Abstract:
A system and method for conditioning a tissue are provided. The system includes a substrate, a plurality of microwells formed in the substrate, and a microsphere associated with each of the plurality of microwells. The system also includes a pair of flexible pillars within each of the plurality of microwells. Each flexible pillar includes a first end bonded to a respective microwell and at least one flexible pillar has a second end bonded to the microsphere. The flexible pillars are configured to deflect when exposed to a magnetic field to controllably stretch microtissue spanning the flexible pillars.

Description:
CROSS REFERENCE 
       [0001]    This application is based on, claims priority to, and incorporates herein by reference, U.S. Provisional Application Ser. No. 62/082,374, filed Nov. 20, 2014 and entitled “SYSTEM AND METHOD FOR CONDITIONING OF ENGINEERED TISSUES.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with government support under HL90747 awarded by the National institutes of Health. The government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0003]    The present disclosure relates generally to mechanical conditioning of engineered tissues and, more particularly, systems and methods for conditioning or controlling processing of microtissues. 
         [0004]    Engineered biological tissues provide the potential for options beyond the traditional treatments, for example, when testing drugs in development, and when performing tissue and organ repair. In addition, these tissues allow for the study of the organization and mechanical and biological function of model multicellular constructs. Static and dynamic mechanical conditioning during the engineering process have been found to enhance tissue structure, mechanical strength, and overall functionality. Mechanical conditioning of these engineered tissues traditionally requires the use of centimeter scale tissue samples and potentially complex bioreactor systems. The large scale of the tissues sets a limit to the imaging which can be performed on the tissue and the ability for pharmacological treatments to diffuse throughout. 
         [0005]    Current cutting edge methods of tissue engineering range from bioreactors to 3D printing. The majority of these methods, however, use relatively large amounts of reagents and do not condition the tissue as it matures. The expense of certain reagents and the rarity of certain cell lines necessitate a method for high-throughput tissue engineering that uses few materials. Furthermore, conditioning is an important step that contributes to the tissues&#39; ultimate mechanical integrity. 
         [0006]    A range of microengineered devices fabricated from soft materials, such as poly(dimethylsiloxane) (PDMS), have been developed that can measure the force generation (contractility) of model tissues of the millimeter and sub-millimeter scales during the process of tissue conditioning. In these devices, cells and extracellular matrix self-assemble under the contractile action of the cells into tissue constructs suspended between a pair of flexible vertical cantilevers, whose deflection reports the net contractile force generated by the cells in the tissue. These microtissue strain gauges have enabled the study of contractility in a range of model tissues, involving fibroblasts, airway smooth muscle cells, and cardiomyocytes. 
         [0007]    The capability of such force measuring devices can be greatly expanded by enabling actuation of the microtissues, as this can allow further exploration of the properties of such tissue constructs. In addition, actuation allows for analysis of both acute and long-term response to mechanical conditioning of the specific tissues. It would be desirable to have a system and method that allows for further actuation of tissue constructs during a conditioning and measuring process. 
       SUMMARY 
       [0008]    The present disclosure overcomes the aforementioned drawbacks by providing a system and method for non-invasive actuation of tissues to mechanically condition such tissue. For example, a system and method is provided for mechanical actuation of an array of microtissues. When paired with a magnetic microtissue tester (MMT), the contractility of the tissue can be accurately monitored during the conditioning process. 
         [0009]    In accordance with one aspect of the disclosure, a system and method for conditioning a tissue are provided. The system includes a substrate, a plurality of microwells formed in the substrate, and a microsphere associated with each of the plurality of microwells. The system also includes a pair of flexible pillars within each of the plurality of microwells. Each flexible pillar includes a first end bonded to a respective microwell and at least one flexible pillar has a second end bonded to the microsphere. The flexible pillars are configured to deflect when exposed to a magnetic field to controllably stretch microtissue spanning the flexible pillars. 
         [0010]    In accordance with another aspect of the disclosure, a method of conditioning tissue is provided that includes introducing sample cells into one or more microwells to extend as microtissue across a plurality of pillars. The method also includes magnetizing the nickel microsphere and pillars using an externally-applied magnetic field to displace the pillars and apply a force to the microtissue and monitoring mechanical properties of the microtissue while repeatedly adjusting the magnetizing of the nickel microsphere and pillars to perform a tissue conditioning process. 
         [0011]    The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1A  is a schematic illustration of a microwell containing the actuation device and a magnetic microtissue tester (MMT) in accordance with the present disclosure. 
