Patent Publication Number: US-2021162416-A1

Title: Multiwell dynamic model for a tumor-immune microenvironment

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/942,414, titled “MULTIWELL DYNAMIC MODEL FOR A TUMOR-IMMUNE MICROENVIRONMENT,” filed Dec. 2, 2019, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Evaluating the efficacy of candidate immunotherapies against diseases such as cancer can be desirable. It can be difficult to predict the efficacy of a candidate immunotherapy due to challenges in mimicking the in vitro microenvironment in which the immunotherapy is to be introduced. For example, it can be challenging to facilitate interactions between immunotherapies and tissue samples, and to evaluate such interactions, in a laboratory setting. 
     SUMMARY 
     The present disclosure describes systems and methods for modeling a tumor-immune microenvironment. A microfluidic device for modeling such an environment can include a well plate having one or more wells (sometimes referred to herein as a “multiwell plate”). In some implementations, the multiwell plate can be a bilayer device that defines a set of microenvironment units. Each microenvironment unit can include a basal layer and an apical layer, which may be separated from one another by a permeable membrane. The microenvironment units can serve as a setting that mimics the tumor-immune microenvironment. In some implementations, each microenvironment unit can also include a mechanism for trapping a tissue sample, such as a section of a tumor on which testing of candidate immunotherapies can be performed. Such a mechanism can be referred to in this disclosure as a mechanical trapping feature. In some implementations, the mechanical trapping features can be formed from portions of channel walls or compartment walls, such as a sidewalls, ceilings, or floors of compartments or channels in each microenvironment unit. While a tissue sample is trapped by a mechanical trapping feature, it can be perfused with a candidate immunotherapy, and the interaction between the trapped tissue sample and the candidate immunotherapy can be observed and monitored over time. In some implementations, the microfluidic device can include a series of micropumps coupled with respective wells and capable of being individually controlled. Thus, perfusion rates, candidate immunotherapies, and other parameters can be varied simultaneously across the microenvironment units of the multiwell plate, thereby increasing the speed and efficiency with which a group of candidate immunotherapies can be evaluated. 
     At least one aspect of the present disclosure is generally directed to a microfluidic device. The microfluidic device can include a well plate comprising one or more wells. The well plate can define a one or more microenvironment units. The microenvironment units can be fluidically coupled with the one or more of wells. Each microenvironment unit can include one or more compartments. Each microenvironment unit can include a trapping feature positioned within the one or more compartments. The trapping feature can be defined by a portion of at least one of a sidewall or a floor of the one or more compartments. The trapping feature can restrict movement of a tissue sample introduced into the one or more compartments and can allow fluid to flow past the tissue sample. The microfluidic device can include one or more micropumps. Each of the one or more micropumps can control introduction of a fluid sample to a respective well of the plurality of wells. 
     In some implementations, the one or more compartments of each microenvironment unit of the microfluidic device can include a basal compartment, an apical compartment, and a membrane separating the basal compartment from the apical compartment. In some implementations, each microenvironment unit of the microfluidic device can include a basal channel having a basal channel inlet and a basal channel outlet. In some implementations, the basal compartment can include a portion of the basal channel between the basal channel inlet and the basal channel outlet. In some implementations, each microenvironment unit of the microfluidic device can include an apical channel having an apical channel inlet and an apical channel outlet. In some implementations the apical compartment can include a portion of the apical channel between the apical channel inlet and the apical channel outlet. 
     In some implementations, at least one micropump of the plurality of micropumps can be coupled with at least two wells of the plurality of wells. In some implementations, the microfluidic device can include a fluid reservoir coupled with at least one micropump of the plurality of micropumps. In some implementations, the microfluidic device can include a controller communicatively coupled with each micropump of the plurality of micropumps. In some implementations, the controller can selectively control each micropump of the plurality of micropumps independently. In some implementations, the plurality of micropumps can include at least a first micropump fluidically coupled with the basal compartment of a first microenvironment unit of the plurality of microenvironment units via a first well and a second micropump coupled with the apical compartment of the first microenvironment unit via a second well. 
     In some implementations, the membrane can include an apical surface and a basal surface. In some implementations, the basal surface can be opposite the apical surface. In some implementations, the membrane can include a functionalized coating applied to the at least one of the basal surface or the apical surface. In some implementations, the functionalized coating can include a gel. In some implementations, the microfluidic device can include a transparent optical layer coupled with the well plate. In some implementations, the transparent optical layer can provide an optical interface into each microenvironment unit of the well plate. In some implementations, the optical interface can have a thickness selected to permit the tissue sample in each microenvironment unit to be observed using a confocal microscope. 
     In some implementations, the trapping feature can be a mechanical trapping feature. In some implementations, the trapping feature of at least one microenvironment unit of the plurality of microenvironment units extends away from the floor of the one or more compartments into the one or more compartments to reduce a cross-sectional area of the one or more compartments. In some implementations, the mechanical trapping feature of the at least one microenvironment unit comprises at least a first step extending a first distance into the one or more compartments and a second step adjacent to the first step and extending a second distance into the one or more compartments. In some implementations, the second distance is greater than the first distance. In some implementations, the second step is positioned downstream from the first step. 
     In some implementations, the trapping feature of the at least one microenvironment unit comprises one or more raised surfaces extending away from the floor of the one or more compartments into the one or more compartments. In some implementations, each of the one or more raised surfaces are positioned on the floor of the one or more compartments in an arrangement that exhibits symmetry about a longitudinal axis of the one or more compartments. In some implementations, at least one of the one or more raised surfaces has a curved shape. In some implementations, the trapping feature of the at least one microenvironment unit does not extend to a ceiling of the one or more compartments, opposite the floor of the one or more compartments. 
     In some implementations, the trapping feature of at least one microenvironment unit of the plurality of microenvironment units comprises a first portion of a first sidewall and a second portion of a second sidewall. In some implementations, the second portion of the second sidewall can be opposite the first portion of the first sidewall. In some implementations, the first portion the first sidewall and the second portion of the second sidewall are each tapered to reduce a cross-sectional area of the one or more compartments. In some implementations, the first portion of the first sidewall and the second portion of the second sidewall are offset from a vertical center of the one or more compartments. 
     At least one other aspect of the present disclosure is generally directed to a method. The method can include introducing a tissue sample into each microenvironment unit of a plurality of microenvironment units defined by a well plate having a plurality of wells. Each microenvironment unit can include one or more compartments. Each microenvironment unit can include a trapping feature positioned within the one or more compartments. The trapping feature can be define by a portion of at least one of a sidewall or a floor of the one or more compartments. The trapping feature can restrict movement of the tissue sample in the one or more compartments and to allow fluid to flow past the tissue sample. The method can include controlling a plurality of micropumps each coupled with a respective well of the plurality of wells to introduce a respective fluid sample into the respective wells. Each of the respective wells is fluidically coupled with at least one of the plurality of microenvironment units. The method can include observing an interaction between the tissue sample of a first microenvironment unit and the fluid sample introduced into a first well of the plurality of wells. 
     In some implementations, at least a portion of the well plate can include a transparent material. In some implementations, observing the interaction between the tissue sample of the first microenvironment unit and the fluid sample introduced into the first well comprises positioning a lens of a microscope in proximity to the first microenvironment unit. In some implementations, the method can include controlling at least one micropump of the plurality of micropumps to introduce a second fluid sample comprising a plurality of cells into an apical compartment of the one or more compartments of the first microenvironment unit. In some implementations, controlling the plurality of micropumps comprises controlling at least two of the plurality of micropumps independently from one another. 
     These and other aspects and implementations are discussed in detail below. The foregoing information and the following detailed description include illustrative examples of various aspects and implementations, and provide an overview or framework for understanding the nature and character of the claimed aspects and implementations. The drawings provide illustration and a further understanding of the various aspects and implementations, and are incorporated in and constitute a part of this specification. Aspects can be combined and it will be readily appreciated that features described in the context of one aspect of the invention can be combined with other aspects. Aspects can be implemented in any convenient form. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are not intended to be drawn to scale. Like reference numbers and designations in the various drawings indicate like elements. For purposes of clarity, not every component may be labeled in every drawing. The foregoing and other objects, aspects, features, and advantages of the disclosure will become more apparent and better understood by referring to the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a top view of an example multiwell plate, in accordance with one or more implementations; 
         FIG. 2  illustrates a top view of a portion of an example multiwell, in accordance with one or more implementations; 
         FIG. 3  illustrates a top view of another example multiwell plate, in accordance with one or more implementations; 
         FIG. 4A  illustrates a cross-sectional view of a portion of a microenvironment unit shown in the multiwell plate of  FIG. 2 , in accordance with one or more implementations; 
         FIG. 4B  illustrates another cross-sectional view of a portion of a microenvironment unit shown in the multiwell plate of  FIG. 2 , in accordance with one or more implementations; 
         FIGS. 5, 6, 7, and 8  show various views of channels having mechanical trapping features, in accordance with one or more implementations; 
         FIGS. 9A, 9B, 9C, and 9D  show various views of channels having mechanical trapping features, in accordance with one or more implementations; 
         FIGS. 10 and 11  show various views of channels having mechanical trapping features, in accordance with one or more implementations; 
         FIGS. 12A and 12B  show various views of a channel having a mechanical trapping feature, in accordance with one or more implementations; 
         FIGS. 13A and 13B  show various views of a channel having a mechanical trapping feature, in accordance with one or more implementations; 
         FIG. 14  shows a cross-sectional view of a channel having a mechanical trapping feature, in accordance with one or more implementations; 
         FIG. 15  shows a flowchart of a method for evaluating an interaction between a tissue sample and a fluid sample, in accordance with one or more implementations; 
         FIG. 16  illustrates a sectional view of an example system for integrating micropumps with a microfluidic device, in accordance with one or more implementations; 
         FIG. 17  shows a cross-sectional view of an example pumping system, in accordance with one or more implementations; 
         FIG. 18  shows a schematic view of an example pumping system, in accordance with one or more implementations; and 
         FIG. 19  shows a cross-sectional view of an example pumping system, in accordance with one or more implementations. 
     
    
    
     DETAILED DESCRIPTION 
     The various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the described concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes. 
