Role of microfluidics in accelerating new space missions

Abstract

Numerous revolutionary space missions have been initiated and planned for the following decades, including plans for novel spacecraft, exploration of the deep universe, and long duration manned space trips. Compared with space missions conducted over the past 50 years, current missions have features of spacecraft miniaturization, a faster task cycle, farther destinations, braver goals, and higher levels of precision. Tasks are becoming technically more complex and challenging, but also more accessible via commercial space activities. Remarkably, microfluidics has proven impactful in newly conceived space missions. In this review, we focus on recent advances in space microfluidic technologies and their impact on the state-of-the-art space missions. We discuss how micro-sized fluid and microfluidic instruments behave in space conditions, based on hydrodynamic theories. We draw on analyses outlining the reasons why microfluidic components and operations have become crucial in recent missions by categorically investigating a series of successful space missions integrated with microfluidic technologies. We present a comprehensive technical analysis on the recently developed in-space microfluidic applications such as the lab-on-a-CubeSat, healthcare for manned space missions, evaluation and reconstruction of the environment on celestial bodies, in-space manufacturing of microfluidic devices, and development of fluid-based micro-thrusters. The discussions in this review provide insights on microfluidic technologies that hold considerable promise for the upcoming space missions, and also outline how in-space conditions present a new perspective to the microfluidics field.

INTRODUCTION

Microfluidics enables the construction of devices and methods for manipulating liquids in the volume range from micro- to nano-liters, as well as the subjects immersed.^1,2 This technology was first introduced in 1965 with the inception of inkjet printing.³ In fact, the mechanics of microfluidics have been mentioned earlier in the studies of Abbé Nollet and Lord Rayleigh's on the electro-hydrodynamics of cone-jets and its break-ups.⁴ The key features of microfluidics that attract researchers are laminar flows, immiscible fluids, and capillary and diffusion dominating features, which make microfluidic flows highly controllable and predictable.⁵ To achieve these features, microfluidic structures must be fabricated with feature sizes on the order of sub-millimeters to sub-microns, including micro-channels, junctions, valves, and reservoirs. Fabrication methods for microfluidic devices were developed from silicon microfabrication, which originated from microelectronics and extended to glass micromachining,² PDMS soft-lithography,⁶ 3D printing,⁷ bioprinting,⁸ and paper-based construction.⁹ With more than 50 years of development, microfluidics has become a game-changer in numerous research and industrial areas. It not only improves existing technologies such as inkjet printing, bio-assay, electronics cooling, and gas/liquid sensing, but also ignites novel technologies, such as lab-on-a-chip, organs-on-a-chip, single-cell assay, liquid biopsy, μ-TAS, etc. Although microfluidics is still viewed as an adolescent discipline,¹ its impacts in bio-engineering, personalized medicine, and microscale biology are noteworthy.

Most microfluidic devices are designed to be used in daily life, but several have proven useful in space-science research projects or as engineering add-ons. In the 1960s, the role of microfluidics in space technologies was first introduced by the development of electrospray thrusters,¹⁰ enabling the fabrication of micro-sized nozzles and controlling liquid propellants at a higher level of precision.¹¹ To date, different types of electrospray thrusters based on capillary and porous surface wetting emitters have been demonstrated on Earth or in space along with micro-valves, flow-rate sensors, and air bubble eliminators.^12–14 Furthermore, microfluidic-based studies were conducted on space stations; several miniaturized “bio-reactor” systems were developed for pioneer research in astrobiology and space pharmacy fields for investigating cells and microorganisms functionalities under conditions involving microgravity and space radiation.¹⁵ As the technology of microfluidic devices in point-of-cares and biological experiments experienced rapid growth in the 1990s and early 21st century, the concept was adopted into space medicine for providing tools for the astronauts' healthcare.¹⁶ Microfluidic chips that monitor physiological indicators of astronauts,¹⁷ diagnose infectious diseases,¹⁸ and oversee living conditions (air, water, and food) have been constructed.¹⁹

The usefulness of microfluidics in space missions stems from its fundamental features. Primarily, a microfluidics-based system along with the samples and reagents involved can be made small, light-weight, and operated automatically with little power consumption, which significantly reduces the cost and difficulties of carrying the system into space. Furthermore, key features of microfluidics are maintained when being examined under microgravity conditions, since the laminar, capillary, and diffusion features in the microscale are not sensitive to gravity, making most microfluidic applications verified on Earth workable in space. Furthermore, microfluidic devices can work without or with little manual operation, making them suitable in unmanned spacecraft such as satellites and probes, which are the most frequently used spacecraft at present. Finally, microfluidics provides platforms capable of investigating biological samples scaling from one cell to entire human beings, which is of great interest to astrobiology, space medicine, and pharmaceutic communities, therefore attracting the attention of scientific communities.

In the past decades, a new wave of mankind's space activities has emerged, which could fundamentally change the role of microfluidics from the “passenger” of a space experiment to the “playmaker” of novel space missions, such as the low-cost but powerful CubeSats for space sciences,²⁰ longer and farther manned space trips,²¹ landing and exploring of another planet,²² ultra-high precision space experiment,²³ and boosting of commercial space activities by companies like SpaceX, SpacePharma, and SPACETY. This trend is envisioned through the successful demonstration of several missions, such as NASA's Gene-Sat,²⁴ Pharma-Sat,²⁵ EcAM-sat,²⁶ O-OREOS missions,²⁷ the space antenna for gravitational wave detection missions of China (Tianqin-1 and Taiji-1 Satellites), and ESA (LISA Project),^14,28,29 or in the proof-of-concept tests of microfluidic devices on space stations. In this review, we will focus on the recent advances of space microfluidic technologies and their impact on the state-of-the-art space missions (Fig. 1). Starting with a brief summary of hydrodynamic theories, we will discuss whether the micro-sized fluid and the microfluidic instruments act as expected in the under microgravity conditions. Next, we outline the reasons why microfluidic components and operations have become so crucial in recent missions by categorically investigating a series of successful space missions accompanied with microfluidic technologies. We aim to provide a comprehensive technical analysis on how microfluidic systems carried on unmanned spacecraft can benefit human being's understanding of space biology as well as developing new drugs in the Lab-on-A-CubeSat section. Then we discuss how microfluidic technologies in manned space missions for providing diagnostic and sensing tools in the Microfluidics in Manned Space Mission section. Considering the needs from future long-term and complex space missions, challenges and methods for manufacturing of microfluidic systems in the space condition are discussed in the In-space Manufacturing for Microfluidics section. Lastly, the micro-propulsion technologies are investigated since microfluidics is playing an important role in these systems and several fluid handling and sensing technologies developed from the area may benefit other in-space microfluidic applications.