           [0013]      FIG. 1B  is schematic illustration of an array of microwells containing magnetic microtissue testers (MMT) to create a magnetic microtissue actuation system to condition and test an array of tissues in accordance with the present disclosure. 
           [0014]      FIG. 2A  is schematic illustration of an actuation device in accordance with the present disclosure in one state of a fabrication process in accordance with the present disclosure. 
           [0015]      FIG. 2B  is schematic illustration of an actuation device in accordance with the present disclosure in another state of a fabrication process in accordance with the present disclosure. 
           [0016]      FIG. 2C  is schematic illustration of an actuation device in accordance with the present disclosure in yet another state of a fabrication process in accordance with the present disclosure. 
           [0017]      FIG. 2D  is schematic illustration of an actuation device in accordance with the present disclosure in still another state of a fabrication process in accordance with the present disclosure. 
           [0018]      FIG. 2E  is schematic illustration of an actuation device in accordance with the present disclosure in still another state of a fabrication process in accordance with the present disclosure. 
           [0019]      FIG. 3A  is a schematic illustration of the geometric characterization of the actuation device in accordance with the present disclosure. 
           [0020]      FIG. 3B  is another illustration of the geometric characterization of the actuation device in accordance with the present disclosure. 
           [0021]      FIG. 4A  is a graph illustrating characteristics of an example for magnetic characterization of a system in accordance with the present disclosure. 
           [0022]      FIG. 4B  is another graph illustrating characteristics of an example for magnetic characterization of a system in accordance with the present disclosure. 
           [0023]      FIG. 4C  is yet another graph illustrating characteristics of an example for magnetic characterization of a system in accordance with the present disclosure. 
           [0024]      FIG. 5A  is an image of an MMT without an external magnetic field applied 
           [0025]      FIG. 5B  is an image of an MMT with an external magnetic field applied 
           [0026]      FIG. 5C  is a graph of the motion of a pillar in response to the external magnetic field in accordance with the present disclosure. 
           [0027]      FIG. 6A  is a graph of stiffness changes of microtissue characterized by force and length during dynamic loading. 
           [0028]      FIG. 6B  is a graph of stiffness changes of microtissue characterized by elastic modulus during dynamic loading. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    An approach has been demonstrated that enables mechanical stimulation of microtissues via magnetic actuation of magnetic microspheres bonded to the cantilevers of arrays of microtissue strain gauges. This approach allows for the measurement of the mechanical stiffness of the tissue constructs, and analysis of the relative contribution of cells and matrix to relevant mechanical properties. Additionally, investigation of the role of boundary conditions and mechanical constraints on tissue formation can be performed. While previous methods demonstrate the potential of magnetically driven microtissue constructs, actuation has been performed using an electromagnetic tweezer device. This approach requires invasive probing as the pole tip of the electromagnet must be inserted into the culture media. 
         [0030]    The present disclosure provides a device for actuating and conditioning microtissues. When paired with a magnetic microtissue tester (MMT), the device acts as a less resource intensive means of tissue engineering and as a method to influence and test tissue mechanical properties. 
         [0031]    The present approach allows for simultaneous magnetically-driven mechanical actuation of an array of microtissue strain gauges (μTUGs)  100 .  FIG. 1( a )  illustrates one microwell  102  which may be used in a broader system that may incorporate more than one microwell  102 , each containing flexible pillars  104  that may be fabricated in a PDMS substrate with, for example, a magnetic Nickel microsphere  106  bonded to one of the pillars  104 . Though a pair of pillars  104  is illustrated, this is a non-limiting example. For example, other numbers of pillars  104  beyond pairs, for example, 4 or 8 pillars, or odd numbers of pillars may be included. A mixture of cells and extracellular matrix (ECM) is introduced into the wells  102 , and as the cells contract the mixture, they form an aligned microtissue  108  spanning the pillars  104 , which bend due to the collective contractile force of the microtissue  108 , providing a read-out of the force. A small (˜1 mm) Nickel bar  110  is microfabricated on a silicon wafer, and is placed near the magnetic pillar  104 . When the sphere  106  and bar  110  are magnetized by an externally applied magnetic field, the sphere is attracted to the bar with a magnetic force, which controllably stretches the microtissue  108 . 