     The present disclosure describes systems and methods for modeling a tumor-immune microenvironment. For example, such a microenvironment can mimic aspects of an in vitro environment that can contain a tissue sample of interest, such as a tumor or a portion of a tumor. A microfluidic device for modeling such an environment can include a multiwell plate. In some implementations, the multiwell plate can be a bilayer device that defines a set of microenvironment units. A bilayer device can include a plurality of chambers or channels separated from one another by a semipermeable membrane. In some other implementations, the multiwell plate can be a single layer device, in which the microenvironment units are defined by single-layer channels or chambers. A microenvironment unit can be a self-contained area of the multiwell plate in which environmental conditions and immunotherapies can be monitored and observed, independently from other microenvironment units of the device. Thus, the device may be used to perform a set of simultaneous experiments or observations under different conditions, thereby allowing multiple immunotherapies to be evaluated in parallel with one another. 
     In some implementations, each microenvironment unit can be fluidically coupled with at least one respective well of the multiwell plate. In some implementations in which the device is a multilayer device, each microenvironment unit of the device can include a basal layer and an apical layer, which may be separated from one another by a permeable membrane. The microenvironment units can serve as a setting that mimics the tumor-immune microenvironment. In some implementations, each microenvironment unit can also include a mechanism for trapping a tissue sample, such as a section of a tumor on which testing of candidate immunotherapies can be performed. Such a mechanism can be referred to in this disclosure as a mechanical trapping feature. In some implementations, the mechanical trapping features can be formed from portions of channel walls or compartment walls, such as a sidewalls, ceilings, or floors of compartments or channels in each microenvironment unit. 
     While a tissue sample is trapped by a mechanical trapping feature in a given microenvironment unit, it can be perfused with a candidate immunotherapy. The interaction between the trapped tissue sample and the candidate immunotherapy can be observed and monitored within the microenvironment unit over time. In some implementations, the microfluidic device can include a series of micropumps coupled with respective wells and capable of being individually controlled. Thus, perfusion rates, candidate immunotherapies, and other parameters can be varied simultaneously across the set of microenvironment units of the multiwell plate, thereby increasing the speed and efficiency with which a group of candidate immunotherapies can be evaluated. 
       FIG. 1  illustrates a top view of an example multiwell plate  100 . The multiwell plate  100  includes a plurality of microenvironment units  102 . The microenvironment units  102  are arranged in a rectangular grid pattern across a surface of the multiwell plate  100 . The multiwell plate  100  is depicted as including 96 microenvironment units  102  arranged in a 12×8 grid, however it should be understood that other arrangements, which may include more or fewer microenvironment units  102 , are also possible. Each microenvironment unit  102  can serve as an area for one or more cell cultures to be introduced. In some implementations, other substances, such as therapeutic substances, can be introduced into the microenvironment units  102 , and their interactions with the cell cultures can be observed or measured. 
       FIG. 2  illustrates a top view of a portion of an example multiwell plate. In some implementations, the multiwell plate depicted in  FIG. 2  can correspond to the multiwell plate  100  illustrated in  FIG. 1 . The portion shown in  FIG. 2  includes a single microenvironment unit  102  of the multiwell plate  100 . The microenvironment unit  102  can include a basal channel  202 . The microenvironment unit  102  can include an apical channel  212 . The basal channel can include two ports  204   a  and  204   b  (generally referred to as ports  204 ). The apical channel can include two ports  214   a  and  214   b  (generally referred to as ports  214 ). The microenvironment unit  102  also includes an overlapping portion  220  in which the basal channel  202  and the apical channel  212  overlap one another. It should be understand that structures referred to as channels in this disclosure may also be referred to as chambers or compartments. For example, the basal channel  202  and the apical channel  212  may also be referred to in this disclosure as the basal compartment and the apical compartment, respectively. 
     In some implementations, tissue fragments such as pieces of a tumor may be contained in the overlapping portion  220  of the microenvironment unit  102 . In some implementations, fluid samples can also be introduced into the basal channel  202  via the ports  204 , as well as into the apical channel  212  via the ports  214 . The fluid samples may include, for example, therapeutic substances such as drugs. For example, the fluid samples may be or may include candidate immunotherapies that are intended to treat patients who exhibit tumors or other tissue similar to the tissue sample contained in the overlapping portion  220 . Interactions between the tissue sample and the substances included in the fluid samples can be observed in the overlapping portion  220  as a way to evaluate the efficacy of the candidate immunotherapy. 
     The overlapping portion  220  can also include additional features selected to mimic a particular microenvironment, such as an in vitro environment in which a tumor to be treated by the candidate immunotherapy is likely to be present. For example, cell cultures may be grown in the overlapping portion  220 . For example, the overlapping portion  220  can include a permeable membrane, as shown in the enlarged view on the left hand side of  FIG. 2 . The membrane can separate the basal channel  202  from the apical channel  212  in the overlapping portion  220 . In some implementations, a cell culture can be introduced on the apical channel side of the membrane. In some implementations, a cell culture can be introduced on the basal channel side of the membrane. In some implementations, cell cultures can be introduced on both the apical channel side and the basal channel side of the membrane. The cell cultures on each side of the membrane can be the same or different from one another. 
     In some implementations, the overlapping portion  220  can include features to help ensure that the tissue sample under test remains secured in place within the overlapping portion  220  over time. For example, such features may include mechanical features formed from the walls of the basal channel  202  or the apical channel  212  that are designed to trap, catch, or otherwise retain the tissue sample in a fixed location within the overlapping portion  220  of the microenvironment unit  102 . Such features are described further below 
     It should be understood that while the example multiwall plate described herein can include one or more multi-layer microenvironment units, that single-layer microenvironments units are also possible. Single-layer microenvironment units can include one or more chambers, which may be separated by a semi-permeable membrane. Single-layer microenvironment units may not include an overlapping portion, and instead may share a wall or a portion of a chamber. For example, a single layer microenvironment unit can be similar to the microenvironment unit  102 , but without the overlapping region across two layers. Instead, a single-layer microenvironment unit can include the apical channel  212  (which may be referred to as a “first channel”) and the basal channel  202  (which may be referred to as a “second channel”) defined within a single layer (e.g., without overlapping, etc.). The first channel and the second channel can be defined in a single layer of a substrate, similar to the substrates described herein. The first channel and the second channel in the single-layer microenvironment unit can share a wall or surface (e.g., a sidewall, etc.). The shared side-wall can include a semi-permeable membrane that allows the channels of the single-layer microenvironment unit to be in fluid communication with one another. Although only two channels are described herein with respect to microenvironment units, it should be understood that any number of channels are possible. For example, a microenvironment unit can include a single channel defined in a single layer. The single channel can include a tissue trap. In some implementations, any number of channels on any number of layers are possible, with any number of tissue trapping features (e.g., as described herein below, etc.), any number of membranes, or any number of overlapping portions, channels, or ports. 
       FIG. 3  illustrates a top view of another example multiwell plate  300 . The multiwell plate  300  can be similar to the multiwell plate  100  depicted in  FIG. 1 . The multiwell plate  300  includes a plurality of ports  304 . The ports  304  are arranged in a 24×16 rectangular array, for a total of 384 ports. Channels are not depicted in the multiwell plate  300  of  FIG. 3  for illustrative clarity, however in some implementations, each port  304  may serve as a port for at least one channel. In implementations in which the multiwell plate  300  is a bilayer device, each port  304  may serve as a port for a basal channel or an apical channel of the bilayer device, similar to the ports  204  and the ports  214  shown in  FIG. 2 . For example, groups of ports  304  each including four ports may serve as the set of ports for a single microenvironment unit such as that shown in  FIGS. 1 and 2 . In some implementations, the multiwell plate  300  having 384 ports can be used to support 96 such microenvironment units, similar to the microenvironment unit  102  shown in  FIG. 2 . In some other implementations, the multiwell plate  300  can be a single layer device rather than a bilayer device. For example, each port  304  may serve as either an inlet or an outlet of a microfluidic channel. For example, the microfluidic channel can be a microenvironment unit having an inlet and an outlet, and therefore groups of two ports  304  may each form a respective microenvironment unit. Thus, the multiwell plate  300  may support 192 such microenvironment units. In still other implementations, each port  304  may be coupled with a respective chamber within the multiwell plate  300 . Such a chamber can serve as a microenvironment unit. Thus, the multiwell plate  300  can support 384 such microenvironment units. In some implementations, the types of microenvironment units may vary across the multiwell plate  300 . For example, some microenvironment units may include channels connected by two or more of the ports  304 , while other microenvironment units include chambers coupled with a single respective port  304 . In some implementations, the ports  304  can each have a circular shape as depicted in  FIG. 3 . In some other implementations, the ports  304  can have a different shape, such as a square shape, a rectangular shape, another polygonal shape, or an irregular shape, rather than the round shape shown in  FIG. 3 . 
       FIG. 4A  illustrates a cross-sectional view of a portion of the microenvironment unit  102  shown in  FIG. 2 . The cross-sectional view of  FIG. 4A  is taken along the line A-A′ of  FIG. 2 .  FIG. 4B  illustrates another cross-sectional view of a portion of the microenvironment unit  102  shown in  FIG. 2 . The cross-sectional view of  FIG. 4B  is taken along the line B-B′ of  FIG. 2 , and shows the microenvironment unit  102  from a perspective that is perpendicular to the perspective shown in  FIG. 4A . Referring now to  FIGS. 4A and 4B , the microenvironment unit  102  can include the basal channel  202 , the apical channel  214 , and a membrane  402  separating the basal channel  202  from the apical channel  212 . In some implementations, a first cell culture can optionally be seeded on a first side of the membrane  402 , and a second cell culture can optionally be introduced on a second side of the membrane  402 . In some implementations, other coatings or features, such as a functionalized coating or gel may be added to either or both sides of the membrane  402 . In some implementations, the basal channel  202  and the apical channel  212  can be defined within a single layer of a substrate material, such that each of the basal channel  202  and the apical channel  212  are position side-by-side. In such implementations, the membrane  402  can form a sidewall of each of the basal channel  202  and the apical channel  212 , such that a desired fluid can flow between each channel. 
     In some implementations, the functionalized coating can be added to any side of any channel or chamber described herein. For example, the functionalized coating can be added to the floor of the basal chamber, or the ceiling of the apical chamber. In some implementations, the functionalized coating can be applied to any portion of any surface of a microenvironment unit. The functionalized coating can be applied to the tissue trapping features described herein, such as the tissue trapping features depicted in  FIGS. 5-11 . 
