Abstract:
A sample testing system includes a plurality of sample tubes, each sample tube coupled to a pumping chamber, a pressure control subsystem and a flow control subsystem. The pressure control system includes a first dynamic pump equipped to induce pulsatile pressure in a mass of pumping fluid coupled to the pumping chambers. The flow control subsystem includes a mean flow pump equipped to generate a flow of sample fluid in a plurality of flow loops. Each of the flow loops conducts the flow of sample fluid between the mean flow pump and one of the sample tubes. The pumping chamber couples pressure from the pumping fluid to the sample fluid.

Description:
BACKGROUND 
     This disclosure relates to multiple-specimen device testing with particle measurement. 
     Equipment for simultaneously testing multiple implantable medical devices, such as stents or stent/grafts, is described in U.S. Pat. No. 5,670,708, incorporated here by reference. 
     SUMMARY 
     In general, in some aspects, a sample testing system includes a plurality of sample tubes, each sample tube coupled to a pumping chamber, a pressure control subsystem and a flow control subsystem. The pressure control system includes a first dynamic pump equipped to induce pulsatile pressure in a mass of pumping fluid coupled to the pumping chambers. The flow control subsystem includes a mean flow pump equipped to generate a flow of sample fluid in a plurality of flow loops. Each of the flow loops conducts the flow of sample fluid between the mean flow pump and one of the sample tubes. The pumping chamber couples pressure from the pumping fluid to the sample fluid. 
     Implementations may include one or more of the following features. 
     Each sample tube is coupled to the pumping chamber at a first end and is coupled to a second pumping chamber at a second end. Each pumping chamber includes a first volume coupled to a pressure inlet, a second volume coupled to a flow inlet and a flow outlet, and a diaphragm separating the first volume from the second volume and deformable such that pressure in the first volume is transferred to the second volume. The diaphragm includes a generally flat membrane separating the pumping chamber into the first volume and the second volume. Each sample tube is located inside the corresponding pumping chamber, the first volume is between the sample tube and outer walls of the pumping chamber, the second volume is internal to the sample tube, and the diaphragm includes the sample tube. Each sample tube is coupled to the flow control subsystem through flow apertures in the pumping chamber, and each of the flow apertures is sized to attenuate pressure changes in the sample fluid such that pressure changes in the sample tube are not communicated to the flow control system. The flow apertures in each pumping chamber include a flow inlet and a flow outlet, one of the flow inlet or the flow outlet of each pumping chamber is sized to attenuate pressure changes in the pumping chamber such that pressure changes in the pumping chamber are not communicated to the flow control system, and the other of the flow inlet or flow outlet of each pumping chamber is sized to communicate pressure changes in the pumping chamber to the sample tube coupled to the pumping chamber. 
     The pressure control subsystem also includes a first pressure manifold coupled to the first dynamic pump through the mass of pumping fluid, and having a plurality of outlets corresponding to the number of sample tubes in the plurality of sample tubes, a second pressure manifold coupled to the first dynamic pump through the mass of pumping fluid, and having a plurality of outlets corresponding to the number of sample tubes in the plurality of sample tubes, and a plurality of pumping tubes, each pumping tube coupling one of the outlets of the first or second pressure manifolds to one of the pumping chambers. The pressure control subsystem also includes a second dynamic pump, equipped to induce pulsatile pressure in a second portion of the mass of pumping fluid, the first dynamic pump inducing pulsatile pressure in a first portion of the mass of pumping fluid, a first pressure manifold coupled to the first dynamic pump through the first portion of the mass of pumping fluid, and having a plurality of outlets corresponding to the number of sample tubes in the plurality of sample tubes, a second pressure manifold coupled to the second dynamic pump through the second portion of the mass of pumping fluid, and having a plurality of outlets corresponding to the number of sample tubes in the plurality of sample tubes, and a plurality of pumping tubes, each pumping tube coupling one of the outlets of the first or second pressure manifolds to one of the pumping chambers. A first pressure balancing tube is coupled to the pressure control system, a second pressure balancing tube is coupled to the flow control system, and a filter housing couples the first pressure balancing tube to the second pressure balancing tube through a filter. 