FIG. 1.

Recently developed in-space microfluidic applications. (i) Microfluidics in space pharmaceutics and biology; (ii) Tests in a space trip or on another planet; (iii) 3D printing and bio-printing in space; (iv) Micro-propulsion technologies.

MICROFLUIDIC DYNAMICS IN MICROGRAVITY

Because of the microgravity condition, microfluidic dynamics in space behaves differently in some cases. The behavior of incompressible fluid is governed by the Navier-Stokes (N-S) equation:

$$\rho {\displaystyle \frac{\mathrm{\partial }\mathit{V}}{\mathrm{\partial }t}+(\mathit{V}\cdot \mathrm{\nabla })\mathit{V}=\rho \mathit{f}-\mathrm{\nabla }p+\mu {\mathrm{\nabla }}^2\mathit{V},}$$ (1)

where ρ is the density of fluid, μ is the dynamic viscosity of fluid, $\mathit{V}$ is the velocity of fluid domain, and p is the pressure of the fluid domain. $\mathit{f}$ is the external force applied on the fluid domain. Taking the z axial as the direction of gravity acceleration, the three axial components of the external force can be described as

$$f_x=F_x,f_y=F_y,f_z=\rho g+F_z,$$ (2)

where $F_i(i=x,y,z)$ are the three axial components of the external force except for gravity. For the electrolyte solution in an electric field, the forces applied on a microdomain of the electrolyte solution are

$$f_x=\rho {\rho }_e{E}_x,f_y=\rho {\rho }_e{E}_y,f_z=\rho g+\rho {\rho }_e{E}_z,$$ (3)

where ${\rho }_e$ is the volume charge density, and $E_i(i=x,y,z)$ are the three axial components of the electric field intensity. If the fluid is doped with magnetic particles, the forces applied on the particles are

$$\begin{array}{rl}& f_x=\rho \eta \left(M_x{\displaystyle \frac{\mathrm{\partial }{H}_x}{\mathrm{\partial }x}+M_y{\displaystyle \frac{\mathrm{\partial }{H}_x}{\mathrm{\partial }y}+M_z{\displaystyle \frac{\mathrm{\partial }{H}_x}{\mathrm{\partial }z}}}}\right),\\ & f_y=\rho \eta \left(M_x{\displaystyle \frac{\mathrm{\partial }{H}_y}{\mathrm{\partial }x}+M_y{\displaystyle \frac{\mathrm{\partial }{H}_y}{\mathrm{\partial }y}+M_z{\displaystyle \frac{\mathrm{\partial }{H}_y}{\mathrm{\partial }z}}}}\right),\\ & f_z=\rho g+\rho \eta \left(M_x{\displaystyle \frac{\mathrm{\partial }{H}_z}{\mathrm{\partial }x}+M_y{\displaystyle \frac{\mathrm{\partial }{H}_z}{\mathrm{\partial }y}+M_z{\displaystyle \frac{\mathrm{\partial }{H}_z}{\mathrm{\partial }z}}}}\right),\end{array}$$ (4)

where $\eta$ is the magnetic permittivity of the fluid, $M_i(i=x,y,z)$ are the three axial components of the magnetization intensity of the magnetic particle, and $H_i(i=x,y,z)$ are the three axial components of the magnetic field intensity. In deep space, the gravitational acceleration g can be neglected, such that the external force in these two situations can be calculated by the following equations, which can be eventually inserted into the calculation of the N-S equation:

$$\mathit{f}=\rho {\rho }_e\mathit{E}\mathrm{or}\mathit{f}=\rho \eta (\mathit{M}\mathrm{\nabla })\mathit{H}.$$ (5)

N-S equation in microgravity conditions has profound influence on space engineering and scientific disciplines. A coupled liquid-solid system in a satellite carrying fuel may lead to satellite motion that is uncontrollable.³⁰ Scardovelli et al. use a field method based on the N-S equation for tracking the position of the liquid and its free surface.³¹ Veldman et al. investigate the influence of sloshing liquid on board spacecraft and satellites by a theoretical/computational model based on the N-S equation for 3D incompressible free-surface flow.³² On-orbit refueling technologies provide a new way to extend the lifespan of satellites that run out of propellant. Because fuel transfer rate significantly depends on refueling pressure, Guang et al. use the N-S equiation combined with the Reynolds number, distance loss, and local loss to describe the relationship between refueling pressure and fuel transfer rate.³³ The growth process of crystals of biological molecules is influenced by two gravity-driven phenomena, namely sedimentation of the crystals, and natural convection in the feeding solution. Carotenuto et al. present an order-of-magnitude analysis of the N-S equation in a time-dependent, incompressible and Boussinesq formulation to make predictions of the conditions in which a crystal is growing in a convective regime, rather than in the ideal diffusive state, even under the typical microgravity conditions of space platforms.³⁴

In many cases, the behavior of fluid in space can be easily predicted by computing some non-dimensional numbers, rather than by solving N-S equations, such as

$$\mathrm{Ma}={\displaystyle \frac{{\sigma }_T\mathrm{\Delta }TL}{\mu \alpha },}$$ (6)

$$\mathrm{Bo}={\displaystyle \frac{\rho a{L}^2}{{\sigma }_0},}$$ (7)

$$\mathrm{Ca}={\displaystyle \frac{{\sigma }_T\mathrm{\Delta }T}{{\sigma }_0},}$$ (8)

$$\mathrm{We}={\displaystyle \frac{\rho v^{2}L}{{\sigma }_0},}$$ (9)

where ${\sigma }_T=-d\sigma /dT$, $\sigma$ is the surface tension; T is the temperature of the fluid domain; ${\sigma }_0$ is the surface tension for $T=T_0$; L is the characteristic length; v is the characteristic velocity; $\alpha$ is the thermal diffusivity; a is the acceleration associated with the body force. The Marangoni number (Ma) measures the temperature gradient induced Marangoni convection, it is—for example–applicable to propellant behaviour calculations in spacecraft tanks or bubble and foam research. The Bond number (Bo) is a measure of the importance of surface tension forces compared to body forces. A high Bond number indicates that the system is relatively unaffected by surface tension effects; a low number (Bo < 1) indicates that surface tension dominates. The capillary number (Ca) gives the relative change of the surface tension due to temperature variations. It involves the mean surface tension and it is an important measure of the dynamic deformability of the free surface. If Ca << 0, the dynamic surface deformation can be neglected. The Weber number (We) is a dimensionless number in fluid mechanics that is often useful in analyzing fluid flows where there is an interface between two different fluids, especially for multiphase flows with strongly curved surfaces. It can be thought of as a measure of the relative importance of the fluid's inertia compared to its surface tension. The quantity is useful in analyzing thin film flows and the formation of droplets and bubbles.

Also, in Eq. (1), the pressure p is independent of gravity; therefore, pressure-induced fluid driving is valid in the space scenario by using a gas pressure difference. However, the pressurized gas must be isolated from the fluid by a layer or membrane placed in between, otherwise the gas easily mixes with the liquid to form bubbles or to float everywhere in the container and fails to drive the fluid, as liquids are not naturally separated with gas inside a container under the microgravity condition.

Apart from the pressure driving method for continuous flows, dielectric wetting (EWOD), dielectrophoresis (DEP), thermocapillary, surface acoustic wave (SAW), and optical tweezer technologies are commonly used for manipulating droplets in microfluidic applications. If the contact between the droplets and substrate is maintained to avoid the floating of liquids, these methods will behave properly in the microgravity condition, as the major driving force is not gravity-dependent.

μ and ρ are both independent of gravity; however, their values are affected by the fluid temperature. Taking water as an example, the dynamic viscosity of water from T = 0 to 100 °C declines by 83.9%, and its density declines by 4.1%. However, the environment temperature in space is −270.4 °C, which is the temperature of the cosmic microwave background,³⁵ and taking the solar radiation into consideration, without temperature controlling system for the spacecraft, the surface temperature of spacecraft in low earth orbit varies from −83.3 °C to 77.1 °C.³⁶ And the solution to the N-S equation is significantly dependent on the parameters ρ and μ. Therefore, to accurately predict the behaviors of the fluid, passive and active temperature controls must be embedded for the device, which indeed significantly increases the system volume and complexity.

Another factor that may affect microfluidic mechanics is the space vacuum (10⁻⁴–10⁻⁷ Pa in low earth orbit).³⁷ First of all, microfluidic devices working in open space must be encapsulated properly, as any small hole or damage would result in a massive leakage of liquids. Second, waste liquids from a microfluidic chip are usually flushed through an open hole on the channel when operated on Earth, whereas in space openings are not allowed on the chip. In practice, one-way pumping of liquids mostly used on Earth must be replaced by circular pumping in this scenario. Finally, the effects of gas dissolving or small air bubbles being immersed in the fluid must be taken into serious consideration. If gases or air bubbles inside the fluid have not been eliminated on Earth, they will appear and enlarge in the fluid when the pressure drops, causing possible blockages in the microchannel and the malfunction of microfluidic devices. An air bubble trap built from porous materials was equipped with an electrospray thruster on the LISA Pathfinder satellite; however, bubble-induced blockage of microchannel was nevertheless observed during the mission.¹⁴ Methods to entirely eliminate air bubbles in the microfluidic system are desperately needed, but lack comprehensive study, complicating the problem for in-space microfluidics.

LAB-ON-A-CUBESAT: MICROFLUIDICS IN SPACE PHARMACEUTICS AND BIOLOGY

Understanding the behaviors of biological systems in space is of great significance for astrobiology, in-space medicine, and the development of new drugs.^15,16,38,39 However, the opportunities to conduct such experiments in space are limited to satiating the curiosity of inquisitive researchers. To date, most in-space biomedical or biochemical experiments are conducted in space stations or space shuttles by manual operation, thereby dramatically increasing the cost and restricting the time devoted to each experiment. The introduction of microfluidics or other automatic fluid handling systems into space would not only minimize the size of experiment systems, but also significantly reduce the needs of manpower for conducting such studies. In the early 1990s, the space biology group of ETH Zürich developed a miniature space bioreactor,^15,40,41 exploiting microfabrication technologies to construct the high-performance micro-instrument for understanding cellular functions in microgravity [Figs. 2(a) and 2(b)]. The bioreactor consists of a silicon micro membrane pump together with a piezo flow sensor to deliver the cell-culture nutrient solution into reaction chambers. Sensors measuring the pH, redox potential, and temperature were integrated inside each chamber. Using this system to cultivate yeast cells in space, no morphological differences were observed between flight and ground samples. However, the percentage of cells with randomly located bud scars left on the mother cell is found to be significantly higher in the space group than on Earth one under the same cultivation conditions, which is a definitive result of cell cultivation in the microgravity condition [Fig. 2(c)].¹⁵

FIG. 2.

Microfluidics applications in space biology. (a) Bioreactor: (i) photograph of the bioreactor. Distributed under a Creative Commons Attribution License 4.0 (CCBY).¹⁵ (ii) cross-section of bioreactor chamber. Reproduced with permission from Enzyme Microb. Technol. 27, 778 (2000). Copyright 2000 Elsevier Science Inc.⁴¹ (iii) photograph of flow sensor and micropump in bioreactor. Distributed under a Creative Commons Attribution License 4.0 (CCBY).¹⁵ (b) Space Shuttle missions, STS-65 in 1994 and STS-76 in 1996. Reproduced with permission from NASA. (c) Percentage of randomly located bud scars vs normal bipolar positioning, F: Flight, G: Ground. Distributed under a Creative Commons Attribution License 4.0 (CCBY).¹⁵

Although these bioreactors consumed little volume, mass, and power in the space shuttle, the opportunity of a space trip is still rare, leaving these studies open for only “high-end” projects that are far beyond the budgets of most researchers. Notably, the development of CubeSats may help researchers design and conduct an in-space experiment into a convenient and low-cost “daily” regime. CubeSats were invented in 1999 as an educational tool,⁴² they have very small weights and sizes (usually less than 10 kg and 1000 cm³) and are equipped with primary communication functions, such that dozens of CubeSats can be deployed in one launch. Therefore, the cost of one CubeSat mission can be significantly reduced by sharing the rocket charge. The first CubeSat mission was conducted in 2003 with the cost about $40,000 for the satellite itself.⁴² Afterwards, the number of

CubeSats launched into orbit has dramatically increased each year (Table I), and most of them are designed for space science studies. According to quotations from commercial space companies, the cost for one CubeSat including the rocket charge can be reduced to less than $70,000 per cube (a 10 × 10 × 10 cm cube comprises one unit of volume, abbreviated as 1 U) including the rocket charge, satellite monitoring, and data transmission services. However, the small size of satellites also means that the space for the scientific payload is considerably limited. All experimental systems, as well as the power and communication system, must be packed into a few Us, without any possibility of manual operation after launching, which has increased the technical difficulties in conducting bio-medical experiments with CubeSats. Indeed, due to the small size, the power provided by the CubeSats is also limited. For example, a 12 U satellite from SPACETY can provide up to 118 W solar power with a 236 Wh battery.⁴³

TABLE I.

Cubesats launched into orbit in recent years. 3 U*: these satellites were slightly larger (10× 10 × 34 cm³) than a standard 3 U cubeSat.

Name	Research object	Purpose	Monitoring system	Size
GeneSat-124	E. coli	Growth of E. coli in spacecraft	Optical: LED-excited fluorescent detection, light scattering measurement; Pressure, humidity, temperature, radiation dose, accelerometer: commercial sensors	10 × 10 × 34 cm, 3.5 kg
PharmaSat25	Saccharomyces cerevisiae	Drug dose response in microgravity	Thermal: temperature sensor Analog Devices AD590; Pressure: a micromachined pressure transducer; Humidity: a thin-film capacitive humidity sensor	10 × 10 × 35 cm, 5.1 kg
EcAMSat26,58	E. coli	Antibiotic resistance of E. coli cells.	The same monitoring system with PharmaSat, except the changes in protocol and modifications to the hardware	6 U
SporeSat-144	Ceratopteris richardii	Gravitational threshold for calcium ion channel activation in the spores of a fern	Calcium signaling activity: microelectrode	3 U*, weight < 5.5 kg
O/OREOS46	Bacillus subtilis, Halorubrum chaoviatoris	Growth, activity, health and ability of microorganisms to adapt to the space environment	Thermal: integrated-circuit temperature transducers	3 U
BioSentinel59	Saccharomyces cerevisiae	DNA double-strand-break (DSB) events caused by ambient space radiation	Optical: LED-light scattering measurement	6 U
TSAT460	Anabaena cylindrica	Influence of long-term exposure to low Earth orbit (LEO) on the development of primary producers.	Optical: photodiode measurements	3 U
WeissSat-161	Extremophile bacteria	Test and validate the survivability of extremophile bacteria in orbit	Optical: fluorescent stain and fluorescent detection	1 U, 1.3 kg

Microfluidic technologies form the core of the “lab-on-a-chip” methods used in CubeSat missions to efficiently conduct biological, medical, or chemical experiments within a small volume. Considering the autonomous operational requirement, as well as the small size provided for the experimental payload, the microfluidics-enabled lab-on-a-chip technology can be skilfully engineered for incorporation within CubeSats for space pharmaceutics and biology, thus making lab-on-a-CubeSat possible. NASA's Ames research center has been the pioneer of this field. In 2006, a 3 U CubeSat “Genesat-1” was launched, carrying a microfluidic card and optical, temperature, and pressure sensors for the in-vitro study of E. coli cells [Fig. 3(a)].²⁴ The culture medium was delivered into the cell chamber by squeezing the PEVA bag, and the temperature was maintained at 34 °C for a 96 h experiment. Light scattering signal and green fluorescence signal (blue light excitation) were acquired for evaluating the bacterial growth rate and protein expression [Fig. 3(b)]. Their following CubeSat, the “PharmaSat” was launched in 2009,²⁵ focusing on investigating the influences of microgravity on yeast cells, which are widely used in pharmaceutical industries. PharmaSat has a similar systematic design with GeneSat-1, but the number of culture wells is increased from 10 to 48, which also increases the complexity of the microfluidic system [Fig. 3(c)]. Actively controlled diaphragm pumps and solenoid-operated valves were integrated. Preliminary results indicate slower growth of yeast cells in microgravity conditions than those on Earth with normal gravity. The E. coli AntiMicrobial Satellite (EcAMSat) mission (launched in 2017) is the successor of PharmaSat,²⁶ which tested the antibiotic resistance of E. coli cells using modified hardware from the PharmaSat payload [Fig. 3(d)]. Results indicate that microgravity affects metabolic activity in the stationary phase in the presence of an antibiotic [Fig. 3(e)]. Other than cells, the SporeSat-1 mission carrying a pair of rotated microfluidic disks with fern spores was conducted in 2010,^44,45 but the experiment failed due to malfunction of the illumination system. As biological systems on orbit not only experience microgravity conditions, but also are exposed to space radiation, in 2010, the O/OREOS 3 U CubeSat was launched to a high-inclination, 650 km Earth orbit, where the space radiation dose is as high as 14 Gy/day to test the long-term survival, germination, metabolism, and growth response of Bacillus subtilis spores and the degradation of organic molecules.^27,46 After 181 days in space, spore germination and growth appeared hindered and abnormal.

FIG. 3.

Microfluidics-enabled lab-on-a-chip technology within CubeSats for space pharmaceutics, and biology. (a) CubeSat “Genesat-1.” Reproduced with permission from NASA. (b) Payload optical detector system on CubeSat “Genesat-1.” Reproduced with permission from IEEE.²⁴ (c) Modified microfluidic, optical, and thermal cross-section of one of 48 wells, each containing 100 μl with an integral 1.2 μm filter membrane at the inlet and outlet to confine the yeast. Reproduced with permission from NASA.²⁵ (d) Schematic of fluidic system showing flow directions through valve board on CubeSat “EcAMSat.” Reproduced with permission from Life Sciences in Space Research 24, 18 (2020). Copyright 2020 Elsevier.²⁶ (e) Average reduction of alamarBlue over time for wildtype uropathogenic E. coli and its isogenic $\mathrm{\Delta }rpoS$ mutant on CubeSat “EcAMSat.” Reproduced with permission from Life Sciences in Space Research 24, 18 (2020). Copyright 2020 Elsevier.²⁶

Adopting CubeSat as a platform with lab-on-a-chip systems for space research is reasonable, as evidenced by these successful demonstrations. However, state-of-the-art technologies remain simple and fail to attract the attention of researchers compared with microfluidics applications demonstrated on Earth, which may be due to several encountered technical challenges. Most chips are designed for laboratory settings, which are equipped with instruments such as microscopy systems, syringe pumps, and precise electronics, while being operated by trained personnel, therefore, it is more difficult to incorporate current chips into CubeSats, which offer an extremely resource-limited condition. To realize the concept of the “lab-on-a-CubeSat,” the invention of advanced tools,^48,50 such as powerful but compact microscopes, flow cytometers, and PCR instruments is required, which must be well engineered for the in-space condition and greatly minimized for being integrated into a CubeSat. Indeed, with more standard tools available for conducting microfluidic researches in space, the cost can be further reduced, as there is no need to design and customize instruments dedicated for a single mission. With more and more cubesat missions being successfully conducted, we foresee the participation of more companies into this area would certainly accelerate this process and offer more standard commercial products for such experiments in space.

MICROFLUIDICS IN MANNED SPACE MISSIONS: HOW TO PERFORM TESTS IN A SPACE TRIP OR ON ANOTHER PLANET?

Other than serving as autonomous platforms for space science projects, microfluidic technologies, particularly lab-on-a-chip originated point-of-care technologies, are considered important in future manned space activities.⁵¹ Microfluidics has tremendous advantages in personalized healthcare and bio-chemistry sensing. Using a tiny microfluidic chip, a number of biomarkers can be screened by dripping a very small volume of liquid samples such as blood, urine, saliva, and sweat, onto the chip.^52,53 Applications including on-chip detection of infectious diseases, blood biopsies, and wearable health monitors are rapidly developed for use in a resource-limited scenario.⁵⁴ In manned space missions, it is highly likely that medical resources are strongly limited by the weight, volume, power, as well as manpower limitations on an interstellar spacecraft; therefore, the tiny microfluidic chip would be of unique significance in these scenarios, even if it can only offer basic biochemical tests for astronauts. In 1999, Weigl et al. from the University of Washington described the potential of microfluidic T-sensor for blood cells analysis in microgravity.¹⁸ However, very few microfluidic tests are able to render the test results directly on the chip and require other systems, like a high-performance fluorescent microscope for signal reading and analysis, which may pose an obstacle to using the chip in space. To develop a standalone and easily read microfluidic system as convenient as a “pH test strips” remains an important task. The strategy was developed by Shi et al.'s work,⁵⁵ which demonstrated a portable microfluidic flow cytometer for in-space healthcare. The chip system is integrated with a mini pump, laser, photomultiplier tubes, and other optical components to achieve the counting of LEUKOCYTE types by itself [Figs. 4(a) and 4(b)]. Other studies striving to achieve the standalone lab-on-a-chip system including the all-on-a-chip pump, lensless microscope, and self-powered liquid actuation and sensing would definitely increase the chances of using microfluidics for future in-space healthcare.

FIG. 4.

Biological tests in a space trip or on another planet. (a) Photograph of microfluidic cytometer system. (Left) portable optical reader, and (Right) microfluidic cartridge. Reproduced with permission from IEEE.⁵⁵ (b) Measured intensities of all leukocyte cells displayed as a scatter plot, each point representing a counted leukocyte. The scatter plot shows four distinct clusters. Reproduced with permission from IEEE.⁵⁵ (c) Fluorescence image of C. elegans: (i) original fluorescence image, (ii) converted grayscale image, (iii) image after filtering process, (iv) image after corrosion process. Reproduced with permission from Electrophoresis 40, 922 (2019). Copyright 2018 WILEY-VCH.⁵⁷ (d) Wearable sensor array monitoring human health by analyzing sweat. Reproduced with permission from Nature 529, 509 (2016). Copyright 2016 Macmillan Publishers Limited.⁶² (e) Microfluidic systems as the water-quality microanalyzer in a living ecosystem for long-term space trips or planet immigration. Reproduced with permission from Acta 995, 77 (2017). Copyright 2017 Elsevier B.V.⁶³

Several agencies have announced their plans of long-term manned space missions, such as the manned Mars mission by SpaceX.⁵⁶ Although in the trip all astronauts are kept away from most pathogens or bacteria as the spacecraft forms a closed environment, the long-term stay under microgravity conditions, as well as exposure to space radiation may have unexpected effects on the astronauts' health, aging, gene expression, and protein production. The evaluation of these influences has become a crucial factor for a secure space trip. Prior to the real trip, studies with living biology samples, such as animals, human cells, worms, and insects are conducted to evaluate the influence of space trips. As discussed in the previous section, it is more feasible to conduct such an experiment with the lab-on-a-chip system. Wang et al. present a microfluidic system to capture and monitor C. elegans on a chip and assess the influence of microgravity through fluorescence and motional imaging [Fig. 4(c)].^47–50,57 The result suggests potential muscle atrophy and compromised nematode physiological activity induced by long-term microgravity. During a long-term space trip, it is necessary to monitor the astronauts' physiological conditions, which can be accomplished by daily multi-parametric tests conducted with wearable microfluidic systems such as watches, contact lenses, smart skins, and masks, taking sweat, tears, blood, and exhaled gas as samples for analysis [Fig. 4(d)].⁶²

Apart from monitoring human physiological parameters, the evaluation of the ecosystem supporting life is another target function for microfluidic systems in a long-term space trip or planet immigration. Water, the most important resource for life, must be carried within the spacecraft and used in a recycled manner to reduce total weight. To maintain drinkable water, recycling using different processes such as nitrification, filtrations, and remineralization must be applied, and microfluidic systems, such as water-quality microanalyzers, must be also developed. For example, researchers from the University of Barcelona presented a lab-on-a-chip system for autonomous and automatic sensing of ions in water based on a continuous potentiometric method. The detection limits of 0.51 mg/l for potassium, 1.58 mg/l for chloride, and 3.37 mg/l for nitrate were demonstrated, fulfilling the requirements of ESA for manned space missions [Fig. 4(e)].^19,63 Microfluidics incorporated with optical and electrical functionalities has been demonstrated with more sensing targets, such as heavy metal ions, proteins, and living samples, which can be considered for future space missions. Apart from serving as the microanalyzer in spacecraft, similar microfluidic systems could also play an important role in the planetary exploration and colonization,⁶⁴ such as in the search for water and evidence of life, analyzing soil elements as well as artificial photosynthesis.^22,65 As an interesting topic, although in current thinking, life does not exist on Mars, Titan, and the moon, the question whether life has ever evolve on these ancient planets remains poorly answered. Several studies have involved microfluidic analytical systems in the search for the existence of biogenic molecules (e.g., organic materials, amino acids, DNA, RNA) on another planet, using techniques such as on-chip chromatography, electrophoresis, mass spectroscopy, DNA amplification, and sequencing.^66–69 Microfluidics systems for the detection of these samples have been intensively studied for on-Earth applications, but to meet the requirements of in-space usages, they require more technical effort in reducing the size and power required, while increasing the level of integration and automation.

IN-SPACE MANUFACTURING FOR MICROFLUIDICS: 3D PRINTING AND BIOPRINTING

New space missions, like extra-terrestrial exploration have long-lasting mission periods and far away destinations, such that it may not be logistically possible to carry numerous instruments onto the spacecraft based on current space technologies. Considering the size of carriers and the outer space environment, it is equally not feasible to equip standard clean-room facilities within the spacecraft or build one on another planet in the coming decades. This challenge may be aggravated in complicated space missions, where careful adjustments of microfluidic instruments or even engineering new devices may be required. In view of these challenges, we discuss other fabrication methods that may be suitable for these space missions. Generally, additive manufacturing, also named as 3D printing technologies, is viewed as the best solution owing to the fabless, flexible, and acceptable precision for most microfluidic devices. 3D printers feature compact sizes, while being able to provide acceptable fabrication precision down to a few microns, meeting the needs for most microfluidic devices. Indeed, the superiority of 3D printing over other fabrication technologies is the ability to produce irregular structures in three dimensions from a model built with computer-aided design (CAD) software. The convenience and flexibility of using 3D printing make it capable to handle “make-for-needs” microfluidic chips in a small and resource-limited spacecraft or settlement on another planet.

On Earth, numerous 3D printing technologies have been studied for fabricating microfluidic devices, such as extrusion-based 3D printing,⁷⁰ stereo-lithography,⁷¹ and poly-jet printing.⁷² Apart from replacing conventional manufacturing protocols, such as photolithography and etching to fabricate the molds of PDMS soft-lithography, 3D printing likewise provides unique ways to directly fabricate curved, complex, and irregular features on a microfluidic chip with suboptimal but acceptable precision. To date, all microfluidic devices tested in space are manufactured and assembled on Earth. Seeing numerous 3D printing or additive manufacturing technologies being conducted in space, we foresee the emergence of in-space manufacturing of microfluidic devices in the very near future. In 2014, NASA launched the first 3D printer to the International Space Station (ISS) in collaboration with Made In Space, Inc.⁷³ This custom-built fused deposition modelling 3D printer uses acrylonitrile butadiene styrene (ABS) plastic as the printer material, and has successfully built a total of 20 objects, including a ratchet wrench [Fig. 5(a)]. In 2020, China's Long March 5B rocket carried a 3D printer onto orbit,⁷⁴ which realized continuous fiber-reinforced composite sample printing in space. To date, 3D printing in space aiming to fabricate microfluidic devices has not yet been put into practice. Compared to numerous printing technologies used for microfluidics on Earth, tested in-space approaches are limited in the extrusion-based printing of plastics, which hardly meets the need for fabricating microfluidic devices or molds with high structural and material complexity. In space, the development of extrusion-based printing faces challenges from microgravity, whereas other printing technologies, such as stereo-lithography and poly-jet printing would not be significantly affected by it, based on their working mechanism. Indeed, in-space 3D printing of microfluidic devices may benefit from the microgravity conditions, since it becomes significantly easier to fabricate a channel, reservoir, or structures that eliminate the need of sacrificial support materials during the fabrication [Fig. 5(b)].⁷⁵ However, more in-space verifications of this idea are required before we can draw an affirmative conclusion.

FIG. 5.

In-space manufacturing for microfluidics. (a) First 3D printing task in space. Reproduced with permission from NASA.⁷³ (b) SEM images of a 3D-printed channel before (i) and after (ii) removing supporting material. Reproduced with permission from Anal. Methods 8, 6005 (2016). Copyright 2016 The Royal Society of Chemistry.⁷⁵ (c) First 3D bio-printing task in space. Distributed under a Creative Commons Attribution License 4.0 (CCBY).⁷⁷ (d) Bio-printing process recorded by a video. Distributed under a Creative Commons Attribution License 4.0 (CCBY).⁷⁷ (e) Macro photography of printed 3D construct returned to Earth. Distributed under a Creative Commons Attribution License 4.0 (CCBY).⁷⁷

Considering the extensive needs of space pharmaceutics and astrobiology, another printing technology may play an important role in the coming decades. Bioprinting, which employs living cells and other biomaterials as the bioink, aiming to directly fabricate biological samples, artificial bio-tissues, and organs with proper functions, has attracted the attention of space agencies and commercial companies. Bioprinting is being applied to regenerative medicine, addressing the need for tissues and organs suitable for transplantation. Common medical technologies, such as beating artificial heart cells, cartilage implants, skin repairs, and functional kidney tissues have been successfully printed on Earth.⁷⁶ In December of 2018, a bio-printer named Organ.Aut was delivered to the Russian segment of the ISS by the rocket “Soyuz-FG.”⁷⁷ The world's first in-space biofabrication experiment was performed during the Space Expedition “ISS 58-59” [Fig. 5(c)]. The bioink used in the bio-printer involved chondrospheres, which are fabricated from human chondrocytes on Earth. The key factor for the biofabrication of 3D tissue constructs is the paramagnetic medium, gadobutrol (Gd³⁺-chelate), the flowing of which is controlled by a valve. When the valve opens, gadobutrol is mixed with the bioink. A nonhomogeneous magnetic field trap is applied in the target zone of the mixed bioink. After culturing for 48 h at 37 °C, tissue spheroids start to fuse and form a stable 3D tissue construct. Figure 5(d) shows the printing process video recording, and Fig. 5(e) shows the macro photograph of the printed 3D construct returned to Earth. In July 2019, the 3D BioFabrication Facility (BFF), developed by the U.S. companies TechShot and nSrypt, was launched to the International Space Station (ISS) onboard the SpaceX CRS-18 cargo mission from the Florida's Cape Canaveral Air Force Station.⁷⁸ The BFF uses adult human cells (such as stem or pluripotent cells) and adult tissue-derived proteins as its bioink to create viable tissue, dedicated to manufacturing human tissue in the microgravity conditions of space.

Indeed, the microgravity conditions in space remarkably improve 3D bioprinting, since microgravity overcomes limitations of monolayer cell growth, making the growth of cell spheroids or organoids much easier compared to Earth models.⁷⁹ For microfluidics, numerous on-chip models for bio-printed tissues, organs, and cells have been demonstrated for studies of drug responses, cell mechanisms, and human-on-a-chips. It is highly likely that mature bioprinting carried out in space will solve the difficulties in limited biological samples, providing an ideal way of conducting space pharmaceutical, medical, and biological studies along with other in-space microfluidic technologies.

MICRO-PROPULSION TECHNOLOGIES: HANDLING FLUIDS IN MICRO-SCALES PROMOTES NEW THRUSTERS FOR SATELLITE

When introduced into aerospace technology, microfluidics was first applied in propulsion technology for developing micro-thrusters.^10,11,14,80 Differently from other space propulsion engines that aim for high thrust to lift the giant rockets or space shuttles into orbit, this novel branch of propulsion technologies aims for small thrust (on the millinewtons or even micronewtons order) and is useful for applications in the moving of CubeSats and attitude control of satellites. Among these micro-thrusters, the electrospray thruster has the closest relationship to microfluidics. Its working is based on the electrospray of conductive liquids or liquid metals [Figs. 6(a) and 6(b)].^81,82 The electrospray thruster was first introduced in 1966 by Krohn et al.¹⁰ The fundamentals of electrospray emission were studied considerably earlier, and can be tracked back to the early 17th century. They were systematically investigated in the 20th century by Zeleny.^83–85 These studies also laid down the foundations for inkjet printing, which is considered as the origin of microfluidics in the 1960s.³ Following 50 years of development, electrospray thrusters have become an important branch of micro-propulsion technologies owing to their superior stability in producing small thrust and high precision in thrust modification. To achieve these features, ideas and methods from microfluidics were continuously introduced into the design of electrospray thrusters. For example, the emitters with smaller feature sizes were constructed to further reduce the thrust.⁸⁶ Consequently, the dimensions of emitters were drastically reduced to sub-millimeters, such that the liquid propellant flows in these thrusters is considered as microfluidics.

FIG. 6.

Handling fluids in micro-scales promotes new thrusters for satellite. (a) and (b) Electrospray of conductive liquids or liquid metals. Reproduced with permission from Rep. Prog. Phys. 71, 0 36 601 (2008). Copyright 2018 IOP Publishing Ltd.⁸¹ Reproduced with permission from J. Aerosol Sci. 25, 1065 (1997). Copyright 1994 Elsevier Science Ltd.⁸² (c) Application of electrospray thruster technology in space gravitational wave detection missions. Distributed under the terms of the Creative Commons Attribution 3.0 License.⁸⁷ (d) and (e) Fabrication of micro capillary, porous emitters, and emitters array for higher flow impedance. Reproduced with permission from J. Micromech. Microeng 24, 0 75 011 (2014). Copyright 2014 IOP Publishing Ltd.⁹² Reproduced with permission from J. Nanomech. Micromech. 7, 0 40 17 006 (2017). Copyright American Society of Civil Engineers.⁹³ (f) Actively controlled proportional valve based on MEMS technology for changing the flow impedance. Reproduced with permission from Sens. Actuators, A 83, 188 (2000). Copyright Elsevier Science S.A.⁹⁴

In recent years, electrospray thrusters have attracted increasing attention, especially after the initiation of in-space gravitational wave detection missions (e.g., Laser Interferometry Space Antenna, LISA project of ESA) [Fig. 6(c)].⁸⁷ The thruster does not serve as an engine, but produces micronewton (μN) level thrust to counter the external disturbance forces on the satellite, such as sunlight pressures, usually in tens of μN, to maintain the satellite in a “drag-free” condition.⁸⁸ The thrust T, generated by the electrospray thruster, can be expressed as

$$T\propto \mathrm{\Delta }{V}^{1/2}{Q}^{3/4},$$ (10)

where $\mathrm{\Delta }V$ and Q are the accelerated voltage and flow rate of the propellant, respectively.^87,89 On the one hand, calculations indicate that achieving an ultra-low thrust on the micronewton level requires the control of the propellant flow rate in tens of picoliters per second (pl/s).⁹⁰ On the other hand, previous research has confirmed that ultra-small flow rates are also able to switch the emission mode between the droplet and ions regimes for microfluidics-based thrusters, where higher specific impulses can be realized.⁹¹ Indeed, the control precision of the propellant flow rate required here is higher than the needs for most microfluidic applications, which contains numerous technical challenges. First of all, ultra-small flow rates down to tens of picoliters per second can be realized by increasing the flow impedance, which can be achieved by further reducing the channel sizes or extending the channel length. Microfabrication on silicon was investigated for microcapillary [Fig. 6(d)]⁹² and porous emitters [Fig. 6(e)],⁹³ as well as the high-density emitters array,^92,93 replacing the metal needle emitters for the higher flow impedance in several prototypes. Next, to modify the thrust, we must either change the driving pressure or the flow impedance. In electrospray thrusters, engineers have chosen the “impedance-changing” method, in which an actively controlled proportional valve is developed based on micro-electro-mechanical system (MEMS) technology [Fig. 6(f)].⁹⁴ The valve's opening is controlled by a piezoelectric actuator, which provides high-precision displacement control to regulate flow rates with resolutions higher than one nanoliter per second. In the real use of the electrospray thrusters, several technical issues must be taken into consideration, such as the propellant's temperature control, air bubble elimination, real-time detection of propellant flow rates, and avoidance of over-spray. Solutions are directly adopted from microfluidics with significant earlier established knowledge in these areas.

The study of electrospray micro-thrusters has benefited significantly from microfluidics in numerous ways, and ongoing research is investigating the state-of-the-art microfluidic technologies to further advance the abilities of liquid propellant's actuation and sensing. We also foresee that the efforts of manipulating liquids in such a precise manner, exceeding the needs of common lab-on-a-chip devices, would eventually benefit microfluidic studies. Furthermore, the investigation of electrospray emission has offered knowledge for the study of microfluidics-based mass spectroscopy, which shares the same working principles. Moreover, as discussed in previous sections, the improvement of other in-space microfluidic technologies must be accompanied with advances in “aerospace quality” microfluidic components, which are being continuously created from studies and tests on electrospray thrusters including actuators, switch-off valves, proportional valves, temperature control units, and flow-rate sensors.

DISCUSSION

Tiny, multi-functional, and highly automatic microfluidic devices have provided better and innovative ways of conducting biological, chemical, and pharmaceutical studies in space. Increasing installations of cost-effective, scientific CubeSats are providing numerous opportunities for exploring the potential of microfluidic technologies to be verified in space. Numerous complex manned space missions and future explorations on other planets will require the invention of more reliable microfluidic analysis systems for medical and environmental detections. The development of liquid propellant-based thrusters has involved an increasing number of technologies from microfluidics and in-return generated technologies for microfluidic device development as well as knowledge of in-space microfluidics. During the next few years, the number of new-wave space missions will show rapid growth, and we thus foresee that microfluidics will continue playing an important role in these missions. This urgently calls for the attention of researchers from microfluidics communities to ponder on various ways they can contribute to aerospace applications. To highlight this need, we have written this review to not only provide a technical analysis of existing technologies, but also to broadcast the trend of in-space microfluidic applications, such as lab-on-a-CubeSat, healthcare for manned space missions, in-space manufacturing of microfluidic devices, and the development of micro-thrusters. Using this review as a discussion platform, we look forward to igniting new microfluidic technologies or transforming existing solutions that are promising for the upcoming space missions. Developing microfluidic devices for extreme space conditions (e.g., cold, vacuum, high radiation, long working time) will likewise extend the boundaries of this young discipline. Further communication and interdisciplinary collaborations between microfluidics researchers and space agencies are expected to raise space microfluidics to the next level.

AUTHOR DECLARATIONS

Conflict of Interest

There are no conflicts of interest to declare.

Author Contributions

S. K. and P. S. proposed this review. N. M. S., Y. W., Y. S., W. R., K. T. Y., and L. T. wrote the article; all authors have approved the final manuscript.

DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.