     Other coatings can also be applied to the tissue trapping region, or other surfaces in the microenvironment units, to facilitate the trapping of tissue samples introduced via fluids. Such coatings can include adhesives that are applied to the internals of the microenvironment units prior to the introduction of a tissues sample. The adhesives can be applied to portions of the tissue trapping region to prevent tissue samples from becoming dislodged from tissue trapping regions. An adhesive coating can also prevent tissue samples from moving around when the tissue sample is exposed to fluid flows (e.g., via the micropumps described herein, etc.). In some implementations, the tissue trapping feature in a microenvironment unit  102  can be a portion of the microenvironment unit that has been treated with an adhesive. In such implementations, the microenvironment may not necessarily include a mechanical trapping feature, and may instead rely on the adhesive coating to trap a tissue sample within a microenvironment unit. 
     Some example adhesive coatings can include any suitable glue that is compatible with both tissue samples and with the materials used to manufacture the surfaces of the microenvironment unit. In some implementations, an adhesive can include one or more materials that can encourage cell attachment. In some implementations, an adhesive may not necessarily be a glue. Example adhesives can include an area treated to encourage cell attachment, a substance that encourages cell attachment, tethered ligands or other adhesive motifs that can attach to tissues or cells, an energetically modified surface (e.g., any surface of the microenvironment units, channels, chambers, trapping features, etc.) that encourages adhesion of cells or tissues, or any other substance that encourages attachment of cells or tissues. Thus, adhesives can be substances other than glues, and can encompass features of the surfaces of the microenvironment chamber that encourage cell attachment. 
     The microenvironment unit  102  can include a wall  430  enclosing the basal channel  202  and a wall  440  enclosing the apical channel  212 . Because the membrane  402  separates the basal channel  202  from the apical channel  212 , the wall  430  can serve as a floor of the basal channel  202  and the wall  440  can serve as a ceiling of the apical channel  212 . Likewise, the membrane  402  can serve as a ceiling of the basal channel  202  and can serve as a floor of the apical channel  212 . The microenvironment unit  102  can also include other walls, such as the sidewalls  432  and  434  that enclose the basal channel basal channel  202  as shown in  FIG. 4B . These sidewalls are not visible in the cross-sectional view of  FIG. 4A . 
     A tissue sample  450  has been introduced into the microenvironment unit  102  in the example shown in  FIG. 4A . The tissue sample  450  can be a group of cultured cells or spheroids, or a tissue fragment. The tissue sample  450  can be human or animal in origin, and may be created from a syngeneic process, a cell line or primary cell source, or may be excised from an animal or human and introduced into the microenvironment unit  102 . In some implementations, the microenvironment unit  102  can be used as a testing mechanism for oncology applications. Thus, the tissue sample  450  may be derived from a tumor. In some other implementations, the tissue sample  450  can be a tissue sample related to or derived from a liver, a kidney, a gut, a lung, a vascular system, a bone, skin, a brain, a connective tissue, a neural or sensory tissue, or another organ from a human or an animal. 
     The microenvironment unit  102  also includes a mechanical trapping feature configured to secure the tissue sample  450  in place within the basal channel  202  of the microenvironment unit  102 . In this example, the mechanical trapping feature is formed from portions of the sidewalls  432  and  434  of the basal channel  202  that taper inwards to reduce a cross-sectional area of the basal channel  202  in the vicinity of the tapered portions. The tapered sidewalls  432  and  434  are depicted in  FIG. 4B . Because of the reduction in cross-sectional area of the basal channel  202  due the tapered sidewalls  432  and  434 , the tissue sample  450  becomes trapped as it flows down basal channel  202 . As a result, the tissue sample  450  is held in place by the tapered sidewalls  432  and  434 . 
     While the tissue sample  450  is trapped by the tapered sidewalls  432  and  434 , space remains above and below the tissue sample  450  to provide a fluid path through the basal channel  202 . Thus, fluid can continue to flow through the microenvironment unit  102  while the tissue sample  450  is trapped. The position at which the tissue sample  450  is trapped can differ from typical cell culture applications in multiwell plates in which the cells are cultured either on the bottom surface of the well or on a Transwell-type membrane that is placed in the well. In this disclosure, the tissue sample  450  can be introduced into a well of the multiwell plate and can become fixed or trapped at a location within the microenvironment unit  102  so that it can be monitored over time while it is perfused with fluid. 
     The tapered sidewalls  432  and  343  can hold the tissue sample  450  in place and can also permit flow toward, through, and around the tissue sample  450 , so that an undue pressure does not build up proximal to the tissue sample  450 . The size of the tissue sample  450  as well as the degree of taper for the sidewalls  432  and  434  (e.g., the total reduction in cross sectional area of the basal channel  202  due to the tapered sidewalls  432  and  434 ) can be selected to achieve either or both of a predetermined rate of fluid flow or a predetermined pressure build up behind the tissue sample  450 . For example, if the tissue sample  450  blocks only a relatively small fraction of the cross-sectional area of the basal channel  202 , then most of the flow may not interact with the tissue sample  450 . On the other hand, if the tissue sample  450  blocks a relatively large portion of the basal channel  202 , then little or no fluid may be able to pass through the basal channel  202  and a large pressure may build up behind the tissue sample  450 . Therefore, a balance can be struck between the ability to maintain reasonable pressures and the need to improve the interaction potential of media components flowing through the basal channel  202  and the tissue sample  450 . In some implementations, the tissue sample  450  may obstruct about 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, or about 90% of the basal channel  202 . 
     In some implementations, the fluid flowing through either or both of the basal channel  202  and the apical channel  212  may include cells that have been introduced into the fluid samples. Such cells may flow toward, through, and past the tissue sample  450 . While many disease models and drug discovery models may require only that drugs introduced into the media interact with the various cell types contained within a captured tissue construct, other applications may benefit from having interactions between mobile cells traversing the microenvironment and the captured tissue sample  450 , in the absence or presence of drugs introduced in the media. Thus, in some implementations, cells such as lymphocytes obtained from tissue fragments from the same tissue source as the tissue sample  450  can be introduced into the flow stream, so that interactions between those lymphocytes and the trapped tissue sample  450  can be observed over time. In some implementations, such lymphocytes could be obtained from digestion of a sample that is then minced to provide the tissue sample  450 , and the lymphocytes could be activated, expanded, and treated with drugs prior to introduction into the system. In addition to lymphocytes (or as an alternative), other immune cells such as dendritic cells, or organ-specific cells or cancer cells could be introduced into either or both of the basal channel  202  and the apical channel  212  for observation of interactions with captured tissue sample  450 . 
     In some implementations, cells can be introduced into the apical channel  212 . These cells can then flow via pumping action across the apical channel  212  and then make their way through the membrane  402  to reach the tissue sample  450  on the basal side of the membrane  402  in the basal channel  202 . To facilitate this, the membrane  402  can have a pore size and a pore density selected to permit such transport, and the membrane crossing may be guided or driven by a cytokine gradient generated by the tissue sample  450  or by a separately controlled mechanism. In some implementations, if the cells typically migrate via crawling or rolling action through tissue sample  450 , then the tissue sample  450  may be positioned proximal to the basal surface of the membrane so that cells are not forced to “swim” through media to reach the tissue sample  450  when they pass through the membrane. 
     The properties of the membrane  402  in device can be guided by the requirements of a particular application. The pore size, pore density, surface properties, thickness and hydraulic permeability of the membrane may be tuned for specific applications, for example either to exclude cell transport or control the transit of large molecules. In some implementations, the properties of the membrane  402  can be selected to permit wide latitude in membrane crossing. The membrane  402  can be a commercially available item that is incorporated into the microenvironment unit  102  without modification. In some other implementations, the membrane  402  can be functionalized by molecules or gels. Such a gel may be integrated onto either or both of the basal or apical sides of the membrane  402 , or into either or both of the basal channel  202  or the apical channel  212  (e.g., either away from or adjacent to the membrane  402 ). Such a gel can also be positioned adjacent to the tissue sample  450  (e.g., on the sidewalls  432  and  434 ) or embedded into the tissue sample  450  itself. 
     In some implementations, the membrane  402  can be or can include any of a membrane, a filter, a mesh, or other substance that allows liquid to be forced through it while trapping cells on it so they can adhere, spread and grow. Pore sizes, mesh spacing, or general transport properties of the membrane  402  can be adjusted to control the relation between cell attachment, fluid flow, and pressure driving the fluid flow. In some implementations, the membrane  402  can be designed for a desired hydraulic resistance along with cell attachment properties. In some implementations, the membrane  402  can also be a porous mesh, gel, or other material that allows preferential transport of liquid through it while limiting cell transport through it. The membrane  402  can be embossed, etched, laser machined, mechanically machined, ablated or otherwise patterned with mechanical surface features to influence cell attachment, adhesion, spreading, or other cell properties. The membrane  402  can also be coated, energetically treated via a plasma or other means, affixed with a self-assembled monolayer, surface deposited, or otherwise modified chemically to have chemical surface features to influence cell attachment, adhesion, spreading, or other cell properties. In some implementations, the membrane  402  can have both mechanical and chemical surface features, with either or both such features placed on the scaffold in selected areas so that cell properties are modified within those areas. For example, some areas of the membrane  402  can have a chemical surface modification to limit cell attachment while others would have a mechanical surface modification to encourage cell attachment. 
     In some implementations, the tissue sample  450  may be introduced and fixed in a mechanical trapping feature (e.g., the tapered portion of the sidewalls  432  and  434  in the basal channel  202 , and this trapping event may fix the tissue sample  450  horizontally (between the sidewalls  432  and  434 ) and vertically (between the floor  430  and the membrane  402  that serves as a ceiling) at a predetermined location. In some implementations, the mechanical trapping feature can be coated with an adhesive, as described herein above. An adhesive coating can secure the tissue sample to the mechanical tissue trapping feature, even when fluid flows over the tissue sample at speeds that would otherwise dislodge or move the tissue sample  450 . 
