The International Space Station (ISS) has transformed humanity’s ability to live and work in low-Earth Orbit (LEO). The station is positioned 400 km (250 miles) above Earth’s surface and travels at a speed of 28,000 km/h (17,500 miles/h) in a constant state of free fall towards Earth, creating microgravity conditions for astronauts and their research. Since the beginning of its assembly in 1998, the ISS has proven to be a valuable platform to understand and characterize the benefits of microgravity. Changes in physical forces include dominance of surface tension and diffusive forces, absence of sedimentation, and the absence of buoyancy-driven convection. The microgravity environment radically reshapes science as we know it on Earth, allowing us to realize possibilities such as containerless processing and creating multi-material products with improved uniformity. Biological phenomena include changes in cell aggregation, cell signaling, stem cell differentiation and proliferation. Additionally, crystallization in microgravity has seen improvements in structural quality, uniformity, and size of proteins, macromolecules and inorganics.

Bioprinting in space is one of the novel promising and perspective research directions in the rapidly emerging field of biofabrication. The National Aeronautics and Space Administration (NASA) [2], the European Space Agency (ESA) [3], and the European Union (EU) [4] are supporting several initiatives that are currently underway. There are several proposed advantages of bioprinting in space. First, under the conditions of microgravity, it may be possible to bioprint constructs using biocompatible bioinks with different viscosities than those used to bioprint on Earth. Second, microgravity conditions could enable 3D bioprinting of tissue and organ constructs of more complex geometries with voids, cavities, and channels without the need of additional support or sacrificial materials. Microgravity presents unique challenges and opportunities for the study of polymer-protein interactions, fundamentally altering their structural dynamics and assembly mechanisms compared to terrestrial conditions. The application of artificial intelligence, particularly in the design and optimization of printable bioinks for use in microgravity, holds transformative potential by leveraging predictive models to simulate these altered interactions. Advanced computational tools, such as Google's AlphaFold, could be adapted to incorporate the distinct physical forces characteristic of microgravity, enabling the precise prediction of protein structures in such environments. By comparing these predictions with Earth-based structures, researchers could identify key differences in folding and stability, guiding the engineering of bioinks with tailored rheological and biological properties suitable for bioprinting applications in space. This synergy of AI-driven modeling and experimental insights could accelerate the development of robust, functional biofabrication systems for extraterrestrial medical and research needs. Third, novel engineering designs could be developed thanks to the conditions of reduced gravity. The ideal space bioprinters must be safe, automated, compact, and user friendly, to allow easy operation while in constrained environments. Thus, there are no doubts that systematic exploration of 3D bioprinting in space will advance biofabrication, and more specifically bioprinting technologies per se. Vice versa 3D bioprinted tissues could be used to study pathophysiological biological phenomena when exposed to microgravity and cosmic radiation that will be useful on Earth to understand ageing conditioning of tissues, and in space for the crew of deep space missions [5].

Multi-levitation biofabrication technology funded under the EU PULSE project is being used to create cardiovascular 3D in vitro models able to better mimic cardiac and vascular physiology compared to organoids. Such models could be used to study cardiac ageing and test the efficacy of anti-inflammatory/anti-oxidative drugs with anti-ageing potential [6, 7]. The potential of using biofabrication in space goes beyond cardiac models but can be expanded to all those tissues and organs that are affected by ageing on Earth and can be affected in space during long term stays. Examples include skeletal, muscle, lung, endocrine, reproductive system, and intestine (microbiome shifting) tissues and organs.

Redwire’s BioFabrication Facility (BFF extrusion bioprinting) was the first American system flown to the ISS that is capable of manufacturing human tissue models in the microgravity conditions of space through pneumatic extrusion-based 3D bioprinting. BFF utilizes a variety of cell types to create viable tissue in space using ultra-fine layers of bioink. BFF was originally launched to the ISS in 2017, returned to the ground in 2019 for upgrades and returned to the ISS in 2022 ready to print. To date the BFF has printed a human knee meniscus and cardiac tissue in microgravity [8]. Upcoming prints include blood vessels and liver tissue. Bioprinting in microgravity allows for the use of less viscous bioinks and printing of scaffold free structures. The printed tissue cannot then be returned to Earth in that state as it would still collapse under its own weight in Earth’s gravity. Instead, 3D printed tissue must be cultured and conditioned on the ISS after printing. Redwire’s ADvanced Space Experiment Processor (ADSEP) is used to culture and condition the tissue printed in BFF. The culturing and conditioning process includes media exchanges and can include chemical, electrical and mechanical stimulation as required by the printed tissue. ADSEP also provides the appropriate culture conditions for the sample. Once tissue is ready for return to Earth it can either be fixed and returned or returned live.

A system offering similar opportunities is currently under development for the European Space Agency ESA [9]. The respective industry consortium is led by Redwire (Europe) as well. The hardware called 3D Biosystem (3DBS) shall consist of a bioprinting device, equipped with two extrusion and one inkjet printhead that can be utilized together within one printing process. This part of the system will be provided by the Finnish company Brinter [10]. It will be combined with a bioreactor unit that allows further cultivation of the bioprinted tissue constructs. The latter shall include opportunities for mechanical and electrical stimulation as well as cultivation under shear stress. The 3DBS shall be installed in the Columbus module at the ISS in 2026 and then shall become available for research.

As noted, the microgravity environment could help the biomedical industry overcome current limitations in additive manufacturing. By printing supportless structures, microgravity could enable 3D printing of unique geometries and high aspect ratio structures. Microgravity also allows for printing with multiple degrees of freedom and the opportunity to layer materials of multiple densities. Because sedimentation is absent on orbit, nanoparticles can be embedded more uniformly into printed structures, as Auxilium Biotechnologies is exploring with their peripheral nerve devices [11].