     The flow control subsystem also includes a first flow manifold coupled to an outlet of the mean flow pump and having a plurality of outlets corresponding to the number of sample tubes in the plurality of sample tubes, and a second flow manifold coupled to an inlet of the mean flow pump and having a plurality inlets corresponding to the number of sample tubes in the plurality of sample tubes, each flow loop connecting one outlet of the first flow manifold to one inlet of the second flow manifold and including one of the sample tubes and the corresponding pumping chamber, a first flow tube coupling the outlet of the first flow manifold to a flow inlet in the pumping chamber, a second flow tube coupling a flow outlet in the pumping chamber to the inlet of the second flow manifold. The first flow tube is coupled to the flow inlet in a first pumping chamber, and the second flow tube is coupled to the flow outlet in a second pumping chamber, the two pumping chambers being coupled to first and second ends of the sample tube. Each of the flow loops also conducts the flow of fluid to a filter array. The filter array includes, for each flow loop a first branch dividing the flow loop into a first flow pathway coupled to a first filter housing and a second flow pathway coupled to a second filter housing, a switch selectively opening one of the first or second flow pathways and closing the other of the first or second flow pathways, a second branch coupling the first flow pathway and the second flow pathway to a flow rate sensor. The flow rate sensor of each flow loop is coupled to the switch in the filter array associated with the corresponding flow loop. 
     In general, in some aspects, a pumping chamber for use with a pressure control system and a flow control system includes a first volume coupled to a pressure inlet, a second volume coupled to a flow inlet and a flow outlet, and a diaphragm separating the first volume from the second volume, pressure in the first volume inducing deformation of the diaphragm to induce pressure in the second volume. 
     In general, in some aspects, a filter module for use in a flow control system includes a first branch dividing an incoming flow into a first flow pathway coupled to a first filter housing and a second flow pathway coupled to a second filter housing, a switch selectively opening one of the first or second flow pathways and closing the other of the first or second flow pathways between the first branch and the first and second filter housings, a second branch coupling outlets of the first and second filter housings to a flow rate sensor. 
     Advantages include obtaining accurate particle counting and capture from multiple specimens without mixing of particles between the specimens. 
     Other features and advantages will be apparent from the description and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows an isometric view of a multi-specimen testing system. 
         FIG. 1B  shows a side view of the multi-specimen testing system of  FIG. 1A . 
         FIG. 2A  shows a side view of a pressure control system for a multi-specimen testing system. 
         FIG. 2B  shows an end view of a pressure control system of  FIG. 2A . 
         FIG. 3  shows a schematic view of a sample fluid system for a multi-specimen testing system with separate flow loops. 
         FIG. 4  shows a pumping chamber for a single sample. 
         FIG. 5A  shows a sample tube and two pumping chambers for a single sample. 
         FIG. 5B  shows a connection between the pressure control system of  FIG. 2  and the sample fluid system of  FIG. 3 . 
         FIG. 6  shows an alternative embodiment of a sample tube and pressure chamber. 
         FIG. 7  shows a pressure sensor located within a sample tube. 
         FIG. 8  shows a switching and filtering system. 
         FIG. 9  shows a pump fluid manifold. 
         FIG. 10  shows a light source located within a pumping chamber. 
         FIGS. 11 and 12  show mounting arrangements for instrumentation 
     
    
    
     DESCRIPTION 
     As shown in  FIG. 1 , a multi-specimen testing system  100  allows an investigator to stimulate a number of samples in parallel. The system of  FIG. 1 , in particular, is used for testing implantable cardiovascular devices, such as stents or stent/grafts located inside sample tubes  102  that serve as simulated blood vessels, also referred to as mock arteries. Latex has commonly been used for this purpose, but in some examples, such as coated stent durability testing, silicone may be preferred for its particulate shedding properties. In some examples, the sample tubes and supporting structures are sized to accommodate sample devices having inner diameters between around 2 to 14 mm and lengths of around 100 to 200 mm for test simulations of coronary stent/grafts. Other example systems, such as for peripheral vessel simulation, accommodate larger devices. Previous systems, such as the ElectroForce™ 9110 from the ElectroForce Systems Group of Bose Corporation, located in Eden Prairie, Minn., have held twelve (12) specimens in a circular configuration, with the sample tubes held horizontally between two motors, as described in U.S. Pat. No. 5,670,708, noted above. In contrast, in the system shown in  FIG. 1 , the sample tubes  102  are located vertically and oriented in a common plane. Each of the sample tubes is connected at its ends to upper and lower pumping headers  104  and  106 , which are in turn connected to upper and lower dynamic pump motors  110  and  112 . A measurement platform  114  positions measurement instruments  116 ,  118  near the sample tubes and allows a user to control the position of the instruments. Other support structures  120 ,  122  hold the above components in position in a conventional manner. 