         [0032]    In one aspect of the disclosure, a PDMS MMT device can be created in conjunction with an actuation device. The PDMS MMT device facilitates the formation of the microtissues, which will be magnetically activated by the actuation device. In one example embodiment of the PDMS MMT device, PDMS molds are used for replica molding. The PDMS molds include pairs of flexible pillars  104  with, as a non-limiting example, separation of 500 μm in wells  102  with, as a non-limiting example, dimensions 800 μm×400 μm ×170 μm deep. In addition, as a non-limiting example, the PDMS may have an elastic modulus of 1.6 MPa. The pillars  104  may have, as a non-limiting example, a length of 115 μm and cross section of 140 μm×35 μm in their flexible sections, giving them an effective spring constant of k=0.9 μN/μm for small deflections. Nickel spheres  106 , as a non-limiting example, may have ˜100 μm diameters and be bonded with PDMS to the top of one pillar in each MMT. 
         [0033]    In one example, the microtissues  108  are formed by introducing suspensions of NIH 3T3 fibroblasts and 2.5 mg/ml unpolymerized rat tail type-I collagen (BD Biosciences) into the wells  102 . The cells are cultured on the MMT devices for two days prior to measurements in high glucose Dulbecco&#39;s Modified Eagle&#39;s Medium, supplemented with 10% bovine serum, 100 units/ml penicillin, and 100 μg/ml streptomycin (all from Invitrogen). 
         [0034]      FIG. 1( b )  illustrates the present approach in an array format, with a schematic of a magnetic microtissue actuation system, with multiple nickel bars  114  that align with the individual μTUGs  116 , microtissues and the inclusion of an array of holes  118  etched through the Si wafer  120  to enable optical access and good exchange of culture media for the microtissues. The actuation device  112  is made up of a through-etched silicon nitride-coated wafer  120  with patterned gold fingers. Nickel islands  114  are created on top of the gold circuitry via electrodeposition in order to transduce an externally applied uniform magnetic field into a local inhomogeneous field near each MMT  122 . That is, the nickel bars  114  may be patterned on a silicon wafer  120 . Alternatively, the nickel bars  114  may be mounted directly on the PDMS MMT device, as generally illustrated in  FIG. 1( a ) . 
         [0035]    The construction of the actuation device can be broken down into three segments: gold circuitry definition, nickel electro-deposition, and wafer through-etching. The component steps in the latter two processes can be interleaved to ensure survivability of the features on the substrate. 
         [0036]    The actuation device is constructed using standard photolithography and electrodeposition techniques, and represents a simple to design, easy to create device for tissue mechanical conditioning and testing. The techniques used for fabrication allow for highly uniform actuation device production. 
         [0037]    A schematic illustration of a fabrication process for the actuation device can be seen in  FIG. 2 . This process includes the steps of fabricating the gold fingers on the silicon nitride-coated wafers, spinning a photoresist layer over the gold fingers, defining the desired shape of the nickel bars in the resist layer, defining a mask for the wafer through-etching, electrodepositing the nickel bars in the previously defined mask, and etching holes in the wafer. 
         [0038]    Turning to  FIG. 2 , one example fabrication process is shown. Patterned metal (Cr(7 nm)/Au (45 nm)) finger-shaped arrays  202  that are 1,600 μm wide were fabricated on double-side polished silicon-nitride coated wafers  204  using standard photolithography, thermal evaporation, and lift-off processing techniques which can be seen in  FIG. 2( a ) . The position and size of the metal fingers were designed to align along the short edge of each individual microwell, and fit to the empty space between two adjacent microwells. A 120 μm thick layer of SU-8 photoresist  206  was then spun between the gold fingers, and patterns corresponding to the shape of the nickel bars were defined in the SU-8 layer on the gold fingers as seen in  FIG. 2( b ) . To ensure clean gold surfaces for the subsequent nickel electrodeposition, the top side of the substrate was reactive-ion etched (RIE) in O 2  for 5 minutes to remove any remaining SU-8 on the exposed gold regions. Next, to define a mask for the wafer through-etching, an array of rectangular holes  208 , each of dimension 1,200 μm by 840 μm, was patterned into the silicon-nitride coating using S1813  210 . The holes  208  were patterned on the side of the wafer opposite the gold fingers via backside alignment, using standard photolithography and reactive ion etching in CF 4  and O 2 , as shown in  FIG. 2( c ) . 
         [0039]    As can be seen in  FIG. 2( d ) , the nickel bars  212  were then electrodeposited onto the previously defined patterns to thicknesses of 50-100 μm, as desired, using the gold finger array as a working electrode. The nickel deposition solution consisted of 80.5 g nickel (II) sulfamate, 6.25 g nickel chloride, 10 g boric acid, and 0.05 g sodium dodecyl sulfate (SDS), in 250 mL water. For deposition, a potentiostat (Model 263A, Princeton Applied Research) was used in galvanostatic mode and was set to −1 V relative to a platinum reference electrode. 
