3D Printing in Organ-on-a-Chip Platforms: Comparison
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Three-dimensional (3D) in vitro models, such as organ-on-a-chip platforms, are an emerging and effective technology that allows the replication of the function of tissues and organs, bridging the gap amid the conventional models based on planar cell cultures or animals and the complex human system. Hence, they have been increasingly used for biomedical research, such as drug discovery and personalized healthcare. A promising strategy for their fabrication is 3D printing, a layer-by-layer fabrication process that allows the construction of complex 3D structures.

  • biofabrication
  • organ-on-a-chip
  • 3D bioprinting
  • in vitro models
  • biosensors
  • biomicrofluidics
  • Lab-on-a-chip

1. Introduction

Over the last decades, conventional manufacturing techniques, such as photo-patterning, self-assembly, and soft lithography, have been used for the manufacturing of microfluidic devices and also OoC. In particular, lithography has been the most common technique performed. Nevertheless, this is a highly expensive process with complicated and time-consuming manual procedures performed in a clean-room environment [10,15,16,17,18,19][1][2][3][4][5][6]. In this sense, recent advances in microfabrication techniques, cell biology, microfluidics, and 3D printing enabled the rapid manufacturing of OoC along with biomimetic tissue micro-architectures, which can provide the basis for preclinical assays with greater predictive power [5][7]. Compared to conventional manufacturing techniques, 3D printing includes the advantages of unlimited design space, freedom of complex geometries, and reduction of waste products [20,21,22][8][9][10]. In particular, 3D bioprinting is an innovative and promising biofabrication strategy that has played an important role since it allows the deposition of biomaterial-encapsulated living cells in the manufacturing of complex 3D structures with high precision, high accuracy, and high throughput [23,24,25,26,27,28][11][12][13][14][15][16]. Considering those characteristics, bioprinters are expected to establish systems that mimic the microenvironment of the human body in a more appropriate way than the animal models and current 2D cell culture environments, enhancing the accuracy of the results and the clinical usage of OoC [25,29,30][13][17][18].

The aforementioned preclinical models used in biomedical investigation are outlined in Figure 1, highlighting the evolution of cell-culture models from simple two-dimensional to complex OoC platforms with 3D bioprinted models.

Sensors 21 03304 g001 550

Figure 1.

Schematic diagram showing preclinical models used in biomedical research.

2. 3D Printing Techniques and Their Applications to Organ-on-a-Chip Platforms

2.1. 3D Printing Techniques

3D printing has become a growing field in different areas and it has gained great interest, because of the ability to build complex structures through a layer-by-layer process with different materials in an affordable way [30]. For these reasons, some authors have been using 3D printing techniques to rapidly fabricate the microfluidic models and holders for OoC devices (

3D printing has become a growing field in different areas and it has gained great interest, because of the ability to build complex structures through a layer-by-layer process with different materials in an affordable way [18]. For these reasons, some authors have been using 3D printing techniques to rapidly fabricate the microfluidic models and holders for OoC devices (

Table 1

).

Table 1.

3D printing techniques used to fabricate OoC platforms.

2.2. 3D Bioprinting Techniques

As previously stated, 3D bioprinting can be described as the spatial distribution in a defined pattern of living cells. The cells are loaded and assembled through layer-by-layer deposition methods assisted by means of a computer, and used for the manufacture of organ analogs and living tissue for a different set of applications, such as pharmacokinetic, tissue engineering, cancer research, and regenerative medicine, among others [39][25]. For this purpose, biocompatible materials, such as alginate, gellan-gum, collagen, fibrin, and gelatin, are usually used to form hydrogels, called bioinks, to encapsulate cells (cf. Table 2) in order to protect them during the printing process.

Table 2.

3D bioprinting techniques used to fabricate OoC platforms.