     The dynamic pump motors  110  and  112  are part of a pressure control system  200 , shown in  FIG. 2 , that generates a cyclic pressure profile in the sample tubes by raising the pressure of the fluid in the system. In some examples, the pressure control system is designed to stimulate the sample devices to induce an radial inner diameter strain of around 3 to 5 percent at frequencies of 1 to 150 Hz. In the pressure control system  200 , each of the dynamic pump motors  110  and  112  applies pressure in a corresponding upper and lower fluid volume  202 ,  204 . Upper and lower dynamic pump manifolds  206 ,  208  divide the fluid volumes  202 ,  204  into separate upper and lower pressure tubes  210 ,  212  for each sample tube  102 . Note that while the pressure tubes  210 ,  212  are shown as thin lines in the figures, they may be any diameter appropriate to the volume and pressure of the fluids used. The upper and lower pressure tubes are coupled to the upper and lower pumping headers  104  and  106 . Each of the pressure tubes, sample tubes, and pumping headers is repeated as many times as there are samples to be tested. In  FIG. 2 , for clarity, only two of each are shown, and only one of each is labeled. Pressure applied by the dynamic pump motors is transmitted through the fluid volumes, manifolds, pressure tubes, and pumping headers into the sample tubes  102 . In some examples, each of the upper pressure tubes  210  is the same length, and each of the lower pressure tubes  212  is the same length, so that the same pressures are applied at the end of each upper or lower tube. When the upper and lower tubes are also the same length as each other, then the same pressure can be applied at each of the upper and lower fluid volumes to apply the same pressures at the upper and lower pumping headers. If the upper and lower tubes are different, then different forces may be applied at the upper and lower fluid volumes to assure that the same pressures are applied at the upper and lower pumping headers. When the same pressure is applied in both pumping headers at the same time, the pressure in the sample tube  102  is the same at both ends, controlling the total pressure without creating a net flow. Such a pressure control system is described in U.S. Pat. No. 5,670,708, noted above. Similar techniques are used in the vertical/planar system shown in  FIG. 1  to those of the horizontal/circular system of that patent. A dynamic pump  213   a  includes the dynamic pump motor  112  and the dynamic pump manifold  208 . A dynamic pump  213   b  includes the dynamic pump motor  110  and the dynamic pump manifold  206 . One advantage of pressurizing the sample tubes from both ends is that the effective length of each sample tube, as viewed from a pressure source at one end, is cut in half, allowing higher-frequency testing with more uniform strain distribution over the length of the sample tube. Another advantage is that each dynamic pump  213   a ,  213   b  must move half as much fluid to achieve a given pressure, allowing greater operating frequencies. 
     As shown in  FIG. 3 , a sample fluid system  300  is connected to the sample tubes  102  and pumps a sample fluid through the sample tubes for transport and analysis of particles shed by the devices under test in the sample tubes. Pumping a sample fluid through the tubes also allows control of environmental factors, such as temperature or pH, to be conducted remotely from the sample. The fluid flow is primarily a constant flow. The sample fluid system includes a mean flow pump  302 , primary tubing  304 ,  305 , and  306 , upstream and downstream sample fluid manifolds  308  and  310 , sample fluid tubing  312 , a filter array  314 , and a sample fluid reservoir  316 . The mean flow pump  302  receives fluid from the reservoir  316  through primary supply tubing  304  and pumps it into the system through the primary delivery tubing  305 . At the upstream manifold  308 , the fluid is divided into as many sample fluid tubes  312   a  as there are sample tubes  102 . We sometimes refer to the individual pathways created by the sample fluid tubes as sample fluid loops. The sample fluid tubes  312   a  deliver the sample fluid to the lower pumping headers  106 , from which it flows through the sample tubes  102  and into the upper pumping headers  104 . In some examples, flowing the sample fluid upward through the sample tubes  102  assists in purging of air from the fluid, discussed below. In some examples, the fluid flows in the other direction, so accommodating particles that are dense enough to fall downward, against an upward flow. The sample tubes do not have to be oriented vertically, as shown, but may also be located horizontally or at any arbitrary angle desired by the user. Description of the components as “upper” and “lower” is for reference only. Leaving the upper pumping header  104 , additional sample tubing  312   b  takes the sample fluid to the filter array  314 , where any particulate now in the sample fluid is filtered out, as described in more detail below, before the separate sample fluid loops in tubing  312   c  rejoin at the downstream manifold  310 . The primary return tubing  306  returns the sample fluid to the reservoir  316 . In some examples, the individual sections of sample tubing  312  making up each sample flow loop are the same length, within each step of the pathway, to assure uniform flow and mean pressure in each of the sample flow loops. One of skill in the art will appreciate that the systems shown in  FIGS. 2 and 3  can be used together to provide both pressure and flow control in the sample tubes  102 . 
