Mass Controller System for Hypoxia and Hyperoxia Testing
Huser, A.J., Kreofsky, C.R.,
Nadler, D.C., Poblocki J.R.
Department of Biomedical Engineering
University of Wisconsin – Madison
March 12, 2004
Brad Hodgeman, Instrument Specialist
Department of Comparative Biosciences
John G. Webster, Professor Emeritus
Department of Biomedical Engineering
Mass flow controllers are used to regulate the flow of gas through chambers, thus controlling the concentrations of gas in an enclosed chamber. A system was designed to test the effects of different concentrations of O and N, within mice. The system has three main variables as 22
outlined by the client: software, mass flow controllers, and interface for communication. A plethora of research has been completed on different types of mass flow controllers and mass flow controller manufacturers. Investigation into different types of communication interfaces and programming software has also been completed.
The purpose of this project is to design a system that can create a reproducible and accurate hypoxic/hyperoxic environment with the capability of oscillating between various concentrations of oxygen and nitrogen.
Our client, Brad Hodgeman, has the following motivations:
1) Determine the physiological mechanism of neural respiratory plasticity. It is widely
believed that neural plasticity is dependent on serotonin 5HT, but the whole mechanism
is yet to be discovered.
2) Purchase new mass flow controllers and develop user-friendly software. The current
mass flow controllers are inaccurate and the software is outdated
3) Increase the automation of the system. Currently, there are manual aspects of the system
that the client would like to get rid of to increase efficiency within the system. Hypoxia Background
The neural respiratory control system’s responses to respiratory stresses such as intermittent & continuous hypoxia along with hyperoxia are being associated to clinical disorders like sudden infant death syndrome (SIDS), apnic sleep disorders, and spinal cord injury. Links between these (and other) clinical disorders and hypoxia/hyperoxia are being investigated by researchers in hopes of finding the mechanisms behind their correlations.
Normal respiration includes 21% atmospheric O, ~78% N, and a very small percentage 22
of all other gases. A lack of inspired O (<21%) can cause a condition called hypoxia, where 2
insufficient amounts of O reach the tissues of an organism. Induced hypoxic conditions are 2
more extreme but analogous to atmospheric oxygen at high altitudes. The physiological and morphological effects from hypoxia can be detrimental to the organism if the O level is down 2
low enough and is induced for long enough periods of time.
Figure 1, shows the phrenic response to continuous hypoxia. As seen here by the steady decline in phrenic amplified from the short-term hypoxic response, no long term facilitation (LTF) is induced from continuous hypoxia. (From Kinkead et al, 1998)
Developmental respiratory control in many mammalian species can be heavily influenced by variation in gas concentrations (Johnson and Mitchell, 2003). Hyperoxia is a condition of ambient O levels being above the standard (low altitude) atmospheric O levels, 21% 22
respectively. Animal models support the conclusion that perinatal changes in O levels induce 2
developmental plasticity: lasting changes in the respiratory control system that can be drawn out only during critical periods of development (Bavis et al, 2003b). Carotid body chemoreceptors
bathe in the arterial blood and measure the P levels, adjusting breathing rate and volume as O2
P changes accordingly (Feldman and McCrimmon, 2003). Neonatal hyperoxia-treated rats, O2
when compared to control rats, had significantly less carotid body volume (Fuller et al, 2002). Smaller volume of carotid bodies and attenuated responses to respiratory stresses of hypoxia later in the rat’s life (>3 months) has researches believing developmental hyperoxia has
detrimental effects to postnatal carotid body morphological and functional maturation (Bavis et
Respiratory plasticity is defined as a future change in performance or persistent change in the neural control system based on prior experience (Mitchell and Johnson, 2003). Intermittent hypoxia and not continuous hypoxia induce long-term facilitation (LTF) the most common and widely studied form of respiratory plasticity. LTF is defined as the augmented phrenic burst frequency and amplitude lasting minutes to hours after episodes of intermittent hypoxia (Baker and Mitchell, 2000). Intermittent hypoxia is necessary to induce but not maintain LTF, thus there are other mechanisms behind the increased drive to breathe, as seen with the increased phrenic output. It is widely accepted among researchers that LTF results from serotonin receptor activation and is maintained with new protein synthesis, enhancing synaptic inputs to phrenic motoneurons (Fuller et al., 2002). Serotonin, or 5-hydroxytryptamine (5Ht), is a neuromodulator that aids in increasing respiratory drive. The exact physiological process in which serotonin elicits LTF is uncertain.
Figure 2, shows the phrenic and hypoglossal (XII) response to 3 episodes of intermittent hypoxia (H1, H2, H3). LTF is the amplified response above baseline (BL) signified at 60 minutes post-intermittent hypoxia. (From Zabka et al, 2001).
In an experimental protocol that involves dynamic entities such as gas flow and control, accuracy is of paramount concern. In our client’s situation, this concern is addressed through the technology of mass flow controllers. Mass flow controllers (MFCs) accomplish accuracy through automating gas flow rates, and thus gas concentrations, to desired levels, for use in further testing. As a desired gas is fed into the mouth of the MFC, it is divided into two different paths. A large fraction flows into the bypass of the device, creating a pressure drop that shunts the smaller, remaining portion (usually 5% of the total mass) of gas up into the thermal sensor (figure 3). The shunted gas is subjected to a pair of heating coils which measure the change in temperature from the beginning to the end of the tube.