     In some implementations, the size and shape of the mechanical feature that traps the tissue sample  450  (in this example, the sidewalls  432  and  434 ) can have a geometry selected based on such a predetermined location. For example, the left sidewall  432  can protrude into the basal channel  202  a greater distance than the right sidewall  434  in order to hold the tissue sample  450  at a position to the right of the horizontal center of the basal channel  202 , or the right sidewall  434  can protrude into the basal channel  202  a greater distance than the left sidewall  432  in order to hold the tissue sample  450  at a position to the left of the horizontal center of the basal channel  202 . Likewise, the vertical position of the tapered portions of the sidewalls  432  and  434  can be selected to achieve a desired vertical position of the tissue sample  450 . In the example shown in  FIG. 4B , the sidewalls  432  and  434  are positioned at the vertical center of the basal channel  202 . However, in other implementations the sidewalls  432  and  434  could offset from the vertical center of the basal channel  202  in order to trap the tissue sample  450  at the offset position. 
     In some implementations, it may be desirable for the microenvironment unit  102  to allow for the ability to perform imaging (e.g., high-resolution imaging) of the tissue sample  450  in real-time, and therefore an unobstructed optical view into the basal channel  202  can be useful. In some implementations, for example in applications in which drugs and small molecule media components are introduced but no flow of cells is added, the tissue sample  450  may be trapped anywhere within the basal flow and chamber. However, if cells are introduced into the basal chamber, then the mechanical trapping features can be selected to secure the tissue sample  450  in a position where cell-tissue interactions are likely. In some implementations, at least a portion of the microenvironment unit  102 , such as the floor  430  of the basal channel  202 , can be formed from an optically transparent material, to permit observation of the tissue sample  450  from outside the microenvironment unit  102 . In some implementations, an additional optical layer (e.g., a layer of glass or transparent polymer) can be attached to a portion of the microenvironment unit  102 . For example, an optical layer can be secured to an exterior surface of the floor  430  of the basal channel  202  to permit optical observation (e.g., via a microscope) of the tissue sample  450  from outside the microenvironment unit  102 . 
     The microenvironment unit  102  includes a mechanical tissue trapping feature, which in the example of  FIGS. 4A and 4B  includes a tapered portion of the sidewalls  432  and  434  of the basal channel  202 . However, in some other implementations, additional or different mechanical trapping features can be used. For example, a mechanical trapping features can be features that are capable of being introduced into a multiwell plate such as the multiwell plate  100  of  FIG. 1 , rather than simply in a channel. Such feature can include channel or chamber constrictions from the sides, similar to the tapered portions of the sidewalls  432  and  434  shown in  FIG. 4B . Such constrictions can be either symmetric or asymmetric in nature (e.g., with respect to a longitudinal axis of the basal channel  202 , or with respect to a vertical height of the basal channel  202 ). In some implementations, such constrictions can be smooth and gradual. In some other implementations, the constrictions can contain sharp or angular features. 
     In addition to these constrictions, or alternatively, other structural components can be added to serve as mechanical trapping features. For example, there may be vertical barriers that reduce the effective channel depth in the region where the tissue sample  450  may become trapped. These features can include ramps, steps, or bumps that may be smooth or sharp, and may be symmetric or asymmetric. In general, such features can be selected to effectively trap and hold the tissue sample  450  while permitting flow past the tissue sample  450 . Examples of such alternative mechanical trapping features are described further below. 
       FIG. 5  shows a perspective view  500  of a portion of a channel  502  having a mechanical trapping feature. In some implementations, the channel  502  can be similar to the basal channel  202  shown in  FIGS. 4A and 4B . For example, the portion of the channel  502  shown in  FIG. 5  can serve as a floor or a sidewall of the basal channel  202  of a microenvironment unit  102 . The channel  502  also includes a mechanical trapping feature that can help to secure a tissue sample, similar to the tissue sample  450  shown in  FIG. 4B , within the channel  502 . In this example, the mechanical trapping feature includes a series of raised surfaces  510  positioned along the channel  502 . 
     The channel  502  includes five raised surfaces  510 . The raised surfaces  510  are arranged in a V-shaped pattern along the channel  502 . Each raised surface  510  extends away from the surface of the channel  502 . As a result, the presence of the raised surfaces  510  can reduce a cross-sectional area of the channel  502  in the area in which the raised surfaces  510  are located. As described above, such a reduction of cross-sectional area can case a tissue fragment similar to the tissue sample  450  of  FIG. 4B  to become trapped in place by the raised surfaces  510  within the channel  502 , while still permitting fluid to flow towards, around, and through the tissue fragment in the channel  502 . Thus, the fluid sample can interact with the tissue fragment, and the results of the interaction can be observed. 
       FIG. 5  illustrates a longitudinal axis of the channel  502 , represented by the broken line  520 . A fluid sample in the in the channel  502  can flow in the direction of the arrow. As illustrated, the raised surfaces  510  can be arranged in a manner that exhibits symmetry with respect to the longitudinal axis  520  of the channel  502 . For example, one of the raised surfaces  510  is positioned along the longitudinal axis  520 , and two raised surfaces  510  are positioned symmetrically on either side of the longitudinal axis  520 . In some other implementations, the raised surfaces  510  may not exhibit symmetry with respect to the longitudinal axis  520 . 
     In some implementations, the raised surfaces  510  may extend upward from the surface of the channel  502  (e.g., a floor of the channel  502 ) towards an opposing surface (e.g., a ceiling of the channel  502 ). In some implementations, the raised surfaces  510  may not extend all the way to the opposing surface. Stated differently, the raised surfaces  510  may extend a distance that is less than a height of the channel  502 . In some implementations, the raised surfaces  510  may extend 1%, 2%, 3%, 4%, or 5% of the distance towards the opposing surface of the channel  502 . In some implementations, the raised surfaces  510  may extend 10%, 20%, 30%, 40%, or 50%, 60%, 70%, 80%, or 90% of the distance towards the opposing surface of the channel  502 . The raised surfaces  510  may have different heights, or each may be the same height. In addition, the cross-sectional shapes of the raised surfaces  510  may be circular as depicted in  FIG. 5 , or may be another shape. In some implementations, the cross-sectional shape may vary across the raised surfaces  510 . Other examples of shapes and arrangements for raised surfaces that can form a mechanical trapping feature are described further below. 
       FIG. 6  shows another perspective view  600  of a portion of a channel  602  having a mechanical trapping feature. In some implementations, the channel  602  can be similar to the basal channel  202  shown in  FIGS. 4A and 4B . For example, the portion of the channel  602  shown in  FIG. 6  can serve as a floor or a sidewall of the basal channel  202  of a microenvironment unit  102 . The channel  602  also includes a mechanical trapping feature that can help to secure a tissue sample, similar to the tissue sample  450  shown in  FIG. 4B , within the channel  602 . In this example, the mechanical trapping feature includes a series of raised surfaces  610  positioned along the channel  602 . 
     The channel  602  includes five raised surfaces  610 . The raised surfaces  610  are arranged in a V-shaped pattern along the channel  602 , similar to the V-shaped patter of the raised surfaces  510  shown in  FIG. 5 . Each raised surface  610  extends away from the surface of the channel  602  to reduce a cross-sectional area of the channel  602  in the area in which the raised surfaces  610  are located, which can help to trap a tissue fragment similar to the tissue sample  450  of  FIG. 4B . Unlike the raised surfaces  510  of  FIG. 5 , the raised surfaces  610  of  FIG. 6  have an elongated oval shape, rather than a circular shape. Thus, the raised surfaces  610  include curved edges. In some implementations, the raised surfaces  610  can be elongated in the direction of fluid flow through the channel  602 . In some other implementations, the raised surfaces  610  can be elongated in a different direction, such as a direction perpendicular to the direction of fluid flow in the channel  602 . 
       FIG. 7  shows another perspective view  700  of a portion of a channel  702  having a mechanical trapping feature. In some implementations, the channel  702  can be similar to the basal channel  202  shown in  FIGS. 4A and 4B . For example, the portion of the channel  702  shown in  FIG. 7  can serve as a floor or a sidewall of the basal channel  202  of a microenvironment unit  102 . The channel  702  also includes a mechanical trapping feature that can help to secure a tissue sample, similar to the tissue sample  450  shown in  FIG. 4B , within the channel  702 . In this example, the mechanical trapping feature includes a raised  710  positioned along the channel  702 . 
     Instead of a series of raised surfaces arranged in a V-shaped pattern, as shown in  FIGS. 5 and 6 , the channel  702  of  FIG. 7  includes a single raised surface  710  that forms a V-shaped pattern. The raised surface  710  extends away from the surface of the channel  702  to reduce a cross-sectional area of the channel  702  in the area in which the raised surface  710  is located, which can help to trap a tissue fragment similar to the tissue sample  450  of  FIG. 4B . The raised surface  710  includes curved edges as well as straight edges and sharp angles. The raised surface  710  is arranged approximately in the center of the channel  702  and exhibits symmetry about the longitudinal axis of the channel  702 . However, in other implementations, the shape, position, and orientation of the raised surface  710  can be different from that depicted in  FIG. 7 . For example, the raised surface  710  may have a first side that is longer than a second side, and may be located at a position offset from the center of the channel  702 . 
       FIG. 8  shows another perspective view  800  of a portion of a channel  802  having a mechanical trapping feature. In some implementations, the channel  802  can be similar to the basal channel  202  shown in  FIGS. 4A and 4B . For example, the portion of the channel  802  shown in  FIG. 8  can serve as a floor or a sidewall of the basal channel  202  of a microenvironment unit  102 . The channel  802  also includes a mechanical trapping feature that can help to secure a tissue sample, similar to the tissue sample  450  shown in  FIG. 4B , within the channel  802 . In this example, the mechanical trapping feature includes two raised surfaces  810  positioned along the channel  802 . 
     The raised surfaces  810  are arranged in a semicircular pattern along the channel  802 , rather than the V-shaped pattern of the raised surfaces shown in  FIGS. 5-7 . Each raised surface  810  extends away from the surface of the channel  802  to reduce a cross-sectional area of the channel  802  in the area in which the raised surfaces  810  are located, which can help to trap a tissue fragment similar to the tissue sample  450  of  FIG. 4B . Unlike the raised surfaces  510  of  FIG. 5 , the raised surfaces  810  of  FIG. 8  have an elongated curved shape and are arranged to form a semicircular pattern. The raised surfaces  810  are arranged approximately in the center of the channel  802  and are mirror images of one another, so that they exhibit symmetry about the longitudinal axis of the channel  802 . However, in other implementations, the shape, position, and orientation of the raised surfaces  810  can be different from that depicted in  FIG. 8 . For example, one of the raised surfaces  810  may be longer than the other, and the raised surfaces  810  may not be centered within the channel  802 . 
       FIG. 9A  shows another perspective view  900  of a portion of a channel  902  having a mechanical trapping feature. In some implementations, the channel  902  can be similar to the basal channel  202  shown in  FIGS. 4A and 4B . For example, the portion of the channel  902  shown in  FIG. 9A  can serve as a floor or a sidewall of the basal channel  202  of a microenvironment unit  102 . The channel  902  also includes a mechanical trapping feature that can help to secure a tissue sample, similar to the tissue sample  450  shown in  FIG. 4B , within the channel  802 . In this example, the mechanical trapping feature includes a series of raised steps  910  along the channel  902 . 
     The raised steps  910  include a first step and a second step. The second step can be adjacent to and downstream from the first step. The second step also can rise to a greater height than the first step. Together, the raised steps  910  can gradually reduce a cross-sectional area of the channel  902  in the area in which the raised steps  910  are located, which can help to trap a tissue fragment similar to the tissue sample  450  of  FIG. 4B . Unlike the raised surfaces  510  of  FIG. 5 , the raised surfaces  810  of  FIG. 8  have an elongated curved shape and are arranged to form a semicircular pattern. The raised surfaces  810  are arranged approximately in the center of the channel  802  and are mirror images of one another, so that they exhibit symmetry about the longitudinal axis of the channel  802 . However, in other implementations, the shape, position, and orientation of the raised surfaces  810  can be different from that depicted in  FIG. 8 . For example, one of the raised surfaces  810  may be longer than the other, and the raised surfaces  810  may not be centered within the channel  802 . 
       FIG. 9B  shows another perspective view  915  of a portion of the channel  902  having steps  920  that form a mechanical trapping feature. The steps  920  differ from the steps  910  shown in  FIG. 1  in size and number. Specifically, there are four steps  920  shown in  FIG. 9B , and each of the steps  920  has a length that is shorter than the lengths of the steps  910  shown in  FIG. 9A . Similarly,  FIG. 9C  shows another perspective view  915  of a portion of the channel  902  having steps  930  that form a mechanical trapping feature. Unlike the steps  910  and  920  of  FIGS. 9A and 9B , the steps  930  include a total of three steps.  FIG. 9D  shows a photograph of a set of steps  940  similar to the steps  930  shown in  FIG. 9C . 
       FIG. 10  shows another perspective view  1000  of a portion of a channel  1002  having a mechanical trapping feature. In some implementations, the channel  1002  can be similar to the basal channel  202  shown in  FIGS. 4A and 4B . For example, the portion of the channel  1002  shown in  FIG. 10  can serve as a floor or a sidewall of the basal channel  202  of a microenvironment unit  102 . The channel  1002  also includes a mechanical trapping feature that can help to secure a tissue sample, similar to the tissue sample  450  shown in  FIG. 4B , within the channel  1002 . In this example, the mechanical trapping feature includes two raised surfaces  1010  positioned along the channel  1002 . 
     The raised surfaces  1010  are arranged adjacent to one another across the channel  1002  with a small gap separating them. Each raised surface  1010  extends away from the surface of the channel  1002  to reduce a cross-sectional area of the channel  1002  in the area in which the raised surfaces  1010  are located, which can help to trap a tissue fragment similar to the tissue sample  450  of  FIG. 4B . In some implementations, the gap between the raised surfaces  1010  can be positioned along a central or longitudinal axis of the channel  1002 . Thus, fluid can flow through the gap between the raised surfaces  1010  and beneath the trapped tissue sample, thereby allowing a portion of the fluid to interact with a portion of a bottom surface of the trapped tissue sample. 
       FIG. 11  shows another perspective view  1100  of a portion of a channel  1102  having a mechanical trapping feature. In some implementations, the channel  1102  can be similar to the basal channel  202  shown in  FIGS. 4A and 4B . For example, the portion of the channel  1102  shown in  FIG. 11  can serve as a floor or a sidewall of the basal channel  202  of a microenvironment unit  102 . The channel  1102  also includes a mechanical trapping feature that can help to secure a tissue sample, similar to the tissue sample  450  shown in  FIG. 4B , within the channel  1102 . In this example, the mechanical trapping feature includes a single raised surface  1110  positioned along the channel  1102 . The raised surface  1110  extends away from the surface of the channel  1102  to reduce a cross-sectional area of the channel  1102  in the area in which the raised surface  1110  is located, which can help to trap a tissue fragment similar to the tissue sample  450  of  FIG. 4B . 
     It should be understood that the examples of mechanical trapping features provided in connection with  FIGS. 4A, 4B, 5-8, 9A-9D, 10, and 11  are illustrative only. Other arrangements may also be possible without departing from the scope of this disclosure. For example, while mechanical trapping features have been primarily described as being positioned in the basal compartment of a microenvironment unit, in some implementations mechanical trapping features may be included alternatively or additionally in an apical compartment of a microenvironment unit. In addition, any of the mechanical trapping features described in this disclosure may be used in connection with one another in the same microenvironment unit. For example, a microenvironment unit may include raised surfaces, as well as tapered channel walls, which together can help to secure a tissue sample in place. Furthermore, it should be understood that in a multiwell plate that includes multiple microenvironment units, the mechanical trapping features can be the same or different (e.g., with respect to position, geometry, etc.) across the microenvironment units of the plate. In some implementations, at least some of the microenvironment units may not include any mechanical trapping features. 
     In some implementations, a microenvironment unit may not include a basal compartment and an apical compartment separated by a membrane, but may instead include a single layer defining a channel, compartment, or chamber. It should be understood that any of the examples of mechanical trapping features of  FIGS. 4A, 4B, 5-8, 9A-9D, 10, and 11  may also be used in such a single-layer microenvironment unit. In some implementations, other mechanical trapping features can also be incorporated into a channel or compartment configured to receive a tissue sample via a dedicated tissue fragment delivery port above the channel or compartment. Examples of such features are shown and described further below in connection with  FIGS. 12A, 12B, 13A, and 13B . 
     Referring now to  FIG. 12A , a cross-sectional view  1200  of a portion of a channel  1202  having a mechanical trapping feature is shown. In some implementations, the channel  1202  can be similar to the basal channel  202  shown in  FIGS. 4A and 4B . In some other implementations, the channel  1202  can be part of a single-layer microenvironment unit. The channel  1202  includes a mechanical trapping feature that can help to secure a tissue sample  1250 , which can be similar to the tissue sample  450  shown in  FIG. 4B , within the channel  1202 . In this example, the mechanical trapping feature includes a series of raised vertical posts  1210  within the channel  1202  that surround a tissue trapping area to secure the tissue sample  1250  within the tissue trapping area. For illustrative clarity, not every post  1210  is labeled with a reference numeral in  FIGS. 12A and 12B . The raised posts  1210  extend vertically between a floor and a ceiling of the channel  1202  to prevent the tissue sample  1250  from moving out of the tissue trapping area when fluid is flowed through the channel  1202 . In the example of  FIG. 12A , fluid can flow left to right in the channel  1202  in the direction shown by the arrows. Because of the spaces between the posts  1210 , fluid can be permitted to flow through the tissue trapping area in the channel  1202 , which allows the fluid to interact with the tissue sample  1250 . In some implementations, the posts  1210  can have thin profiles, as shown, which may allow for improved fluid flow characteristics in the channel  1202  relative to other mechanical trapping features, such as constructions of the channel  1202 . 
       FIG. 12B  shows a top down view  1230  of the portion of the channel  1202  shown in  FIG. 12A . As shown, the mechanical trapping feature can include six posts  1210 . However it should be understood that the precise number, shape, and arrangement of the posts  1210  shown in  FIGS. 12A and 12B  is illustrative only. For example, while  FIG. 12B  shows six posts  1210  arranged in a radially symmetric configuration, the mechanical trapping feature could instead include more or fewer posts  1210  than depicted, and the arrangement of the posts  1210  could be altered as well. In some implementations, the spacing between the posts  1210  can be selected to be smaller than a diameter of the tissue sample  1250 . Such a spacing can help to ensure that the tissue sample  1250  is kept within the tissue trapping area defined by the posts  1210 , and is not permitted to escape by moving between adjacent posts  1210 . In some implementations, the posts  1210  can have circular cross-sectional shapes as depicted. In some other implementations, the posts  1210  can have other shapes, such as triangular, rectangular, hexagonal, or other geometric cross-sectional shapes. 
     Referring again to  FIG. 12A , the posts  1210  can extend between a floor and a ceiling of the channel  1202 . In some implementations, the posts  1210  may be mechanically coupled with either or both of the floor or the ceiling of the channel  1202 . In some implementations, the posts  1210  may not extend the full distance between the ceiling and the floor of the channel  1202 . For example, the posts  1210  can be coupled with the floor of the channel  1202 , but may not extend all the way to the ceiling of the channel  1202 , such that there is a gap between an end of the posts  1210  and the ceiling of the channel  1202 . Likewise, in some implementations, the posts  1210  can be coupled with the ceiling of the channel  1202 , but may not extend all the way to the floor of the channel  1202 , such that there is a gap between an end of the posts  1210  and the floor of the channel  1202 . In some implementations, some of the posts  1210  can be coupled with the floor of the channel  1202  while other posts  1210  can be coupled with the ceiling. 
     The channel  1202  can be coupled with a port  1215 , as shown in  FIG. 12A . The port  1215  can be used to introduce the tissue sample  1250  into the channel  1202 . Thus, the tissue sample  1250  is introduced from above the channel  1202 , rather than into an inlet of the channel  1202  in which fluid flow through the channel  1202  carries the tissue sample toward the mechanical trapping feature. In some implementations, the port  1215  can be similar to one of the ports  304  of the multiwell plate  300  shown in  FIG. 3 . The port  1215  can also serve as an access point for retrieving the tissue sample  1250  and removing it from the channel  1202 . In some implementations, the channel  1202  can include a ceiling feature that covers at least a portion of the channel  1202  underneath the port  1215  after the tissue sample  1250  has been introduced into the channel  1202 . For example, such a ceiling feature can serve to prevent fluid from leaking out of the channel  1202  through the port  1215  during operation of the device. In some implementations, such a ceiling may be configured to be removed to allow the tissue sample  1250  to be retrieved from the channel  1202  via the port  1215  at a later time. 