     The sample fluid system  300  shown in  FIG. 3  also includes an optional in-line particulate measurement system  318  for counting and/or measuring particles in the sample fluid. The particulate measurement system  318  includes light-obscuration particle counters that measure the number and, in some cases, size of the particles passing through each flow loop. In some examples, the sample fluid system includes a filter  320  between the mean flow pump  302  and the upstream manifold  308  to capture any particulate shed by the mean flow pump  302  so that it does not contaminate the fluid flowing through the sample tubing  312 . 
     Fluid Segregation 
     When multiple samples are to be tested and particulate shed from the samples is to be monitored, it is desirable to keep the particles shed from the samples separated, so that each sample can be measured individually. The systems shown in  FIGS. 1 ,  2 , and  3  apply a common pressure and a common fluid flow to all the samples, while maintaining segregation between the flow loops so that the particles shed from each sample can be measured or captured without mixing. 
     There are two separate areas of segregation. First, as shown in  FIGS. 4 and 5 , the pumping fluid that is used to drive pulsatile stimulation of the samples is isolated from the fluid that flows through the samples and transports any shed particles. This prevents any stray particles or bubbles from the pulsatile pumping system from entering the sample flow loops or vice versa. It also prevents particles shed by one sample from entering the flow loop of another sample through the pumping fluid. This isolation is designed such that there is no fluidic communication, but enables the transfer of pulsatile/dynamic pressure from the dynamic pumps to the flow loops. Second, as shown in  FIG. 3 , the fluid that flows through the samples is driven by a common mean flow pump but divided by the upstream sample fluid manifold into separate loops for each sample. The sample fluid flowing through each sample flow loop is passed through particle counting systems, if used, and then through individual particle filters. After the sample fluid is filtered, the flow paths rejoin at the downstream sample fluid manifold and are returned to the sample fluid reservoir. By filtering the flow loops individually, any particles that are shed are associated with the sample they originated from, while a single fluid reservoir and mean flow pump can be used to drive the sample fluid through all the samples. In some examples, a pathway exists for shed particles to move upstream to the first sample fluid manifold, where they could mix. In such a system, additional upstream filters can be used in each flow loop to maintain segregation of the flow loops. Alternatively, separate flow pumps could be used to provide entirely segregated flow loops. 
       FIG. 4  shows a cross-section of one of the pumping headers  104  from  FIGS. 1 ,  2 , and  3 . Inside the chamber, a diaphragm  400  separates pumping fluid, on the left, from sample fluid, on the right. A pressure fluid port  402  admits dynamic pumping fluid from the pressure control system  200  (not shown). This controls the pressure in a left chamber  404  of the pumping header  104 . If the pressure in the left chamber  404  is raised or lowered relative to the pressure in a right chamber  406 , the diaphragm  400  flexes to equalize the pressure in the left and right chambers. When the pressure in the left chamber is higher than that on the right, the diaphragm moves to the right, increasing the pressure in the right chamber  406 . When the pressure in the left chamber is lower than that on the right, the diaphragm moves to the left, decreasing the pressure in the right chamber  406 . The right chamber is coupled to the sample tube  102  (not shown) through a sample fluid inlet port  408 . Sample fluid leaves through a sample fluid outlet port  410 . When the pressure in the right chamber is raised or lowered by action of the diaphragm, that pressure is communicated to the sample tube through the sample fluid shared between the right chamber and the sample tube. This allows pressure from the pressure control system  200  to be transferred to the sample fluid system  300  without mixing of the fluids or any transfer of particulates. The lower pumping header  106  is constructed and connected similarly to the upper pumping header, such that pressure is controlled at both ends of the sample tube, as shown in more detail in  FIG. 5 . In some examples, the diaphragm is designed with an area and stroke to displace the fluid volume required to extend the sample tube radius along the effective length of the tube. It is preferable for the diaphragm to have sufficient compliance and durability to yield effective linear pressure transfer over the duration of many tests. In some examples, one test typically involves 400 million cycles to perform a 10 year simulation. 