         [0040]    Finally, as seen in  FIG. 2( e ) , the holes  214  in the wafer were produced by etching in a 30% KOG solution at 150° C. for approximately 6 hours to obtain holes on the side of the wafers with the nickel bars that match the dimensions of the wells on the MMTs. The KOH bath had the added effect of removing any remaining photoresist adhered to the wafer. 
         [0041]    To characterize the properties of the actuation device, several measurement devices were used. The lateral dimensions of the device features were verified using optical microscopy. The thickness and uniformity of the nickel bars were measured by optical profilometry (VK-VX100, Keyence). Finally, the magnetic properties of the nickel bars were determined by removing individual nickel bars from the silicon wafers, and measuring the bars in a vibrating sample magnetometer (VSM) (DMS Model 10; ADE Technologies, Westwood, Mass.). 
         [0042]    To test the fabrication and measurement procedures for the actuation devices, arrays containing up to 20 electrodeposited nickel bars and corresponding through-holes on segments of silicon-nitride coated silicon wafers may be produced.  FIG. 3( a )  illustrates a portion of one such array, showing three nickel bars  302 , their underlying Au strip  304 , and corresponding through-holes  306 . The nickel bars are fabricated with pointed ends to concentrate magnetic flux and create larger field gradients in the neighborhood of the magnetic pillars. The bars shown have length 1,600 μm, width 450 μm, and tip width 90 μm. To achieve alignment with the MMT arrays, in this configuration, the nickel bars are laid out on a rectangular grid with center-to-center spacing of 3,200 μm along the bars&#39; long axis and 1,200 μm along the short axis. 
         [0043]    The dimensions of the nickel bars may be characterized via optical profilometry.  FIG. 3( b )  shows the height profile of a nickel bar. The lateral dimensions of the electrodeposited bars vary by &lt;0.6% along the long axis, and &lt;2% along the short axis in this configuration. Individual bars are flat, with height variations of less than 2%. A variation of ˜10% is found in the bars&#39; thicknesses t=60±7 μm (N=27), likely from variations in the deposition current across the arrays. 
         [0044]    The PDMS-MMT device together with actuation device provide a system for mechanically activating the microtissue, and calculating mechanical properties such as stress, strain, and stiffness. By mounting the actuation device on the PDMS-MMT device, a system is created that can allow for electromagnetic activation of the actuation device and stimulation of the microtissues that have been formed on the PDMS-MMT device. 
         [0045]    In one example, measurements and probing of microtissue occur when the actuation device is mounted and aligned under a microscope on the MMT device, and the arrays are actuated with a microscope-mounted dual-coil programmable electromagnet. The electromagnet used is capable of producing magnetic fields of up to 50 mT with a uniformity of 3% over the largest arrays studied. 
         [0046]    Images of individual MMTs and microtissues may be obtained using phase contrast microscopy with a 10× objective on a Nikon TE-2000E inverted microscope. For mechanical measurements of individual microtissues a quasi-static stretching protocol is used, during which images were recorded with a CoolSnap HQ (Photometrics) camera. To characterize sinusoidal actuation, movies are recorded at 100 frames/sec using a Prosilica GX (Allied Vision Technologies) camera. The pillar deflections are determined from the images using Image) (NIH) for the quasi-static measurements and via custom tracking software written in IgorPro (WaveMetrics) for the dynamic actuation studies. 