     Referring now to  FIG. 13A , a cross-sectional view  1300  of a portion of a channel  1302  having a mechanical trapping feature is shown. In some implementations, the channel  1302  can be similar to the basal channel  202  shown in  FIGS. 4A and 4B . In some other implementations, the channel  1302  can be part of a single-layer microenvironment unit. The channel  1302  includes a mechanical trapping feature that can help to secure a tissue sample  1350 , which can be similar to the tissue sample  450  shown in  FIG. 4B , within the channel  1302 . In this example, the mechanical trapping feature includes a series of horizontal structures  1310  within the channel  1302  that surround a tissue trapping area to secure the tissue sample  1350  within the tissue trapping area. For illustrative clarity, not every horizontal structure  1310  is labeled with a reference numeral in  FIGS. 13A and 13B . Each horizontal structure  1310  extends inward from a sidewall of the channel  1302 , rather than vertically between a floor and a ceiling of the channel  1302 , as depicted in  FIGS. 12A and 12B . Together, the horizontal structures  1310  can prevent the tissue sample  1350  from moving out of the tissue trapping area when fluid is flowed through the channel  1302 . In the example of  FIG. 13A , fluid can flow left to right in the channel  1302  in the direction shown by the arrows. Because of the spaces between the horizontal structures  1310 , fluid can be permitted to flow through the tissue trapping area in the channel  1302 , which allows the fluid to interact with the tissue sample  1350 . In some implementations, the horizontal structures  1310  can have thin profiles, as shown, which may allow for improved fluid flow characteristics in the channel  1302  relative to other mechanical trapping features, such as constructions of the channel  1302 . 
       FIG. 13B  shows a top down view  1330  of the portion of the channel  1302  shown in  FIG. 13A . As shown, the mechanical trapping feature can include six sets of horizontal structures  1310  arranged in columns, and each column can include four horizontal structures  1310 . However it should be understood that the precise number, shape, and arrangement of the horizontal structures  1310  shown in  FIGS. 13A and 13B  is illustrative only. For example, while  FIG. 13B  shows six sets of horizontal structures  1310  arranged in a radially symmetric configuration, the mechanical trapping feature could instead include more or fewer sets of horizontal structures  1310  than depicted, and the arrangement of the horizontal structures  1310  could be altered as well. In some implementations, the spacing between the horizontal structures  1310  can be selected to be smaller than a diameter of the tissue sample  1350 . Such a spacing can help to ensure that the tissue sample  1350  is kept within the tissue trapping area defined by the horizontal structures  1310 , and is not permitted to escape by moving between adjacent horizontal structures  1310 . In some implementations, the horizontal structures  1310  can have rectangular cross-sectional shapes as depicted. In some other implementations, the horizontal structures  1310  can have other shapes, such as circular, triangular, hexagonal, or other geometric cross-sectional shapes. 
     The horizontal structures  1310  can project inwards toward an interior of the channel  1302  from a sidewall of the channel  1302 . In some implementations, the horizontal structures  1310  may be mechanically coupled with the sidewall of the channel  1302 . In some implementations, opposing horizontal structures  1310  do not extend to span the full distance between their respective sidewalls of the channel  1302 . As a result, a gap remains in the interior space defined by the ends of the horizontal structures  1310 , and this gap can define the tissue trapping area in which the tissue sample  1350  becomes trapped. Thus, in some implementations, each horizontal structure can extend less than 50% of the distance between the opposing sidewalls of the channel  1302 . For example, in some implementations each horizontal structure  1310  can extend 10%, 20%, 30%, or 40% of the distance between the opposing sidewalls of the channel  1302 . In some implementations, some of the horizontal structures  1310  can have different lengths than others. The shape, size, and arrangement of the horizontal structures  1310  need not be uniform as shown in  FIGS. 13A and 13B . 
     The channel  1302  can be coupled with a port  1315 , as shown in  FIG. 13A . The port  1315  can be used to introduce the tissue sample  1350  into the channel  1302 . Thus, the tissue sample  1350  is introduced from above the channel  1302 , rather than into an inlet of the channel  1302  in which fluid flow through the channel  1302  carries the tissue sample toward the mechanical trapping feature. In some implementations, the port  1315  can be similar to one of the ports  304  of the multiwell plate  300  shown in  FIG. 3 . The port  1315  can also serve as an access point for retrieving the tissue sample  1350  and removing it from the channel  1302 . In some implementations, the channel  1302  can include a ceiling feature that covers at least a portion of the channel  1302  underneath the port  1315  after the tissue sample  1350  has been introduced into the channel  1302 . For example, such a ceiling feature can serve to prevent fluid from leaking out of the channel  1302  through the port  1315  during operation of the device. In some implementations, such a ceiling may be configured to be removed to allow the tissue sample  1350  to be retrieved from the channel  1302  via the port  1315  at a later time. 
       FIG. 14  shows a cross-sectional view  1400  of a portion of a channel  1402  having a mechanical trapping feature. In some implementations, the channel  1402  can be similar to the basal channel  202  shown in  FIGS. 4A and 4B . In some other implementations, the channel  1402  can be part of a single-layer microenvironment unit. The channel  1402  includes a mechanical trapping feature that can help to secure a tissue sample similar to the tissue sample  450  shown in  FIG. 4B , within the channel  1402 . In this example, the mechanical trapping feature includes a tissue trapping region  1420  having a geometry selected to trap the tissue sample with a tissue trap  1410  as the tissue moves from an upstream portion of the channel  1402  (e.g., the left-hand side of the channel  1402  shown in  FIG. 14 ) toward a downstream portion of the channel  1402  (e.g., the right-hand side of the channel  1402  shown in  FIG. 14 ). The tissue trapping region  1420  also includes two branch channels  1440   a  and  1440   b  branching off from the upstream portion of the channel  1402  in opposing directions at a junction near the tissue trap  1410 . 
     The tissue trapping region  1420  is configured to trap a tissue sample in a fixed location while a fluid sample is flowed through the channel  1402 . For example, in some implementations, the tissue trapping region  1420  is shaped such that, when the fluid sample flows through the channel  1402 , a stagnation zone exists in at least a portion of the area of the tissue trap  1410 , causing the tissue sample to become trapped in the tissue trap  1410 . In operation, the tissue sample can become trapped in the tissue trap  1410  in a manner that allows the fluid sample to continue flowing through the upstream portion of the channel  1402  to the branch channels  1440   a  and  1440   b , while a portion of the fluid sample contacts the tissue sample in the tissue trap  1410  as it flows. 
     In some implementations, the tissue trap or trapping zone  1410  can have a bottom wall that is positioned at a lower depth than the bottom of the upstream portion of the channel  1402  that leads up to it. That is, the tissue trap  1410  can be stepped down relative to the bottom surface of the upstream portion of the channel  1402 . Thus, the tissue trap  1410  can serve as a pocket for catching, trapping, holding, immobilizing, or securing the tissue sample. In some implementations, the shape of the tissue trapping region  1420 , including the tissue trap  1410 , is selected to catch or otherwise facilitate trapping of the tissue sample while the fluid sample passes through the channel  1402 . For example, the tissue trap  1410  can have a diameter that is larger than that of the inlet upstream portion of the channel  1402 . In some implementations, the tissue trap  1410  can have a diameter that is about twice that of the upstream portion of the channel  1402  that leads up to the tissue trap  1410 . 
     The trapping of the tissue sample in a manner that allows the fluid sample to continue flowing through the device while contacting the tissue sample can allow the interactions between the tissue sample and agents within the fluid sample to be evaluated, as described above. For example, in some implementations fluorescent materials can be added to either the fluid sample or the tissue sample, and the visual characteristics of the tissue sample and the fluid sample can be observed over time. To facilitate such observation, the microfluidic device that includes the channel  1402  can be formed from a material that is transparent and optically clear, at least in the region of the device near the tissue trap  1410 . This area can serve as an optical interface that can be examined by an optical instrument, such as a camera or a microscope, which is brought into proximity with the microfluidic device. It should be understood that such an optical interface can also be included in microfluidic devices that include any of the mechanical trapping features described above to allow observation of a tissue sample that is trapped by the mechanical trapping features. 
       FIG. 15  shows a flowchart of a method  1500  for evaluating an interaction between a tissue sample and a fluid sample. The method  1500  can include introducing tissue samples into microenvironment units of a microfluidic device (BLOCK  1505 ). In some implementations, the microfluidic device can be or can include a multiwell plate, similar to the multiwell plate  100  shown in  FIG. 1 . Each well can be coupled with a microenvironment unit similar to the microenvironment unit  102 . In some implementations, introducing the tissue samples can include introducing a respective tissue sample into each microenvironment unit of the microfluidic device. 
     As described above, each microenvironment unit can include a basal compartment and an apical compartment. A membrane can separate the basal compartment from the apical compartment in each microenvironment unit. Each microenvironment unit can also include a mechanical trapping feature. For example, the mechanical trapping feature can be positioned within the basal compartment. In some implementations, the mechanical trapping feature can be defined by a portion of at least one of a sidewall or a floor of the basal compartment. The mechanical trapping feature can be configured to restrict movement of the tissue sample in the basal compartment and to allow fluid to flow past the tissue sample. For example, the mechanical trapping feature can be any of the mechanical trapping features shown and described in connection with  FIGS. 4A, 4B, 5-8, 9A-9D, 10, and 11 . In some implementations, the mechanical trapping feature can include a combination of any of those mechanical trapping features, which may be included together in a single microenvironment unit. The tissue samples can be introduced by injecting the tissue samples into the microenvironment units via respective wells. In some implementations, the tissue samples can include individual tumors, which may be either animal or human. Such tumors can be broken into fragments from which at least some tissue samples can be introduced into the wells of the multiwell plate system. Other tissue fragments may be used, for example, as a source for immune cells obtained via digestion of the sample. 