     The sample fluid tubing  312  (not shown) will be coupled to the right chamber  406  at the outlet port  410 , to accommodate the flow of fluid from the sample tube on through the sample flow loop, but in some examples, this connection is made through a small aperture that does not efficiently transmit rapid changes in pressure, so the fluid pressure in the flow loops outside of the pumping headers and sample tube remains relatively constant and the dynamic pressure changes are maximized in the sample tube region. Access ports  412  and  414  allow various functions, such as bleeding gasses from the right and left chambers, or inserting probes or materials into the fluid loops, as described below. 
       FIG. 5A  shows a matched pair of upper and lower pumping headers  104  and  106  with a sample tube in-between and pumping and sample fluid tubing attached. A sample device  500  is located in the sample tube  102 . Like the upper pumping header  104 , the lower pumping header  106  includes left and right chambers  424  and  426  separated by a diaphragm  420 , and pressure inlet  422 . The roles of sample fluid ports  428  and  430  are reversed relative to corresponding ports  408  and  410  in the upper chamber, as sample fluid flows in through port  430  and out, to the sample tube  102 , through port  428 . Access ports  432  and  434  function similarly to ports  412  and  414 . In operation, the dynamic pressure control system applies pressure simultaneously to the two left chambers  404  and  424  through tubing  210  and  212 . Through the diaphragms  400  and  410 , this creates a uniform pressure in the right chambers  406  and  426 , thereby transmitting the pressure to the sample tubes simultaneously from both ends and applying internal radial pressure to the sample device  500 . While the sample tubes are shown directly connected to protrusions from the pumping headers, in implementation various types of fittings may be used depending on the nature of the sample tubes and the types of fluid in use. Sample fluid flows into the right chamber  426  of the lower pumping header  106  from the sample tubing  312   a , upward through the sample tube  102  to the right chamber  406  of the upper pumping header  106 , and out through the sample tubing  312   b.    
     In some examples, to avoid applying a long-term bias on the membrane, the mean pressures on the two sides of the membrane are kept balanced. In the example of  FIG. 5B , pressure balancing tubing  440  connects the bulk fluid in the pressure system  200  and the sample fluid system  300  through a filter  442 . The plumbing  440  and filter  442  are sized to balance pressures while minimizing net flow of fluids between the two systems. This keeps the mean pressures equalized, with little effect on the dynamic performance within the sample tubes  102 . The filter  442  maintains cleanliness of the sample fluid. 
     In some examples, the diaphragms  400 ,  420  are actuated by a structural force source, rather than through fluid. For example, a motor may directly press on the diaphragms, or may do so through a mechanical linkage. Using a mechanical actuator rather than pumping fluid may simplify the pumping side of the system, removing the need to manage the pumping fluid.  FIG. 6  shows an alternative embodiment for segregating the pump fluid. In this example, the sample tube  102  is located within a chamber  502  filled with the pump fluid. The pump fluid in the chamber  502  is driven by the pulsatile pumping system (not shown) and applies pressure to the exterior of the sample tube, thereby applying external radial pressure to the sample device  500 . The sample tube itself maintains the separation between the pump fluid and the sample fluid, which flows through the sample tube as before. In some examples, the sample tube is located upstream or downstream of the concentric tubes shown in  FIG. 5 , and when pressure is applied to the inner tube, fluid is pumped into our out of that tube, and thereby out of or into the sample tube. 