The Auxilium Microfabrication Platform (AMP-1 Digital Light Processing, DLP) funded under the NASA InSPA program is the first DLP-based bioprinter deployed in orbit, launched aboard the SpaceX 31 Commercial Resupply mission (SpX CRS-31) to the International Space Station on November 4, 2024. The printer demonstrated mass-production capabilities, fabricating multiple medical devices per day in microgravity with ultra-high resolution (5 µm pixel size), continuous printing functionality, and unprecedented print speeds (up to 1 mm/min in the z-direction) [12]. Auxilium's 3D printer produces human-scale nervous system implants within minutes (10–15 min), eliminating the need for external support structures, utilizing a bioink that has been validated through in vivo studies in both small and large animal models of nervous system disorders [11, 13]. The platform incorporates a modular cartridge with microfluidic capabilities, enabling perfusion of tissue culture media through newly printed constructs. The initial application of this platform will focus on manufacturing medical devices for peripheral nerve injury repair.

Lawrence Livermore National Laboratory, in partnership with Space Tango, had been selected for their proposal to adapt their terrestrial volumetric 3D bioprinting device for use in microgravity to demonstrate production of artificial cartilage tissue in space [14]. This project is no longer moving forward due to complexities and challenges in bringing this technology to space. The Volumetric Additive Manufacturing (VAM) technology is a revolutionary, ultra-rapid 3D printing method that solidifies a complete 3D structure from a photosensitive liquid resin in minutes. Because of the absence of settling and gravity-driven buoyancy and convective flows in the prepolymer, the cartilage tissues manufactured and matured in microgravity were expected to have superior structural, organizational, and mechanical properties suitable for use in long-term tissue repair and replacement.

The Wake Forest Institute for Regenerative Medicine team was awarded 1 st and 2nd place in the NASA Vascular Tissue Challenge for demonstrating viability and function of thick tissues (> 1 cm) that could be maintained for more than one month in ground studies. The relevance of this technology to 3D bioprinting is there could be tissue constructs that are printed on Earth and then taken up to microgravity to mature where the benefits of greater stem behavior could be used to create more complex and larger tissue building blocks or there could be accelerated differentiation protocols developed to create more mature and functional tissue engineered structures in a shorter time frame than what can currently be achieved on Earth. Funding and resource allocation by the ISS National Laboratory allowed the teams to develop a microgravity demonstration of their technologies supported by Redwire. The payload is flight ready and under discussion for inclusion on an upcoming mission. The WFIRM team flew thick liver and kidney tissue constructs on the second private astronaut mission conducted by Axiom Space (Ax-2) to the ISS. This precursor mission, supported by Axiom Space, was designed to evaluate vascularization of thick tissue in microgravity. These missions provide directional data supporting the upcoming NASA InSPA missions demonstrating proof-of-concept for creating building blocks of thick tissue as a bridge to transplantation in patients awaiting donor organs.

Simulated Microgravity. While opportunities for research on the ISS provide access to true microgravity conditions, given the limited number of missions and costs associated with spaceflight, groundwork using simulated microgravity rotation devices (Clinostats, Random Positioning Machines (RPM), Rotating Vessel Wall Bioreactors) has also been conducted. Microgravity simulators are developed to study cellular-level effects similar to those experienced during or after space travel [15]. These simulators help confirm that such models can be used to investigate the body's responses under conditions other than Earth's standard gravity. By combining advanced numerical calculations with physical modeling methods, real microgravity environments can be simulated, allowing for a deeper understanding of various phenomena while reducing experimental costs and time. However, the introduction of shear forces produced by simulated conditions doesn’t correlate directly [16,17,18] with results obtained in a true microgravity environment. And in humans, where gravity plays a crucial role in biological functions, including those of the heart, nerves, and bones, the impact of microgravity on the development and function of these tissues remains largely unexplored. These tissues are interconnected within the body, meaning changes in one can influence others. For instance, bone loss in microgravity can indirectly affect heart function by altering bone marrow activity, which impairs red blood cell production [19]. Similarly, cardiovascular changes can affect blood flow to tissues like the brain, potentially disrupting neural function [20]. Studying these tissues under simulated microgravity enables researchers to better understand their interactions and the systemic effects of altered gravity environments [19]. The use of hydrogels as artificial extracellular matrices mixed with cells into bioprinted constructs or gel-cell microcapsule complexes holds significant potential for in vitro tissue construction development. For example, it has been demonstrated that use of simulated microgravity culture condition can enhance the proliferation of rat hepatocytes and promote the formation of cell colonies in the silk fibroin microcapsules [21]. Studies using simulated microgravity devices for induced pluripotent stem cells (iPSCs) based 3D tissue model have demonstrated diverse effects on distinct biological behaviors of various stem cells, encompassing migration, adhesion, proliferation, differentiation and apoptosis [22]. However, as the volume in common microgravity simulators like Clinostats or Random Positioning Machines (RPM) is very limited, large equipment like bioprinters cannot be integrated. To investigate the effects of microgravity conditions on the bioprinting processes or the behavior of bioinks of various viscosities during extrusion parabolic flights need to be performed. These offer multiple 20 s long microgravity periods in which meaningful bioprinting experiments can be performed. Additionally, suborbital flights, offering up to 8 min of microgravity exposure, provide alternative opportunities for substantial bioprinting experiments as a more cost effective and available option.