OoC Platform Printing Method Schematic Representation Cells Types Bioink Ref.
Nervous System-on-a-Chip Micro-extrusion 3D printing strategies Sensors 21 03304 i008

Reprinted with permission from ref. [40][26]. Copyright 2001 Royal Society of Chemistry.		 Schwann cells, superior

cervical ganglia and hippocampal neurons and epithelial cells		 Embedded extrusion bioprinting- Sensors 21 03304 i025

Reprinted from ref. [61][42][40][26]
. Central nervous system-on-a-chip Magnetic bioprinting Sensors 21 03304 i009 Spinal cord cells Neural spheroids [41][27]
Multi-tissue OoC with liver, heart and lung organoids Microextrusion bioprinting Sensors 21 03304 i010

Reprinted from ref. [42][28].		 Hepatocyte; stellate; Kupffer iPS; lung fibroblasts, epithelial, and endothelial cells. Spherical organoids with HA-gelatin hydrogel (liver) and fibrin-gelatin bioink (cardiac). [42][28]
3D vascularized tissue-on-a-chip Microextrusion bioprinting Sensors 21 03304 i011

Reprinted from ref. [43][29].		 hMSCs; hNDFs; HUVECs Vascular ink (pluronic and thrombin) and cell-laden ink (gelatin–fibrin) ][43][29]
Liver-on-a-chip Direct write

bioprinter		 Sensors 21 03304 i012 HepG2/C3A cells Hepatic spheroids and GelMA [44][30]
Liver-on-a-chip Microextrusion bioprinting Sensors 21 03304 i013

Reprinted from ref. [10][1].		 HepG2; HUVECs. Gelatin and liver dECM bioinks (collagen type 1) [10][1]
Liver-on-a-chip Microextrusion bioprinting Sensors 21 03304 i014 HepaRG and HUVECs Gelatin and liver dECM bioinks (collagen type 1) [45][31]
Liver Fibrosis-on-a-Chip Microextrusion bioprinting Sensors 21 03304 i015

Reprinted with permission from ref. [46][32]. Copyright 2020 American Chemical Society.		 HepaRG, HUVECs and hepatic stellate

cells		 Gelatin and liver dECM bioinks (collagen type 1) [46][32]
Convoluted 3D renal proximal tubules-on-a-chip Extrusion custom-designed, multi-material 3D bioprinter
Sensors 21 03304 i017

Reprinted with permission from ref. [48][34]. Copyright 2017 American Chemical Society		 L929 fibroblasts; endothelial cells and smooth muscle cells Cell-laden alginate filaments [48][34]
Vessel-on-a-chip - Sensors 21 03304 i018 HAECs; HASMC and NIH/3 T3 fibroblast cell lines GelMA [49][35]
[61 Heart-on-a-Chip Direct write

bioprinter with a customized coaxial nozzle		 Sensors 21 03304 i019

Reprinted with permission from ref. [50][36]. Copyright 2016 Elsevier
]
][42]
Embedded extrusion bioprinting Sensors 21 03304 i026 [62][43]
LIFT printing Sensors 21 03304 i029

Reprinted with permission from ref. [64][46]. Copyright 2019 Royal Society of Chemistry.		 [64][46]
SLA and Bioprinting (BioScaffolder) Sensors 21 03304 i030 [65][47] Sensors 21 03304 i016

Reprinted from ref. [47][33HUVECs		 Alginate-GelMA [50][36]
Myocardium-on-a-chip Extrusion-based 3D bioprinting Tumor array-on-a-chip On-demand array printing Sensors 21 03304 i023 MDA-MB-231 breast tumor cells showed GelMA [54][40Human placental cell line and hMSCs

Some researchers identified in the literature did not study OoC platforms by specifying the target organ; the results are summarized in

Table 3

.

Table 3.

Novel approaches of 3D bioprinting/printing techniques suitable for the manufacturing of OoC platforms.