Figure 3, air flow of sample gas Figure 4, schematic of sensor tube
Once in the sensor, the thermal properties of the gas are used to measure the mass flow rate (Sierra Instruments, 2004) (figure 4). The thermal measurement technique is made possible due to two basic chemical principles: specific heat and the first law of thermodynamics. The specific heat of the gas is important, because it is a constant that can be utilized against a variable such as temperature. When heat is added to a gas within the sensor, a temperature change can be monitored, and the flow rate F can then be solved for by the thermodynamic relationship: F =
q/Cp x δT, where q is the heat lost to the gas flow, Cp is the specific heat at a constant pressure,
and δT is the net change in gas temperature throughout the length of the sensor tube. Under empirical circumstances, the downstream coil, composed of thermal sensitive wiring, has a higher temperature and thus more resistance (Qualiflow, 2004) (figure 5).
Figure 5, Wheatstone Bridge;
Schematic layout of the main functions of the electronic circuitry of a thermal mass flow controller
The coils are part of a bridge circuit that has an output voltage proportional to that of the change in the two resistances. Ultimately, a Wheatstone bridge (figure 6) is used for the resistance to voltage conversion, which can be further calibrated to a relative flow rate.
Figure 6, increased resistance downstream of the sensor tube
Current System in Use
The current system used by our client has many components involved to achieve the testing environments desired. The gas for the rat chamber environments, oxygen and nitrogen, are provided from large refillable metal cylinders. They output a desired pressure controlled by a valve and indicated by a needle gauge. The gasses flow through standard plastic hosing to mass flow controllers.
The mass flow controllers used currently were manufactured by Aalborg Instruments & Control, model AFC3600. These are analog controlled devices that set their flow rate based on a 0-5V input signal, which indicates the percent of max flow rate for that controller. The actual flow rate is indicated by an output signal, which uses the same scale. The analog signals, as well as the power, are supplied from an Aalborg Command Module. This module can support up to four controllers, communicates their flow set points, and displays their flow rate. The module is being controlled by a desktop computer with HyperTerminal, a piece of software packaged with the Windows operating system. The client writes command line macros that communicate his experiment protocols to the Command Module.
Figure 7, client’s current system schematic
The mass flow controllers are used to output a certain flow of each gas, oxygen and nitrogen, so that when they are mixed, they make a desired concentration. In the current system, there are two sets of two flow controllers that produce two outputs with two gas concentrations. Each of these outputs with the desired concentration is split into two lines with a manual mass flow controller. The manual flow controller allows the client to separate the gas into two lines, while still keeping the same flow in each line. These lines then feed into the rat holding chambers where the specimens are exposed to the gas. The holding chambers are made from Plexiglas and are not much bigger than the rat itself. The chamber has an approximately one inch hole opposite of the gas input which does not give much resistance to the gas flow. The
chambers were designed and fabricated by our client, and could possibly be improved in a separate project.
Overall there are four testing chambers in the current system. Our client can run all four chambers on one protocol or run two experimental protocols at once, with two chambers running on each protocol. The concentration of gasses within the chambers is tested periodically for accuracy in the lab. The mass flow controllers have tended to drift off of their calibration over time. Adjustments have been made in the set points to compensate for this problem.
The design must vary the concentrations in of oxygen and nitrogen in an enclosed chamber. The concentration of oxygen must vary between 11% and 21%, and the concentration must vary between 89% and 79%. Switching between different concentrations must be accomplished quickly. The mass flow controllers used in the system must be as accurate as possible. There is not a specific interface, analog or digital, that the client would like to use. The mass flow controllers should also be as quiet as possible so as not to disturb the rats.
The software used to control the mass flow controllers must be user-friendly. The software must have a graphical interface and have customizable features for different experiments. The software should also have a time component to start experiments automatically at different times.
Finally, hose with a uniform resistance should be used to transport the gas from the tanks to the mass flow controllers and from the controllers to the rat chambers. The system should include the capability to expand to allow for the use of carbon dioxide and for more rat chambers. Software Consideration
One of the main objectives of this project is to design new software that will increase the efficiency of the experiments. There were three programs that we considered for this design: LabVIEW, Agilent VEE Pro, and XControls. LabVIEW and Agilent are both programming environments that allowed the user to manipulate the logic via graphical representations of instruments; while XControls is an add-on program that allows the user to put a graphical interface to current data acquisition software.
Figure 8, Screenshot of a LabVIEW project
Both LabVIEW and Agilent VEE Pro are two different pieces of software but they are a lot alike. Each environment comes with a myriad of modules and programming tools that allow the user to setup an interface that can control various instruments. The programs both have a small learning curve allowing for programmers and non-programmers to use the software efficiently. Agilent and LabVIEW have both been used in research and industry and continue to be very successful products. For example, Agilent was used to test the communication system of the rover used in the Mar’s mission. LabVIEW and Agilent both have the same system
requirements for the computer that will run the programs. LabVIEW had one distinct advantage over Agilent and that was the customer service and support guarantee (Sweet 2004). LabVIEW has a sales representative, Adam Sweet, who comes to the University of Wisconsin-Madison