     The method  1500  can include controlling a plurality of micropumps to introduce fluid samples into the microfluidic device (BLOCK  1510 ). In some implementations, each of the micropumps can be coupled with a respective well of the plurality of wells of the microfluidic device. Thus, controlling the micropumps can allow for a respective fluid sample to be introduced into each respective well. In some implementations, each well can be fluidically coupled with at least one of the plurality of microenvironment units. In some implementations, the fluid samples can include therapeutic substances, such as candidate immunotherapies whose efficacies are to be evaluated. 
     In some implementations, the micropumps may be used to perfuse the trapped tissue samples with the candidate immunotherapies, which can afford a competitive advantage over traditional systems by significantly extending the duration of viable function of the tissue sample as compared to static systems. In addition, tissue samples perfused according to the techniques of this disclosure can be exposed to a dynamic microenvironment in which nutrients from flowing media are continually introduced to the tissue samples, waste products are continually removed, and the concentration of soluble factors and drugs can be controlled and maintained in a dynamic fashion. The pumping action of the micropumps can be made steady, or can be pulsed or varied in a time-dependent manner. Drug dosing and gradients of soluble factors and bioactive molecules can be controlled using micropumps in an individually targeted manner. In some implementations, individual micropumps can be used to address each well. In some other implementations, groups of wells can be addressed by a single micropump. In some implementations, the micropumps can be integrated pumps rather than external laboratory pumps that are used to drive flow through and past the tissue traps in each well. For example, the micropumps can be integrated with the multiwell plate. 
     In some implementations, the method  1500  can include controlling at least one micropump of the plurality of micropumps to introduce a second fluid sample into the apical compartment of the first microenvironment unit. For example, the second fluid sample can include a plurality of cells. In some implementations, controlling the plurality of micropumps can include controlling at least two of the micropumps independently from one another. Controlling the micropumps of the systems described can be performed using a controller, such as the controller described herein below. The controller can be communicatively coupled (e.g., via one or more electric traces, wires, or connections, etc.) to one or more of the components of the systems described herein, such as the system  1600 , the system  1700 , the system  1800 , or the system  1900 , among others. The controller can be communicatively coupled with one or more of the microfluidic pumps  1610 , the valves  1715   a  and  1715   b , the pump  1710 , the valve  1815   a  and a second valve  1815   b , the pressure sources of the system  1800 , the agitator  1910 , or any combination thereof, among others. The controller can transmit or provide one or more signals to the valves (e.g., the valves  1715   a  and  1715   b , the valve  1815   a , the second valve  1815   b , etc.) as described herein to cause the valves to open or close in accordance with the implementations described herein. The controller can transmit or provide signals to the pumps (e.g., each of the pumps  1610  individually, the pump  1710 , etc.) that cause the pumps to move fluid throughout the systems described herein. For example, the controller can cause the microfluidic pumps  1610  to either provide or remove fluid from one or more microenvironment units via a port or a sipper, as described herein. 
     The controller can be in communication with more than one component simultaneously, and can control each component independently from other components with which the controller communicates. For example, each of the pumps  1610  can be controlled individually to cause, for example, fluids to flow over a tissue sample that has been trapped in a trapping region in a microenvironment unit. Thus, the controller can transmit or provide independent signals to one or more of the components in the systems described herein, allowing the components of the systems to work independently to achieve a desired outcome. In some implementations, the controller can provide or transmit signals over one or more communication buses, such as a serial bus, a parallel bus, or any other type of communication bus. The processor of the controller can be communicatively coupled to one or more communication buses. The components of the systems described herein can be communicatively coupled to one or more of the communication buses, and which can transport signals from the processor of the controller to a respective component. For example, in a serial bus arrangement, the controller can communicate with a particular component by using an address value assigned to that component (e.g., each component on a bus can be assigned an address value, etc.). In a parallel bus arrangement, one or more transmission lines in the parallel bus can be communicatively coupled to a respective component. Thus, controlling the micropumps using the controller (e.g., via providing one or more instructions to the controller that cause the controller to provide signals to the pumps, etc.) can cause fluid to flow through the systems described herein in a desired manner. As described herein, the pumps can be coupled to one or more reservoirs or fluid sources, and thus can be used to provide fluids to other portions of the system, such as the microenvironment units described herein. 
     The method  1500  can include observing an interaction between the tissue sample of a first microenvironment unit and a corresponding fluid sample (BLOCK  1515 ). In some implementations, the corresponding fluid sample can be a fluid sample introduced into a first well of the plurality of wells that is coupled with the first microenvironment unit. In some implementations, observing the interaction can include optically imaging the interaction, either with the human eye or by using optical equipment such as a microscope. For example, in some implementations at least a portion of the multiwell plate can include a transparent material. Observing the interaction between the tissue sample of the first microenvironment unit and the fluid sample introduced into the first well can include positioning a lens of a microscope in proximity to the first microenvironment unit and observing the interaction through the transparent material. In some implementations, the transparent material can include an optical layer that is coupled with the multiwell plate. 
       FIG. 16  illustrates a sectional view of an example system  1600  for integrating micropumps with a microfluidic device. The system  1600  can include a housing  1615 . The housing  1615  can be an enclosure that surrounds or partially surrounds other components of the system  1600 . The system  1600  can include a multiwell plate  1605 . The well plate  1605  can include a plurality of wells, which may be interconnected by a network of channels within the well plate  1605 . In some implementations, the multiwell plate  1605  can be the same as or similar to the multiwell plate  100  shown in  FIG. 1 . 
     The system  1600  can also include a series of microfluidic pumps  1610  (sometimes referred to herein as “micropumps”). Each pump  1610  can be coupled with a respective port defined by the well plate  1605 . Thus, the pumps  1610  can control the introduction of fluid samples into the wells of the well plate  1605  via the ports with which the pumps  1610  are coupled. In some implementations, the system  1600  may also include additional or different components than those depicted in  FIG. 16 . For example, the system  1600  can include associated electronic components to control the pumps  1610 , such as a controller (not pictured). At least some of these electronic components can be enclosed within the housing  1615 . For example, the electronic components can be positioned above the pumps  1610  inside the housing  1615 . 
     The controller can be integrated with one or more of the systems described herein, including the system  1600 , the system  1700 , the system  1800 , or the system  1900 . The controller can include at least one processor and at least one memory or other computer-readable storage medium, e.g., a processing circuit. The memory can store processor-executable instructions that, when executed by the processor, cause the processor to perform one or more of the operations described herein. The processor may include a microprocessor, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), etc., or combinations thereof. The memory may include, but is not limited to, electronic, optical, magnetic, or any other storage or transmission device capable of providing the processor with program instructions. The memory may further include a floppy disk, CD-ROM, DVD, magnetic disk, memory chip, ASIC, FPGA, read-only memory (ROM), random-access memory (RAM), electrically erasable programmable ROM (EEPROM), erasable programmable ROM (EPROM), flash memory, optical media, or any other suitable memory from which the processor can read instructions. The instructions may include code from any suitable computer programming language. 
     The controller can be communicatively coupled (e.g., via one or more electric traces, wires, or connections, etc.) to one or more of the components of the systems described herein, such as the system  1600 , the system  1700 , the system  1800 , or the system  1900 , among others. The controller can be communicatively coupled with one or more of the microfluidic pumps  1610 , the valves  1715   a  and  1715   b , the pump  1710 , the valve  1815   a  and a second valve  1815   b , the pressure sources of the system  1800 , the agitator  1910 , or any combination thereof, among others. The controller can transmit or provide one or more signals to the valves (e.g., the valves  1715   a  and  1715   b , the valve  1815   a , the second valve  1815   b , etc.) as described herein to cause the valves to open or close in accordance with the implementations described herein. The controller can transmit or provide signals to the pumps (e.g., each of the pumps  1610  individually, the pump  1710 , etc.) that cause the pumps to move fluid throughout the systems described herein. For example, the controller can cause the microfluidic pumps  1610  to either provide or remove fluid from one or more microenvironment units via a port or a sipper, as described herein. 
     The controller can transmit one or more signals to a motor that causes the agitator  1910  to actuate according to the implementations described herein. For example, the controller can provide one or more signals that indicate a rotation speed (e.g., a serial packet signal, a pulse-width modulation signal, an analog voltage signal, etc.) to a motor that causes the motor to rotate the agitator  1910 . Likewise, the controller can transmit one or more similar signals to the valves (e.g., the valves  1715   a  and  1715   b , the valve  1815   a , the second valve  1815   b , etc.) of the systems described herein to cause the valves to open or close by varying degrees. For example, the controller can transmit or provide one or more signals to one or more of the valves that indicate an amount by which the valves should open or close. The signals can be, for example, serial packet signals, pulse-width modulation signals, pulse-frequency modulation signals, or analog voltage signals, among others. The controller can transmit signals to the pumps (e.g., each of the pumps  1610  individually, the pump  1710 , etc.) described herein that cause the pumps to transport specified amounts of fluid through one or more of the systems described herein. For example, the controller can provide a signal to at least one of the pumps that indicates an amount of fluid to pump. In some implementations, the signal provided by the controller can indicate a direction (e.g., pump into a well, pump out of a well, etc.) that particular pump should transport fluid. 
     The controller can be in communication with more than one component simultaneously, and can control each component independently from other components with which the controller communicates. For example, each of the pumps  1610  can be controlled individually to cause, for example, fluids to flow over a tissue sample that has been trapped in a trapping region in a microenvironment unit. Thus, the controller can transmit or provide independent signals to one or more of the components in the systems described herein, allowing the components of the systems to work independently to achieve a desired outcome. In some implementations, the controller can provide or transmit signals over one or more communication buses, such as a serial bus, a parallel bus, or any other type of communication bus. The processor of the controller can be communicatively coupled to one or more communication buses. The components of the systems described herein can be communicatively coupled to one or more of the communication buses, and which can transport signals from the processor of the controller to a respective component. For example, in a serial bus arrangement, the controller can communicate with a particular component by using an address value assigned to that component (e.g., each component on a bus can be assigned an address value, etc.). In a parallel bus arrangement, one or more transmission lines in the parallel bus can be communicatively coupled to a respective component. 