3D (Bio)Printing Technology Schematic Representation Ref.
Embedded extrusion bioprinting
]
.
PTECs-TERT1
Two-part silicone elastomer; Pluronic and

thrombin. [47][33]
Vessel-like structures-on-a-chip Coaxial nozzle-assisted extrusion-based bioprinting
Sensors 21 03304 i020

Reprinted with permission from ref. [51][37]. Copyright 2020 John Wiley and Sons. hiPSC-CSs Non-mulberry silk-based ink GelMA and PEGDMA [51][37]
Gut-on-a-chip Dual cell-printing system supplemented with a core-shell nozzle Sensors 21 03304 i021

Reprinted with permission from ref. [52][38]. Copyright 2018 American Chemical Society.		 Caco-2 cells and HUVECs Cell-laden collagen bioinks [52][38]
Thrombosis-on-a-chip Embedded extrusion bioprinting Sensors 21 03304 i022

Reprinted with permission from ref. [53][39]. Copyright 2016 Royal Society of Chemistry.		 HUVECs GelMA [53][39
]
Placenta-on-a-chip Extrusion-based 3D bioprinting Sensors 21 03304 i024

Reprinted with permission from ref. [55][41]. Copyright 2016 American Chemical Society.		 GelMA [55][41]
iPS—induced pluripotent stem cells; HA—hyaluronic acid; hMSCs—human mesenchymal stem cells; hNDFs—human neonatal dermal fibroblasts; HUVECs—human umbilical vein endothelial cells; HepG2—human hepatocellular carcinoma; HepaRG—terminally differentiated human hepatocellular carcinoma cells; dECM—decellularized extracellular matrix; PTECs—proximal tubule epithelial cells; TERT1—human telomerase reverse transcriptase; HAECs—primary human aortic endothelial cells; HASMC—human aortic smooth muscle cell line CRL1999; GelMA—gelatin methacryloyl; hiPSC-CSs—human-induced pluripotent stem cell-derived cardiac spheroids; PEGDMA—polyethylene glycol dimethacrylate; BMECs—human bone marrow endothelial cells.

It can be seen that 3D printing techniques are versatile and they can be applied to obtain a variety of OoC or multi-organ-on-a-chip, such as nervous-system-on-a chip, vascularized tissue-on-a-chip, liver-on-a-chip, renal tubule-on-a-chip, vessel-on-a-chip, myocardium-on-a-chip, gut-on-a-chip, thrombosis-on-a-chip, and tumor array-on-a-chip. These different cases are now presented.

2.3. New Approaches and Other Applications of 3D (Bio)Printing to Fabricate OoC Platforms without Specifying the Target Organ

Sensors 21 03304 i027


Reprinted with permission from ref. [60][44]. Copyright 2019 Elsevier. [60][44]
DLW and DLIP Sensors 21 03304 i028

Reprinted from ref. [63][45].		 [63][45
Device Printing Method Application Main Observations Ref.
Vessel-on-a-chip - Produce molds with diverse forms of channels. Sensors 21 03304 i001

Reprinted with permission from ref. [32][19]. Copyright 2018 John Wiley and Sons.		 A simple and cytocompatible approach was developed for fabricating hydrogel-based user-defined chips, suitable for the growth of organ or vascularized tissue models. [32][19]
Lung cancer-on-a-chip Inkjet 3D-printed chip holder and elastomeric microfluidic channels and microfluidic connectors for cell culture media routing on the higher part of the glass. Sensors 21 03304 i002

Reprinted with permission from ref. [33][20]. Copyright 2020 Elsevier.		 This lung cancer-on-chip system, includes integrated biosensors for real-time monitoring of physiological events, can be used with any organ tissue or monolayer micro-tumor models for on-chip toxicity studies. [33][20]
Metastasis-on-a-Chip Plaster-based 3D printing 3D-printed inverted chamber/channel structures as molds. Sensors 21 03304 i003

Reprinted with permission from ref. [4][21]. Copyright 2016 John Wiley and Sons.		 This system supports some aspects of the phenomena of metastasis, allowing to study the translocation of metastatic tumor cells from the primary tissue site to the downstream tissue site. [4][21]
Vessel-on-a-chip Extrusion-based 3D printing 3D printing of channel prototypes with carbopol gel Sensors 21 03304 i004 It is presented a highly affordable and practical approach in the manufacture of PDMS devices with closed fluid channels, which have great potential to reconstitute a human endothelium-on-a-chip [15][2]
Kidney-on-a-chip FDM 3D-printed template for conventional soft lithography fabrication of PDMS-based OoC Sensors 21 03304 i005