     Direct Pressure Monitoring and Control 
     In some examples, control of the system is improved by providing direct measurement of the pressure in the sample tube, as compared to, for example, monitoring the pressure remotely from the sample and inferring the true sample pressure.  FIG. 7  shows one design for directly measuring the pressure in the sample tube. In this example, a catheter-type probe  520  is inserted through a sealing fitting  521  in the access port  414  in the upper pumping header  104  and into the sample tube  102 . A sensor  522  at the tip of the probe  520  is located at the mid-point of the sample tube or other location of interest, based on the sample device  500  being tested. Sensor leads  524  are connected to whatever monitoring and control system is in use, or to intervening electronics to convert readings of the sensor to usable signals. In some examples, a T-joint is provided within the sample tube to provide direct access for the pressure sensor to the sample tube. The pumping headers with separation diaphragm from  FIGS. 4 and 5  are used for illustration in  FIG. 7 , but the technique of  FIG. 7  is generally applicable to any pumping strategy. In some examples, when diaphragms are utilized, the pressure sensor may be located in either of the pumping chambers ( 404  or  406 ,  424  or  426 ) to isolate the sensor from the flow loop while effectively measuring the dynamic pressure at the tube inlet. 
     If the pressure control system is an enclosed system, pressure measured at the sample may be used as a direct feedback signal in the control loop. In an open-to-atmosphere pressure control system, the sample pressure may be used in a mixed mode-controller where the pressure is a feedback signal for a dynamic controller and the displacement of the powerheads is a feedback signal for the mean controller. This controller and sensor configuration enables a higher performing direct controller on pressure amplitude. This has the advantages of reducing the additional dynamics present in direct displacement control that often limit controller bandwidth and reducing the effects of changing test system characteristics over the duration of a long-term fatigue test. This additional bandwidth and direct control enables improved testing at higher frequency with higher pressure waveform fidelity of the sample pressure signal over the extended duration of a test. In either situation, control of the pressure is advantageously improved over removed pressure measurement solutions. This is especially valuable in systems where the pressure is varied at high frequencies, where pressure values can vary significantly and dynamically throughout the system. 
     Non-Disruptive Filter Switching 
     As mentioned above, in some applications, filters are used to capture particles shed within each flow loop before the flow loops rejoin in the second sample fluid manifold. Such filters may eventually become sufficiently full of particles that they obstruct the flow and need to be changed. In addition, in some applications, it is desirable to remove the filters for analysis of the captured particles while the test continues. In addition to the number and size of the particles, which may also be determined by in-line particle counters, filters allow the investigator to observe the makeup of the captured particles. Based on the particulate makeup, the investigator might infer the source and biocompatibility of the particulate to evaluate its impact.  FIG. 8  shows a filter switching system  600  that allows filters to be changed without disrupting the test. In some examples, such a filter switching system is used in the filter array  314  of  FIG. 3 . 
     In the example of  FIG. 8 , the sample fluid tubing  602  branches and flows through two filters  604   a ,  604   b.  The tubing in  FIG. 8  represents only one sample fluid flow loop—the same system  600  will be repeated for each flow loop in a multiple-sample system. After branching, two sections of sample fluid tubing  606   a  and  606   b  pass through a switching valve  608 . In some examples, the switching valve  608  is a solenoid pinch valve, which is a non-invasive valve that is less likely to cause particulate shedding itself than more invasive valves. The switching valve  608  selectively closes one of the tubes passing through it and leaves the other open. Each of the tubing sections  606   a  and  606   b  ends at a filter holder  610   a ,  610   b.  After the filters, a new tubing section  612  rejoins the two pathways and enters a flow meter  614 , which may be an impeller-style flow meter or any other in-line flow measurement device. After the flow meter the final section of tubing  616  leaves the filter system  600  and continues in the main sample fluid loop, i.e., becoming the final section on sample tubing  312   c  in  FIG. 3 . 