       FIG. 17  shows a cross-sectional view of an example pumping system  1700 . In some implementations, the pumping system  1700  can be implemented in connection with the system  1600  shown in  FIG. 16 . The pumping system  1700  includes a pump  1710  positioned in a series arrangement between two valves  1715   a  and  1715   b . The pumping system  1700  is coupled with a channel  1702 . Fluid flows through the channel  1702  in the direction illustrated by the arrows. For example, an outlet of the pumping system  1700  (e.g., coupled with the valve  1715   a ) can introduce fluid into an inlet of the channel  1702 , while an inlet of the pumping system  1700  (e.g., coupled with the valve  1715 ) can receive fluid from an outlet of the channel  1702 . 
     The channel  1702  includes a mechanical trapping feature  1713  that traps a tissue sample  1750  within the channel  1702 . In this example, the mechanical trapping feature  1713  is depicted as including a post or partial wall that extends vertically from a floor of the channel  1702 . However, it should be understood that the mechanical trapping feature  1713  could instead be implemented using a different type of trapping feature or structure, such as any of the mechanical trapping features described above, without departing from the scope of this disclosure. The mechanical trapping feature  1713  and the trapped tissue sample  1750  partially occlude the channel  1702 . In order to flow fluid through the channel  1702 , the pumping system  1700  can be configured to supply enough pressure while the channel  1702  is partially occluded. For example, gaskets  1755   a  and  1755   b  can be included to form seals at the points where the channel  1702  couples with the pumping system  1700 . These seals can allow the pump  1710  to build sufficient pressure to flow fluid through the channel  1702 . 
     In some implementations, in order to move fluid through the channel  1702 , the valve  1715   a  can be opened and the valve  1715   b  can be closed while the pump  1710  exerts pressure to drive fluid towards the inlet of the channel  1702  (e.g., towards the left-hand side in  FIG. 17 ). As a result, fluid can flow towards, around, and through the fluid sample  1750  toward an outlet of the channel  1702 . The pumping system  1700  can also include a sipper  1760  that is partially submerged in a downstream portion of the channel  1702  near the outlet. In some implementations, the valve  1715   a  can be closed, the valve  1715   b  can be opened, and the pump  1710  can retrieve fluid from the channel  1702  via the sipper  1760 . Then, the pumping cycle can be repeated. 
       FIG. 18  shows a schematic view of an example pumping system  1800 . In some implementations, the pumping system  1800  can be implemented in connection with the system  1600  shown in  FIG. 16 . The pumping system  1800  includes two reservoirs  1812   a  and  1812   b  positioned on either side of a mechanical trapping feature  1813 . In some implementations, the reservoirs  1812   a  and  1812   b  can be on-chip reservoirs that are part of a multiwell plate that includes a microenvironment in which the mechanical trapping feature  1813  is positioned. The reservoirs  1812   a  and  1812   b  can each be coupled with a pressure source that causes a pressure labeled P 1  in the reservoir  1812   a  and a pressure labeled P 2  in the reservoir  1812   b . For example, the pressure source can be a pneumatic line directly coupled with either or both of the reservoirs  1812   a  and  1812   b . In some implementations, the pressure source may be coupled with either or both of the reservoirs  1812   a  and  1812   b  through a distensible membrane that serves as a diaphragm separating the liquid and gas phase. 
     The pumping system  1800  also includes a first valve  1815   a  and a second valve  1815   b . Together, the valves  1815   a  and  1815   b  can allow for selection of the fluid flow path through the mechanical trapping feature  1813 . For example, when the pressure P 2  of the reservoir  1812   b  is greater than the pressure P 1  of the reservoir  1812   a , the valve  1815   a  can be opened and the valve  1815   b  can be closed to cause fluid to flow from the reservoir  1812   b  through the mechanical trapping feature  1813  and to the reservoir  1812   a  (e.g., towards the right-hand side in the depiction of  FIG. 18 ). Eventually, this fluid flow can deplete the fluid in the reservoir  1812   b . Prior to this depletion, the valve  1815   a  can be closed and the valve  1815   b  can be opened, and the pressure differential between the reservoirs  1812   a  and  1812   b  can be reserved such that the pressure P 1  of the reservoir  1812   a  is greater than the pressure P 2  of the reservoir  1812   b . As a result, fluid can be returned from the reservoir  1812   a  to the reservoir  1815   b  via the valve  1815   b , thereby enabling fluid to be recirculated. 
     It should be understood that, in this example, the mechanical trapping feature  1813  is depicted as including a set of posts arranged to trap the tissue sample  1850 , similar to the posts  1210  of  FIGS. 12A and 12B . However, it should be understood that the mechanical trapping feature  1813  could instead be implemented using a different type of trapping feature or structure, such as any of the mechanical trapping features described above, without departing from the scope of this disclosure. 
       FIG. 19  shows a cross-sectional view of an example pumping system  1900 . In some implementations, the pumping system  1900  can be implemented in connection with the system  1600  shown in  FIG. 16 . The pumping system  1900  includes an agitator  1910 . The agitator can be inserted into a well of a multiwell plate, such as the multiwell plate  300  shown in  FIG. 3 . In the example of  FIG. 19 , the well is defined by a well insert  1907  that is disposed in a well plate  1909 , however it should be understood that other arrangements are also possible. The agitator  1910  is positioned above a tissue trapping region of the well insert  1907 , in which a tissue sample  1950  is trapped. For example, the tissue sample  1950  can be trapped by any of the mechanical trapping features described above. A permeable substrate  1917  can be positioned between the agitator  1910  and the tissue sample  1950 . In some implementations, the permeable substrate  1917  can include a mesh, a membrane, or a porous plate. The permeable substrate  1917  can be supported by a portion of the well insert  1907 . 
     The tissue sample  1950  can be introduced into the well defined by the well insert  1907  for example via a microfluidic channel or by deposition into a cavity of the well. A fluid sample can be added to the well, and the agitator  1910  can be activated. For example, in some implementations the agitator can be or can include a propeller, a magnetic stir bar, or a set of spinning cones coupled with a rotating shaft that spins in the direction shown by the arrows in  FIG. 19 . The agitation of the fluid sample by the agitator  1910  can cause at least a portion of the fluid to be pushed through the permeable substrate  1917  and to interact with the tissue sample  1950 . 
     The agitator  1910  can have a surface geometry selected to promote agitation and/or mixing of the fluid sample when the agitator  1910  is activated. The surface geometry of the agitator can also be selected to mimic various physiological conditions in the interaction between the fluid sample and the tissue sample  1950 . For example, spinning cones or other surfaces can be selected or designed to provide shear, interstitial pressure, and other conditions that may exist in a physiological environment (e.g., a patient) to be treated. As a result, the design of the agitator  1910  can help to increase the utility of experimental results obtained by causing and observing an interaction between the fluid sample and the tissue sample  1950  in implementations in which the interaction is observed for the purposes of identifying or evaluating a candidate therapeutic substance contained within the fluid sample 
     Implementations of some of the subject matter and the operations described in this specification, for example, those related to the controller described herein above, can be implemented in digital electronic circuitry, or in computer software embodied on a tangible medium, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Implementations of the subject matter described in this specification can be implemented as one or more computer programs, e.g., one or more components of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, data processing apparatus. The program instructions can be encoded on an artificially-generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. A computer storage medium can be, or be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal, a computer storage medium can include a source or destination of computer program instructions encoded in an artificially-generated propagated signal. The computer storage medium can also be, or be included in, one or more separate physical components or media (e.g., multiple CDs, disks, or other storage devices, any other storage media described herein, etc.). 
     While operations are depicted in the drawings in a particular order, such operations are not required to be performed in the particular order shown or in sequential order, and all illustrated operations are not required to be performed. Actions described herein can be performed in a different order. 
     The separation of various system components does not require separation in all implementations, and the described program components can be included in a single hardware or software product. 
     Having now described some illustrative implementations, it is apparent that the foregoing is illustrative and not limiting, having been presented by way of example. In particular, although many of the examples presented herein involve specific combinations of method acts or system elements, those acts and those elements may be combined in other ways to accomplish the same objectives. Acts, elements, and features discussed in connection with one implementation are not intended to be excluded from a similar role in other implementations. 
     The phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” “having,” “containing,” “involving,” “characterized by,” “characterized in that,” and variations thereof herein is meant to encompass the items listed thereafter, equivalents thereof, and additional items, as well as alternate implementations consisting of the items listed thereafter exclusively. In one implementation, the systems and methods described herein consist of one, each combination of more than one, or all of the described elements, acts, or components. 
     As used herein, the terms “about” and “substantially” will be understood by persons of ordinary skill in the art and will vary to some extent depending upon the context in which they are used. If there are uses of the term which are not clear to persons of ordinary skill in the art given the context in which it is used, “about” will mean up to plus or minus 10% of the particular term. 
     Any references to implementations or elements or acts of the systems and methods herein referred to in the singular may also embrace implementations including a plurality of these elements, and any references in plural to any implementation or element or act herein may also embrace implementations including only a single element. References in the singular or plural form are not intended to limit the presently disclosed systems or methods, their components, acts, or elements to single or plural configurations. References to any act or element being based on any information, act, or element may include implementations where the act or element is based at least in part on any information, act, or element. 
     Any implementation disclosed herein may be combined with any other implementation or embodiment, and references to “an implementation,” “some implementations,” “one implementation,” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described in connection with the implementation may be included in at least one implementation or embodiment. Such terms as used herein are not necessarily all referring to the same implementation. Any implementation may be combined with any other implementation, inclusively or exclusively, in any manner consistent with the aspects and implementations disclosed herein. 
     The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” 
     References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all the described terms. For example, a reference to “at least one of ‘A’ and ‘B’” can include only ‘A’, only ‘B’, as well as both ‘A’ and ‘B’. Such references used in conjunction with “comprising” or other open terminology can include additional items. 
     Where technical features in the drawings, detailed description, or any claim are followed by reference signs, the reference signs have been included to increase the intelligibility of the drawings, detailed description, and claims. Accordingly, neither the reference signs nor their absence has any limiting effect on the scope of any claim elements. 
     The systems and methods described herein may be embodied in other specific forms without departing from the characteristics thereof. The foregoing implementations are illustrative rather than limiting of the described systems and methods. Scope of the systems and methods described herein is thus indicated by the appended claims, rather than the foregoing description, and changes that come within the meaning and range of equivalency of the claims are embraced therein.