Reprinted with permission from ref. [34][22]. Copyright 2016 Elsevier.		 It is demonstrated the application of a 3D-printed template and a common cutter machine to provide a simple and affordable fabrication of OoC. [34][22]
Multi-Organ-On-a-Chip Laser SLA with epoxy resin Produce master models for the chambers and channels of the fluidic device. Sensors 21 03304 i006

Reprinted from ref. [35][23].		 This technology allows the design and rapid mass production of OoC devices. [35][23]
Lung-on-a-chip DLP 3D-printed molds to manufacture a chip model with an open well design and with lower and upper layers to mimic the human lung. Sensors 21 03304 i007Reprinted from ref. [36][24]. The fabrication technique allows the chip to be fabricated in less than a day, and the molds can also be utilized for repeated PDMS casting. Therefore, the technique is robust, cost-effective, and simple. [36][24]
SLA—stereolithography; FDM—fused deposition modelling; DLP—digital light processing.

The usage of 3D printing techniques to fabricate OoC is simple, cost-effective, robust, and allows the mass manufacture of customized OoC devices. However, attention must be taken in the selection of the 3D printing technique to obtain molds for PDMS casting. For example, molds printed via SLA/DLP methods may not be appropriate for PDMS casting because residual oligomers and monomers on the top of the 3D-printed pieces hamper PDMS polymerization [36][24]. Hence, the development of optimized surface treatments is crucial for ensuring long-term cell viability in OoC devices. Furthermore, the material utilized in 3D printing must be selected taking into account the curing temperature of the casting material in order to prevent material strain and microstructure deformation, which consequently can affect the cell viability.

3.2. 3D Bioprinting Techniques

DLW—direct laser writing; LIFT—laser-induced forward transfer; DLIP—direct laser interference patterning.

3. Other Challenges in Organs-on-Chip Devices: Sensors Integration

For a standard laboratory practice, OoC platforms require an accurate control and monitoring of the cell metabolism and environment, as well as of the biomarkers released by the organ models into the feeding medium. Currently, this monitoring is mainly achieved by off-line post-analysis, which besides being time-consuming is prone to contamination and sample degradation. To overcome this limitation, micro(bio)sensors have been investigated to be incorporated into those platforms to allow for real-time, robust, and autonomous monitoring of the organ models.

In the near future, multi-organ-on-a-chip and, ultimately, human-on-a-chip platforms are expected to be developed. For that purpose, a platform with integrated biosensors will be a huge step towards the advance of OoC platforms, providing physiological metabolism parameters of the organ model, as presented at

Figure 4 [70]. In this way, innovative experimental studies will be possible and as a result it will help to improve our understanding about the evolution of certain pathologies and how they affect the overall system [70]. A more comprehensive review on this topic can be found elsewhere [9].

[48]. In this way, innovative experimental studies will be possible and as a result it will help to improve our understanding about the evolution of certain pathologies and how they affect the overall system [48]. A more comprehensive review on this topic can be found elsewhere [49].

Sensors 21 03304 g004 550

Figure 4. Representation of the biosensors for a human-on-a-chip platform. Reprinted from ref. [70][48].

4. Future Perspectives

Although great efforts for developing new and feasible 3D (bio)printing techniques have been made, wide-scale adoption and validation are still to be achieved. Through advances in 3D printing technologies, more physiologically relevant OoC models are expected and this will accelerate the commercialization of these models and their practical use in drug discovery to overcome several human diseases. Although the focus of this work lies in 3D (bio)printing, it should be mentioned that the variable “time” has also been integrated, giving rise to 4D bioprinting, where printed items (for example, responsive biocompatible materials or cells) are able to change their functionalities or shapes with time once an external stimulus is imposed [72].

Although great efforts for developing new and feasible 3D (bio)printing techniques have been made, wide-scale adoption and validation are still to be achieved. Through advances in 3D printing technologies, more physiologically relevant OoC models are expected and this will accelerate the commercialization of these models and their practical use in drug discovery to overcome several human diseases. Although the focus of this work lies in 3D (bio)printing, it should be mentioned that the variable “time” has also been integrated, giving rise to 4D bioprinting, where printed items (for example, responsive biocompatible materials or cells) are able to change their functionalities or shapes with time once an external stimulus is imposed [50].

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