     Whenever the active filter, e.g., filter  604   b  in  FIG. 8 , needs to be changed, the switching valve  608  opens the currently-closed tubing ( 606   a ) and closes the currently-open tubing ( 606   b ). This cuts off sample fluid through the filter to be changed, and begins flowing sample fluid through the other filter ( 604   a ). The now-inactive filter is then removed and replaced. In some examples, a quick-disconnect fitting is used to attach the filters, effectively shutting off the tubing and avoiding back flow when the filter is removed. In some examples, other measures are used, such as one-way flow valves or a second switching valve downstream of the filters. While we illustrate two filters, any number of filters may be used with an appropriate switching arrangement, depending on the timing needs of the study and the, allowing the manual filter replacement to be less frequent. 
     In some examples, the switching valve  608  is triggered automatically based on a detected need to change the filters. As the active filter collects particles from the sample fluid, it begins to restrict flow through the active side of the filter system. The flow meter measures the flow through the system, thus the control system (not shown) will detect when the flow is restricted to the point that the filter needs to be changed. When a threshold flow condition or other parameter is met, the control system will trigger the switching valve  608  to change which path is open, and will alert the operator or an automated filter switching system to change filters in the now-inactive path. In addition to the mean or instantaneous flow through the system, other parameters that may trigger a filter switch include cycle count in the test system, elapsed time, or total flow volume since the last filter change or over some other time period. In a system with a separate flow pump for each loop, the pressure across the filter may also be a useful measure of whether a filter needs to be changed. A user may program any combination of these parameters, or may use fully manual control and change the filter when desired by manually triggering the switching valve  608 .  FIG. 8  shows the flow meter downstream of the filters. In general, the flow meter may be located anywhere along the flow path, but locating it downstream of the filters avoids particulate shed by the flow meter itself contaminating the sample. 
     Gas Management 
     Fatigue testing systems for stents typically rely on an incompressible fluid for transmitting pressure pulses from a dynamic pump to a flexible tube, which contains the stent. These pulses cause radial distention in this tube, as well as the stent implanted within it, typically at an accelerated rate (&gt;40 Hz) when compared to the physiologic rate (˜1.2 Hz). Any compressible gasses within the system attenuate these pressure waves, reducing the efficiency of the system, which lowers either the testing frequency (increases test duration), or the achievable distention levels. In addition, it is advantageous to remove dissolved gasses from the sample fluid when particulate counters are in use, as any bubbles that form in the sample fluid may be mistaken for particles. Several approaches may be combined to limit the amount of dissolved gasses and avoid gasses coming out of solution to form bubbles. 
       FIG. 9  shows a cross-sectional view of a manifold  700  for use in the pressure control system of  FIG. 2  (e.g., manifolds  206  and  208 ). A conical internal profile  702  leads up to a central degassing bleed port  704 . This profile  702  encourages any bubbles in the main bulk of the pressure fluid to rise to the bleed port  704  rather than exiting through the other ports  706  into the pumping fluid tubes that lead to the pumping headers  104  and  106  (see, e.g.,  FIGS. 1 and 2 ). Connectors  708 ,  710  are shown for reference only—any appropriate method of connecting valves or tubing to the manifold ports  704  and  706  may be used. 
     Access ports  412 ,  414 , and  432  for both the pressure fluid side and the sample side of the system are shown in the pumping headers  104  and  106  in  FIG. 5A . By using these access ports located at the highest points in the sample fluid flow loops (see  FIG. 1 ) and pressure system (see  FIG. 2 ) as bleed ports, gasses coming out of solution in the sample fluid and any bubbles getting or originating past the manifold bleed port  704  in the pressure fluid will rise to the bleed ports in the pumping headers, and the system will not have to be moved around or otherwise disturbed to get the gasses to the bleed ports. 
     Given the use of flexible tubing which is typically gas permeable to at least some extent, it may be necessary to continuously or periodically remove gas from the system, as opposed to simply doing it once during system setup. In some examples, a fluid degassing system is used to remove gasses at a controlled point and keep the total level of dissolved gasses low enough that the gasses will remain in solution in the rest of the system, including any of the various points where changes in pressure and temperature might encourage gasses to come out of solution. In some examples, tubing with high gas permeability is combined with a differential pressure, such as by locating it at a high pressure position relative to the environment and/or by pulling a vacuum on the outside of the tubing. Commercially available in-line degassing equipment may also be used. One example is the Liqui-Cel® line of membrane contactor from Membrana-Charlotte, a division of Celgard, LLC, located in Charlotte, N.C. In membrane degassing systems, the fluid to be degassed separated from a vacuum by a semi-permeable membrane through which dissolved cases are pulled by the vacuum. In some examples, the membrane forms tubes passing through the fluid, with the vacuum presented in the tubes. In some examples, this is reversed, with the fluid passing through membranous tubes that are located within a chamber under vacuum. 
