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International Journal of Bioprinting Bioprinted skin for testing of therapeutics

1. Introduction on-demand (DoD) processes inkjet printing, microvalve
bioprinting, and laser-based bioprinting. Extrusion-based
The limited availability of human tissue for preclinical bioprinting uses pneumatic or mechanical mechanisms,
assays compounds the need for the use of animal-based allowing close control of bioink material flow, making the
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studies during the drug development process. In addition process ideal for the deposition of pre-mixed hydrogels. 14,15
to the ethical concerns associated with the use of animal Inkjet bioprinting is DoD process, which typically uses
models, such studies may not be representative of the thermal actuators or piezoelectric actuators to dispense
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outcome of subsequent first in human studies. This individual droplets of bioink in the picoliter range. This
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contributes to the high failure rate that the pharmaceutical makes inkjet printing ideal for low-viscosity bioinks with
industry experiences when taking drugs to human clinical lower cell densities. Laser-based systems use the principle
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trials. In vivo animal models ultimately differ in physiology of laser-induced forward transfer to dispense individual
to humans. Even in the case of larger animal models such droplets of material, which are typically cell-laden
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as non-human primates, many underlying differences hydrogels. Like inkjet bioprinters, microvalve bioprinters
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remain, influencing the efficacy of animal models as pre- dispense low-viscosity bioinks, but it has also been
clinical tools. When adverse immune reactions (which demonstrated that microvalves can be arranged in a print-
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may cause type IV hypersensitivity reactions) of a drug are head such that they can print materials such as hydrogel
not identified in preclinical studies but become apparent precursors laden with a high density of cells. For low-
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during clinical trials, the consequences for participants can viscosity bioinks, microvalve can deposit high droplets of
be seriously life-threatening. Furthermore, the financial high-cell-concentration solutions in the nanoliter range,
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burden resulted from clinical trial failure may further and so for the development of micro-tissue models offers
hinder drug developers. With high rates of failure at clinic an efficient way of depositing the numbers of cells that
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and a reported decline in both pharmaceutical research and such models require.
development productivity and investment, the mantra of
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“fail early, fail fast” is becoming increasingly important to The bottom-up nature of biofabrication has led to the
drug developers. production of a range of engineered tissue-like constructs
or scaffolds suitable for tissue engineering such as skin
Following the signing of the FDA Modernization Act equivalents, constructs for bone repair, cartilage-like tissue,
2.0, drug developers are under increasing pressure to and cardiac models. 21-24 However, these examples typically
investigate the use of non-animal-based assays. While in rely on allogeneic materials to create 3D tissue constructs.
vitro and ex vivo assays are used during the preclinical Examples of bioprinted tissues where autologous cells and
development process, there is a growing need for tools that autologous materials are applied largely fall within the
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can provide greater physiologically relevant complexity scope of regenerative medicine. 25,26 In contrast, it has been
and interactions. Ex vivo assays can be used to determine demonstrated that using traditional top-down approaches
adverse immune responses to systemic therapies. However, to tissue engineering can produce fully humanized
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human-based ex vivo assays that seek to bridge the gap constructs, which may be adapted to autologous settings.
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between preclinical animal studies and human clinical In particular, the use of inert permeable plastic scaffolds
trials are challenging to scale up. Looking beyond the scope with porous substrates, such as Transwell® and Alvetex®
of drug development, access to human tissue remains a scaffolds, has become almost ubiquitous as the method of
barrier even for medical and biological research. With an creating bi-layered keratinocyte-fibroblast co-cultures for
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overall movement toward the 3Rs (Reduction, Refinement, skin equivalents. However, traditional approaches to tissue
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and Replacement), there is growing interest, however, in engineering lack the benefit of automation and therefore the
human in vitro three-dimensional (3D) models. scalability offered by biofabrication. In considering scalability,
In recent years, the fields of in vitro tissue engineering the use of standard well-plate formats is a key enabler as it
and 3D cell culture have benefited from the growth allows for scale up and scale out in a format familiar in most
of biofabrication techniques. 10,11 Within the tissue microbiology labs and which is easy to interface with many
engineering community, biofabrication is considered to downstream processes. As such, there is limited availability
be the intersection of additive manufacturing, the layer- of scalable, fully humanized and autologous tissue available
by-layer process of manufacturing a 3D construct, and for preclinical in vitro drug screening.
tissue engineering. To biofabricate tissue constructs, Here, we demonstrate how the permeable scaffold
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bioprinting is commonly used to organize and print cells approach can be combined with microvalve-based
or cell aggregates to create a 3D tissue model. Bioprinting biofabrication to develop a fully human, autologous skin
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itself can be categorized into four commonly used equivalent which can be co-cultured with autologous
methods: extrusion-based bioprinting and three drop- immune cells in a 96-well format. This approach provides

Volume 10 Issue 2 (2024) 477 doi: 10.36922/ijb.1851