to print models of various soft structures [3,36-38] .
on extrusion . In an extrusion-based technique called Most of these operate with pressurized material reservoirs
[3]
fused filament fabrication (FFF), a thermoplastic filament with controllable valves or syringe extruders to deposit
is pushed into a heated extruder, and deposited through soft materials. The rheological properties of the printed
a nozzle . This is mostly used for bone modeling and material, such as viscosity or thixotropy, are decisive
[21]
mold making in the field of anatomic models [3,9] . FFF for maintaining the shape of the printed object. Creating
is cheaper than most other AM technologies due to its closed air inclusions is theoretically possible with FFF
[20]
relative simplicity and fierce competition between several and certain DIW techniques .
manufacturers. These systems can process a large variety Silicone rubbers offer a range of mechanical
of hard thermoplastic filaments but are limited in their properties that may be ideal to represent soft tissues
[33]
ability to handle soft materials. A large proportion of in anatomic models . Certain silicone AM (SAM)
available medical image-based anatomic models are technologies are already being applied to anatomical
made of hard plastic using FFF . models in some research endeavors and early-stage
[20]
Liquid photopolymers can be deposited and commercial services [33,39-47] . These are summarized
solidified in small droplets via material jetting, which in Table 1. The collaboration of Wacker Chemie AG
is called inkjet printing (IJP). Among others, it has (Munich, Germany) with ACEO (Burghausen, Germany)
been used to create surgical training models of aortic led to a droplet-based silicone printing technology, which
aneurysms, kidney tumors, skulls and fetuses [15,22,23] . relies on curing each layer of silicone with UV light, in a
IJP can also use multi-colored inks to make full-color similar fashion to IJP [39,48] . Dow Inc. (Midland, MI) and
objects [9,20] and even use multiple hard and soft materials German RepRap GmbH (Feldkirchen, Germany) created
in a single print job [14,16,17,24,25] . The deposited droplets an extrusion-based technology called Liquid AM (LAM)
can be understood as voxels. Given the proper printhead, which deposits silicone with extrusion and cures it layer-
this allows multiple voxels of multiple materials to be wise using a heat source [40,49] . Another SAM process is

Table 1. Summary of relevant commercial soft material printing technologies
Group name Process name Principle Material
Stratasys Ltd. J750 Droplet jetting Photopolymers
Wacker Chemie AG ACEO Droplet jetting Silicone rubbers
Dow Inc./GermanRepRap GmbH LAM Extrusion Silicone rubbers
Fripp Design Ltd. Picsima Extrusion Silicone rubbers
Spectroplast AG Spectroplast Vat photo-polymerization Silicone rubbers

146 International Journal of Bioprinting (2021)–Volume 7, Issue 4