     Strobe Illumination of Sample Tubes 
     In some applications, the rates at which the samples are stimulated exceed the ability of a human observer to see changes in the device under test. One common solution to such a situation is to illuminate the sample with a strobe light, tuned to provide a static image of the sample (if the strobe frequency is an integral fraction of the stimulation frequency) or a slowly evolving image (if the strobe frequency is near an integral fraction of the simulation frequency). With such an image, the observer may be able to see movement of the sample that was not apparent at its true cycle speed. As shown in  FIG. 10 , one way to provide such illumination is to locate an LED or other small strobe light within one anchor of the sample tube. In particular, in  FIG. 10 , an LED  750  is located in the lower pumping header  106 , seated in the lower sample fluid access port  434  with a plug  752 . Lead lines  754  control the LED. By connecting the light source to the same controller (not shown) as the pressure source, the frequency of the strobe can be automatically tuned to provide a steady or slowly evolving image of the sample. Tying the strobe to the controller has advantages over a manually controlled strobe, including eliminating user-to-user variability and providing a consistent light pattern to each sample in the system and throughout the duration of the test. Due to internal reflection at the fluid-surface interfaces, the light from such a source will travel through the sample fluid and illuminate the device  500  under test without significant leakage of light through the rest of the sample tube  102 . 
     Multiple-Plane Sample Tube Measurement 
     In the system shown in  FIG. 1 , an optical emitter  116  and detector  118  move laterally along the measurement platform  114 , moving among the line of sample tubes  102  to measure the diameter of each sample tube in turn. The platform  114  can also move up and down along the length of the sample tubes, positioning the emitter  116  and detector  118  to measure the diameter of each sample tube at multiple points. As shown in  FIG. 1 , the emitter and detector only measure in a single direction, measuring the tube diameter perpendicular to that direction at whatever position they are moved to. In some applications, it is desirable to measure the diameter along different directions, i.e., providing the cross-sectional width of the tube in different planes. This is useful, for example, for identifying non-symmetric failures of the devices under test.  FIGS. 11-13  show several different embodiments for providing these measurements. 
     In some examples, as shown in  FIG. 1 , mounting holes  800  are located in the measurement platform  114  at various angles around each sample tube. The emitter  116  and detector  118  may be located at any matched set  802  of mounting holes  800  to be accurately positioned to measure the sample tube. In the example of  FIG. 11 , the sets of mounting holes are positioned at  60  degree angles along a circular path, so that the emitter and detector will be positioned the same distance from the sample tube in each position. Alternatively, the mounting holes may be located at differing distances. For example, the mounting holes may all be located in a single line. This would allow narrower measurement platform, but would require the measurements to be adjusted for the differing distances involved from each position. Other angles may also be used. In the example of  FIG. 1 , the mounting holes are configured to allow some of the mounting holes to be used for multiple samples, by locating the emitter and detector on the same holes but in different directions. In the example of  FIG. 1 , three pairs of emitters and detectors are shown around one sample tube, with alternative positions of one of the pairs shown in broken lines. If the detectors and emitters are indeed small enough to occupy multiple positions without interfering with each other, several may be used to produce multiple measurements simultaneously. Alternatively, one set may be repositioned between each of the positions as desired. 
     In some examples, as shown in  FIG. 12 , a first track  820  is provided along which the emitter  116  and detector  118  may be repositioned, while the first track itself moves along a second track  822  to reach multiple sample tubes  102 . The curved track shown is merely one example—other configurations, such as a single straight track, may be used, if the emitter and detector can pivot as needed to face the sample tube. In addition to allowing a greater number of measurement angles, the continuous tracks, however implemented, allow the repositioning of the emitter and detector to be automated, for example by motors  824  and wheels  826 . The vertical movement of the measurement platform (see  FIG. 1 ) may also be controlled by a motor and thereby automated. 
     Other implementations are within the scope of the following claims and other claims to which the applicant may be entitled.