         [0047]    The stress, strain, and elastic modulus of each microtissue is determined from the quasi-static stretching data. Briefly, the force on each microtissue is found by tracking the deflection of the non-magnetic MMT pillar and calculating a force based on its spring constant. The stress in the central region of each microtissue is then obtained from the measured dimensions of the microtissue. The strain is measured locally in the microtissues&#39; centers from sequential phase contrast images, using a texture correlation analysis algorithm. The elastic modulus is determined from the slope of the resulting stress-strain curves 
         [0048]      FIG. 4( a )  shows the magnetic moment μ Bar  vs. applied magnetic field μ 0 H for a representative nickel bar removed from the array and measured using a vibrating sample magnetometer (VSM). As shown in  FIG. 4( a ) , the magnetic hysteresis of the bars is small, with a remnant moment  402  approximately 10% of the bars&#39; saturation moment μ Sat    404 . It can be noted that the measured value of μ Sat =1.72 μAm 2  agrees with the value expected for the design dimensions and the room temperature saturation magnetization of nickel.  FIGS. 5( a ) and 5( b )  show the deflection of a magnetic pillar  502  due to the force produced by the nickel bar on the actuation device in an applied external magnetic field B Ext =40 mT. To assess the variability of the force generation, a set of MMTs on an array is measured, and deflections of 17.4±2.5 μm (F Mag =15.7±2.3 μN) are found in a field B Ext =34 mT (N=9).  FIGS. 4( b ) and 4( c )  show a finite-element modeling computation in the vicinity of one end of a nickel bar of the component B x  of the magnetic field parallel to a bar&#39;s long axis in an external field of 34 mT, using the COMSOL Multiphysics package. Hysteresis effects are not included. From this, the force on a nickel bead on an adjacent MMT pillar may be estimated as FMag=(μ Sph (B)·B), where μ Sph  is the field-dependent magnetic moment of the nickel sphere in the total field B=B Ext +B Bar . At a bar-sphere spacing of 150 μm, the calculation shown in  FIGS. 4( b ) and 4( c )  yields B=51.1 mT and dB x /d x =180 T/m. At this field, the nickel spheres have μ Sph =0.075 μA m 2 , which yields F Mag ˜14 μN, in reasonably good agreement with our measured value. 
         [0049]    In another embodiment of the disclosure, the actuation device applies a quasi-static load on the microtissue. By applying a magnetic field to the actuation device, the magnetic pillars are pulled toward the nickel bars. 
         [0050]    To determine the suitability of the device for AC stimulation, the response of the magnetic pillars to sinusoidal external fields is measured. An example of a magnetic pillar&#39;s motion in response to a 0.5 Hz AC magnetic field of amplitude 20 mT is shown in the inset to  FIG. 5( c ) . Since F Mag =∇ (μ Sph (B)·B), the force and displacement are approximately quadratic in B EXt . This leads to a frequency doubling for a sinusoidal driving field, and so the motion of the pillar is at 1 Hz. This motion is nearly sinusoidal with minimal distortion. Indeed, for the data shown in the inset of  FIG. 5( c ) , the Fourier amplitude of the largest harmonic present (at f=2 Hz) is only 5% of the 1 Hz fundamental ( FIG. 5( c ) , main panel), and thus despite the modest hysteresis of the bar ( FIG. 4( a ) ) and the nickel sphere, it can be seen that this system can apply clean periodic signals at physiologically relevant frequencies. 
         [0051]    In another embodiment of the disclosure, the system allows for dynamic loading of the tissue. Cyclic loading of the actuation device causes motion of the pillars, inducing an active load in the tissues by the pillar and a tension force on the pillars by the microtissue. 
         [0052]    The dynamic loading capacity of the actuation device is evaluated by applying a sinusoidal magnetic external field of amplitude 20 mT at 1 Hz to the actuation devices on MMTs. To verify that the microtissues undergo periodic actuation in response to this stimulation, the microtissues are observed for brief intervals (˜15 sec) while recording their motion at 100 frames/sec.  FIG. 6( a )  shows the left pillar displacement (microtissue force)  602  and overall length (difference in pillar positions)  604  vs. time for a microtissue following initiation of actuation. Both the force and length are predominantly sinusoidal with second harmonic content &lt;6% of the 2 Hz fundamental, similar to that observed for AC actuation of magnetic pillars without microtissue as shown in  FIG. 5 . 
         [0053]    To test the actuation device&#39;s potential for longer-term actuation, the elastic modulus of a set of microtissues is first measured by quasi-static loading with the actuation device as described in earlier. Cyclic loading at 2 Hz (1 Hz external field) is then applied simultaneously to the tissues for 15 min, and the stiffness is re-measured. While there is some variability in the degree of stiffness change, all microtissues measured exhibit an increase in modulus, with an average increase of 31%, which can be seen in  FIG. 6( b ) . 
         [0054]    This stiffness change reflects one of a reorganization of the collagen matrix, as it has been shown that cells play a minor role in fibroblast microtissue stiffness, or an actuation of internal force generation machinery. Thus, the ability of the device to influence tissue stiffness through AC stimulation has been demonstrated. In a bioengineering context, the device and associated apparatus are small enough to fit into a standard incubator. Using the device to mechanically condition tissues with AC stimulation as they are maturing is a simple extension of current protocols. Due to the versatility and efficiency of the present device, a possible application is for pharmacological mechanical testing that requires using expensive drugs and rare cell lines. 
         [0055]    The present invention has been described in terms of one or more preferred embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention.