Microfluidics vs. Microgravity - Jad Haddad
Microgravity
Microgravity, often referred to by the term micro-g environment, is a measure of degree in which objects and people appear to be weightless and have zero gravity. It has various applications in engineering and environmental studies and is also used in oil, gas, and mineral exploration. Even though microgravity is referred to a condition at zero gravity, gravity forces are not at zero, but rather very small; hence, the term "micro-" [1]. The prefix "micro-" does not just depict low gravity forces, but it also indicates accelerations equivalent to one-millionth (10^−6) of the force of gravity at Earth’s surface [2]. As a unit of measure, microgravity is expressed in microgravities, μg. In microgravity, the effects are shown when astronauts and objects float in space. Heavy objects can move around easily allowing astronauts to move equipment with just their fingertips.
Nature of Gravity
Gravity is defined as the gravitational force exerted on a unit mass on Earth's surface, or a point in space [3]. This attraction is known as the force of gravity, one of the basic forces in nature. It holds the moon in orbit around the Earth and allows the Earth to orbit around the sun. The nature of gravity was first defined by Sir Isaac Newton. According to Isaac Newton's universal law of gravitation, the force between any two point masses is portrayed below (Figure 1). Furthermore, objects on Earth fall at a normal gravity or 1g. Many people believe there is no gravity in space; however, there is small amounts of gravity everywhere. For example, an object dropped on Earth falls at 1g. An object dropped in space is also falling, but just does not seem like it. The object falls around the Earth at a rate of zero gravity, or more accurately, microgravity.
Free Fall
Microgravity conditions refers to when an object is in free fall. Free fall means an object falls faster and faster accelerating with the acceleration due to gravity (1g on Earth). In other words, it refers to the movement of the body where gravity is the only force acting upon it in Newtonian physics. An object is said to be weightless when an it is in the state of free fall. Moreover, based on free fall, orbital experiments can achieve low gravity. Although, local vibrations can produce accelerations that can be significant.
Applications
Zero Gravity
NASA uses numerous facilities to create microgravity conditions[4]. One of the most popular ways in creating these conditions is flying an aircraft in parabolic arcs. This creates tests and simulations that less a few seconds to create microgravity. Another facility used to test and create microgravity conditions is NASA's drop towers. Their Zero Gravity Research Facility, located at the Glenn Research Center in Cleveland, Ohio, is a large shaft 500 feet deep blew the ground with a vacuum drop chamber [5]. This facility allows test packages to free fall in a vacuum for over five seconds in which weightlessness can be observed.
Studies in Cells and Tissues
Flight studies offer opportunities to assess the effects of gravity on cell and tissue function. Such studies can distinguish "effects of reduced gravity from the complex milieu of factors involved in space flight, and direct effects of gravity from the effects of the associated changes in mass transfer and shear rates" [6]. Additionally, microgravity culture vessels were constructed to achieve operating characteristics of culture systems used for microgravity studies. These include a rotating bioreactor vessel, a rotating wall perfused bioreactor vessel, and a perfused chamber. These bioreactors developed at NASA's Johnson Space Center (JSC) are portrayed in Figure 2.
The rotating vessel constructed at JSC has a annular space holding 110 mL of culture medium and twelve tissue constructs. In a horizontal plane, the bioreactor is operated by solid body rotation while rotational flow field is induced by viscous coupling. This induction allows tissue contracts to be suspended. The fluid flow is laminar with a convective mass transfer. Overall, the purpose of a rotating vessel used for microgravity studies is to randomize the effects of unit Earth gravity on freely suspended biological specimens exposing them to fluctuations in velocity, pressure, and shear [6].
On the contrary, the rotating wall perfused bioreactor developed by JSC is part of a biotechnology system that consists of a bioreactor connected to a recirculation loop. This loop contains a silicone gas exchanger and a peristaltic pump. The bioreactor is configured as a annular space holding 125mL of culture medium between two cylinders, the inner cylinder serving as a viscous pump. The fluid flow is laminar with a convective mass transfer due to differential rotation of the two cylinders.
Human Health Effects
Exposure to microgravity, or weightlessness, has been denoted to have many effects on the human health, both long and short term effects. The most common effects observed in astronauts is bone resorption. This happens mainly to astronauts who experience long term spaceflight. It has been known that when an astronaut is placed in microgravity, the bone immediately remodels [7]. Astronauts experience a reduction of mechanical stress on the skeletal system. Members of the 84 day Skylab 4 mission lost an average of four percent of bone in that 84 day period [8]. Increased mission length can cause severe bone loss in spite of daily exercise. Cells sense changes in physical forces caused by gravity and have to respond by bone remodeling.
A common effect experienced in the initial hours of space time is Space Adaptation Syndrome, or SAS. This syndrome is often referred to space sickness where the body experiences symptoms, such as nausea, vomiting,vertigo, headaches, and overall malaise [8]. Space sickness usually lasts for no more than 72 hours in which the body then adapts itself to a new physical environment. Another effect experienced in the initial hours of space time is muscle atrophy where muscle mass, strength, and performance are reduced; however, these effects reverse rapidly once astronauts return to Earth.
Microfluidics vs. Microgravity
In microfluidics, small length scales are utilized to enable attenuation of gravity effects. These scales increase the surface to volume ratio. Microgravity is said to have structural benefits allowing the improvement of crystal growth in protein crystallization such that these crystals grow large and in high quality.
Microgravity studies are essential in determining what happens to people and equipment in space. NASA studies microgravity in order to keep astronauts safe and healthy, which is another reason why there are multiple facilities that study the science behind weightlessness. These experiments helps NASA learn and interpret microgravity since it is impossible to learn on Earth with "normal gravity".
References
[1] Dunbar, B. What Is Microgravity? https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-microgravity-58.html (accessed Mar 9, 2020).
[2] Harland, D. M. Microgravity. https://www.britannica.com/science/microgravity (accessed Mar 5, 2020).
[3]Saibi, H. Microgravity and Its Applications in Geosciences. https://www.intechopen.com/books/gravity-geoscience-applications-industrial-technology-and-quantum-aspect/microgravity-and-its-applications-in-geosciences (accessed Mar 5, 2020).
[4]Micro-g environment. https://en.wikipedia.org/wiki/Micro-g_environment (accessed Mar 9, 2020).
[5]Dunbar, B. What is Microgravity? https://www.nasa.gov/centers/glenn/shuttlestation/station/microgex.html (accessed Mar 9, 2020).
[6] Searby, N.; Luis, J. D.; Freed, L. E. NYAS Publications. https://nyaspubs.onlinelibrary.wiley.com/doi/full/10.1111/j.1749-6632.2002.tb05927.x (accessed Mar 5, 2020).
[7] Ingber, D.; Vorselen, D.; Ranjeva, R. How cells (might) sense microgravity. https://www.fasebj.org/doi/full/10.1096/fasebj.13.9001.s3 (accessed Mar 9, 2020).
[8] Hughes-Fulford, M.; Lewis, M. L. Effects of Microgravity on Osteoblast Growth Activation. https://reader.elsevier.com/reader/sd/pii/S0014482796901160?token=50D420674510C6C8BB81034F2361BD1B40CDB92BA5874D960842B99EE9D98736476DF2A996BA64AED40D1CA2FE60E6B2 (accessed Mar 9, 2020).
