Abstract:
An apparatus for measuring a characteristic of a sample includes a sample measurement apparatus ( 1404 ), which includes a light source ( 1406 ) configured to illuminate the sample; and a detector ( 1412 ) configured to receive light from the sample. The sample measurement apparatus is sized and dimensioned to fit within a centrifuge receptacle, the centrifuge receptacle ( 1416 ) coupled to a spindle configured to rotate the centrifuge receptacle to apply a force to the sample.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 61/350,088, filed Jun. 1, 2010, and entitled “Spinning Force Apparatus,” the contents of which are incorporated herein by reference. 
         [0002]    This application is related to U.S. application Ser. No. 12/326,279, filed Dec. 2, 2008, and entitled “Spinning Force Apparatus,” the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0003]    This invention relates to measurement of forces relating to molecules. 
         [0004]    The ability to quantify interactions between biomolecules is of great interest for scientific and medical research, as well as for drug development. Examples of measurable characteristics of a biomolecular interaction include the affinity (e.g., how strongly the molecules bind/interact) and the kinetics (e.g., rates at which the association and dissociation of molecules occur) of the interaction. Traditionally, such characteristics are measured in solution, using methods such as calorimetry, stop-flow imaging, or surface plasmon resonance. These bulk measurements are limited in many ways, including 1) they report only average behavior and thus may lose important details associated with metastable states and rare events, and 2) they measure chemistry in the absence of externally applied mechanical stress, which can be dramatically different from crowded and dynamic environments in living systems. 
         [0005]    Recent development in single molecule measurement methods offers a different approach in quantifying molecular interactions by examining the behavior of individual molecules rather than measuring the properties of bulk solutions. This approach enables the observation of rare or fleeting events that can be obscured by ensemble averaging. The resulting detailed information of molecular transitions helps researchers to identify metastable states and to study the transitions rates and the chemical pathways between such states. Furthermore, heterogeneities between molecules in a population and within the behavior of a single molecule can both be quantified. 
         [0006]    Currently, force probes that apply single molecule measurement methods include atomic force microscopes (AFM), optical traps, magnetic tweezers, biomembrane force probes, and flow chambers. Despite many advantages, these devices still have several limitations. For example, due to technical complexities, some systems require a large investment of money and time (e.g. optical trap systems typically cost $150 k or more). Additionally, molecular interactions are studied one molecule at a time in most cases. Statistical characterization of these interactions is therefore slow and painstaking, requiring hundreds or thousands of measurements which are typically performed in a serial manner. 
       SUMMARY 
       [0007]    In a general aspect, an apparatus for measuring a characteristic of a sample includes a sample measurement apparatus, which includes a light source configured to illuminate the sample; and a detector configured to receive light from the sample. The sample measurement apparatus is sized and dimensioned to fit within a centrifuge receptacle, the centrifuge receptacle coupled to a spindle configured to rotate the centrifuge receptacle to apply a force to the sample. 
         [0008]    Embodiments may include one or more of the following. 
         [0009]    The sample measurement apparatus further comprises a lens configured to receive light from the sample, and wherein the detector is configured to receive light from the lens. The lens includes an aspheric lens. The sample measurement apparatus includes only one lens. 
         [0010]    The apparatus further includes the centrifuge receptacle. The apparatus further includes a base disposed within the centrifuge receptacle and configured to received the sample measurement apparatus. The centrifuge receptacle includes at least one of a centrifuge tube, a swing bucket, and a rotor. The centrifuge receptacle is a centrifuge tube having a volume of less than or equal to about 50 mL. 
         [0011]    The sample measurement apparatus further includes a lens tube, wherein the light source, the lens, and the detector are disposed within the lens tube. The apparatus further includes a battery coupled to the centrifuge receptacle and electrically connected to the light source and the detector. The battery is disposed within the centrifuge receptacle. 
         [0012]    The apparatus further includes a recording device coupled to the centrifuge receptacle, the recording medium configured to record a signal received from the detector. The recording device is disposed within the centrifuge receptacle. 
         [0013]    The apparatus further includes a wireless communications module configured to wirelessly transmit a signal received from the detector. The apparatus further includes a computing device electrically connected to the detector. The computing device is external to the centrifuge receptacle. The detector is electrically connected to the computing device via a rotary joint. 
         [0014]    The apparatus further includes a computing device external to the centrifuge receptacle. The detector is optically connected to the computing device via a fiber optic rotary joint. 
         [0015]    The spindle forms part of a centrifuge. An optical axis of the sample measurement apparatus is substantially perpendicular to an axis of rotation of the centrifuge receptacle. 
         [0016]    Among other advantages and features, the spinning force system and methods described herein can provide one or more of the following advantages. 
         [0017]    A small-scale spinning force system uses a benchtop centrifuge, which is standard equipment in many scientific laboratories. The use of standard equipment decreases the cost and increases the accessibility of the spinning force approach to molecular characterization. Furthermore, the components of the small-scale spinning force system are themselves relatively inexpensive. 
         [0018]    The system and methods of operation can provide massively-parallel high-throughput single-molecule force measurements at a low cost. More specifically, rotation-induced forces (e.g., centrifugal forces and viscous drag forces) can be used to manipulate single molecules (e.g., proteins or DNAs) or molecular complexes (e.g., receptor-ligand protein pairs), enabling forces to be applied simultaneously to many subjects. Each subject can be observed directly and independently for true single-molecule detection. The efficiency of experiments can be improved, reducing the time to conduct an experiment from days to minutes. More than simply speeding up experiments, this efficiency also enables new experiments such as near equilibrium measurements that observe interactions with hour-long lifetimes, which would be unfeasible with sequentially collected statistics. Furthermore, with larger statistical sets more easily attainable, more detailed characterizations, model testing, and observations of population heterogeneity are possible. Parallel measurements can also be used to test families of interactions simultaneously (e.g., multiple drug candidates could be tested simultaneously against a target receptor). 
         [0019]    The system and methods of operation can provide accurate force control in a wide range of directions and magnitudes. Through force control, the system and methods of operation can be used to quantify force dependent interactions, including measuring the force dependence of kinetic parameters (e.g., K on  and K off ) and molecular subtleties which would be invisible from population averaging. Using this system, the mechanical properties of biomolecular complexes (e.g., compliances of DNAs and proteins) and cellular targets (e.g., elasticity of stress-bearing cells) can be studied, yielding valuable information into both the structure and the function of those subjects. 
         [0020]    The centrifugal force field applied to a sample in the system and methods of operation describe herein is macroscopically uniform, stable without the need for active feedback, calibration-free, and dynamically controllable in an essentially deterministic way. Thus, a desired force history can be applied to an ensemble of single molecules without the need for active feedback. The force field conveniently couples to mass density, eliminating the possibility of radiative damage and expanding the range of systems that can be studied with force (e.g., beads or objects made of any material can be used, as long as they have a different mass density than their surroundings). Furthermore, by varying the bead type, bead size, and rotation speed, a wide range of forces, at least from sub-femtoNewtons to nanoNewtons, can be achieved. 
         [0021]    The system and methods of operation can be conveniently integrated with various types of force probes to generate forces in multiple dimensions with high flexibility. For example, the system can be used in conjunction with optical traps, magnetic tweezers and/or microfluidic devices to generate a combination of forces (such as gradient and scattering forces, magnetic forces, hydrodynamic forces, and centrifugal forces). Each force can be applied to a sample in a different direction, with a different magnitude, and/or at a different test stage. 
         [0022]    The system and methods of operation can also be conveniently integrated with various imaging techniques to provide real-time observation with high temporal and spatial resolution. For example, using interference techniques and diffraction analysis, the position of individual particles in a sample can be ascertained with sub-nanometer accuracy. Also, fluorescent imaging enables visualization of subtle molecular transitions during experiment. Moreover, using video tracking by high-speed CCD cameras, molecular events can be detected on the scale of microseconds. 
         [0023]    The systems and methods described herein are also more cost effective and simpler to use than other common methods of molecular spectroscopy. The material cost of a spinning force system is generally less than the cost of a typical laboratory microscope. Furthermore, experiments using this system are straightforward, with a pre-preprogrammed force protocol, minimal setup, and little or no need for user intervention. 
         [0024]    Other features and advantages of the invention are apparent from the following description and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1  is a schematic representation of one embodiment of a spinning force system. 
           [0026]      FIG. 2A  is a schematic representation of a sample coupled to the spinning force system of  FIG. 1 . 
           [0027]      FIG. 2B  shows a centrifugal force applied to the sample of  FIG. 2A . 
           [0028]      FIGS. 3A and 3B  are transmitted light images of a sample generated at two different focal depths, respectively. 
           [0029]      FIGS. 4A-4C  show the interference patterns of a sample imaged by a reflection interference contrast microscope at three different heights, respectively. 
           [0030]      FIGS. 5A-5D  are schematic representations of a procedure for preparing a sample to be measured by the spinning force system of  FIG. 1 . 
           [0031]      FIGS. 6A-6D  are schematic representations of another procedure for preparing a sample to be measured by the spinning force system of  FIG. 1 . 
           [0032]      FIGS. 7A-7D  are schematic representations of a further procedure for preparing a sample to be measured by the spinning force system of  FIG. 1 . 
           [0033]      FIG. 8  shows an alternative embodiment (schematic in part) of the spinning force system of  FIG. 1 . 
           [0034]      FIG. 9  shows an additional alternative embodiment of the spinning force system of  FIG. 1 . 
           [0035]      FIG. 10  shows a further alternative embodiment of the spinning force system of  FIG. 1 . 
           [0036]      FIGS. 11A-11D  are schematic representations of a procedure for preparing a sample to be measured by the spinning force system of  FIG. 1 . 
           [0037]      FIGS. 12A-12D  are images of a spinning force experiment using the sample of  FIGS. 11A-11D . 
           [0038]      FIG. 13  is a graph of data collected during the experiment of  FIGS. 12A-12D . 
           [0039]      FIG. 14  is a block diagram of a small-scale spinning force system. 
           [0040]      FIG. 15  is a block diagram of an alternative arrangement of optical components in a small-scale spinning force system. 
           [0041]      FIG. 16  is a block diagram of an alternative embodiment of a small-scale spinning force system. 
       
    
    
     DETAILED DESCRIPTION 
     1 System Overview 
       [0042]    Referring to  FIG. 1 , a spinning force system  100  includes a rotary arm  120  mechanically coupled to a rotary stage  110  (for instance, part numbers ADRT-150 and CP20, Soloist available from Aerotech (Pittsburgh, Pa.)) configured to rotate about a central axis  102  at an adjustable angular velocity ω. Rotary stage  110  is housed and supported on a stationary base  104  immobilized on a platform (not shown) such as a vibration-free optical table. The motion of rotary stage about central axis  102  is computer controlled. 
         [0043]    Rotary arm  120  extends radially outward from central axis  102  to support a set of optical, mechanical, and electrical components for detecting characteristics (e.g., motion, optical, and geometric characteristics) of a sample  140  to be measured by system  100 . These components include, for example, a light source  130 , an objective  150 , a light detector  160 , and a media converter  170 . In operation, these components are moved by rotary arm  120  to rotate about central axis  102  at the same angular velocity ω. Rotary arm  120  may also carry one or more positioning elements (e.g., adjustment screws for coarse adjustment, such as screw AJS100-02H available from Newport (Irvine, Calif.) and electromechanical stages such as piezoelectric positioners for fine adjustment, such as piezo AE0505D08F available from Thorlabs (Newton, N.J.)) for adjusting the position of each component coupled to arm  120 . Examples of positioning elements will be described in greater detail below. 
         [0044]    In this example, light source  130  is mounted at a distal end of rotary arm  120  for emitting a light beam  132  to illuminate a selected region of sample  140 . Examples of light source  130  suitable for use in system  100  include various types of lamps (such as LED bulbs (e.g., LED lamp LED528E available from Thorlabs) and xenon arc lamps) and lasers (such as single- and multiple-wavelength lasers). Light source  130  may also include a set of optical components such as lenses, mirrors, and filters (not shown) for controlling the characteristics of its outgoing beam  132 . For example, a condenser with diaphragms may be used for tuning the emission intensity of beam  132 , and a color filter may be used for transmitting light at only selected wavelengths. 
         [0045]    Sample  140  is mounted onto rotary arm  120  with a sample holder (not shown) fastened to the arm. Depending on the particular implementation, sample  140  may include an acrylic sample chamber (not shown) in which experiment subjects (such as cells, biomolecules, and DNA strands) are sealingly contained. The sample chamber may consist of two parallel cover glasses separated by a 1 mm o-ring, forming an enclosed volume that can be filled with buffer and beads. In some implementations, sample  140  is oriented such that the surfaces of the cover glasses are aligned in parallel to central axis  102 . When rotary arm  120  rotates, the contents of sample  140  experience a centrifugal force normal to the cover glasses. In other implementations, sample  140  is oriented at a selected (and possibly adjustable) angle with respect to central axis  102 , enabling the centrifugal force to be applied in any given direction. 
         [0046]    Light beam  142  exiting sample  140  is received by objective  150  to produce a real image of the illuminated region of sample  140 . The optical characteristics of objective  150  (e.g., magnification and numerical aperture) are selected depending on the particular implementation. For example, a 20× air-immersion objective may be used for applications that require a wide field of view, whereas a 100× oil-immersion objective may be preferred for applications that require a high spatial resolution. 
         [0047]    Preferably, the relative position of objective  150  with respect to sample  140  is adjustable in three dimensions (x-, y- and z-directions shown in the figure), allowing images of different regions of the sample to be collected at various focal depths. In this example, objective  150  is staged on a piezoelectric positioner  190 , which can be translated along each of the x-, y-, and z-directions by an external control signal (e.g., provided by a computer). In other examples, sample  140  (instead of objective  150 ) may be staged on positioner  190  for linear translation in those three dimensions. 
         [0048]    Images formed by objective  150  are received by detector  160  and subsequently converted into electronic signals  162 . One example of a detector suitable for use is a charge-coupled device (CCD), such as a 12-bit 5 megapixel CCD camera (e.g., part number GC 2450 available from Prosilica (Newburyport, Mass.)). Another example of a suitable detector is a CMOS detector. Preferably, detector  160  is capable of acquiring successive images at a speed sufficiently fast to enable video tracking of sample  140  at a high temporal resolution (e.g., 1 kHz). In some examples, light from objective  150  is first transformed through an intermediate optical system (not shown) before reaching detector  160 . The intermediate optical system may include one or more elements such as lenses, filters, polarizers, and pinholes. 
         [0049]    Electronic signals  162  from detector  160  are delivered, for example, using electrical, optical, or wireless transmission means, to be passed onto a computer  180 . In this example, signals  162  are transmitted sequentially through an electronic-to-optical media converter  170 , an optical fiber  174 , and an optical-to-electronic media converter  172 . Exemplary media converters are available from IMC Networks, Foothill Ranch, Calif. (part numbers 855-10734 and 855-10735). Both media converter  172  and computer  180  are positioned on a stationary platform (not shown). Using proper interfacing software, computer  180  decodes electronic signals  176  from media converter  172  to reproduce images of sample  140  on a screen. Optionally, optical fiber  174  is coupled to rotary stage  110  through a fiber optic rotary joint (not shown; e.g., part number MJX-155-28-SC available from Princetel, Pennington, N.J.), which can be further integrated into an electrical slipring  106  (e.g., part number SRF24 available from Princetel) of rotary stage  110 . Power for detector  160 , media converter  174 , light source  130 , and positioner  190  may be transmitted through slipring  106 , allowing dynamic control of these components during rotation of system  100 . 
         [0050]    Computer  180  is used for viewing and processing images of sample  140 . In addition, computer  180  is also configured to provide various control signals to control individual components of spinning force system  100 . For example, computer  180  may be coupled to an electric motor (not shown) for controlling a rotational drive force to change the angular speed ω of rotary stage  110 . Computer  180  may also be coupled to a positioning device (not shown) for adjusting a distance between light source  130  and sample  140 , or coupled to positioner  190  for translating objective  150  in each of x-, y-, and z-directions to select detection regions and to control focal depth. Computer  180  may also be configured to control the optical characteristics of light source  130  (e.g., the brightness and the frequency range of output beam  132 ) as well as the image acquisition variables of detector  160  (e.g., readout rate, integration time, and electronic gain). Additionally, computer  180  may provide control signals based on previously acquired data, enabling real-time feedback control. 
       2 Operation 
     2.1 Force Application 
       [0051]    Referring to  FIGS. 2A and 2B , when rotary arm  120  is in operation, sample  140  rotates at an angular velocity ω and at a distance R from the center of axis  102 . For illustrative purposes, in this example, sample  140  includes a cover slip  140 ′ having an inner surface  140   a  and an outer surface  140   b  with respect to axis  102 . Both surfaces are aligned in parallel with axis  102 . A particle  240  (e.g., a bead) adheres to outer surface  140   b  through a chemical bond  243  formed between molecule A  242  and molecule B  244 . In this example, molecule A is a receptor chemically linked to outer surface  140   b , and molecule B is a ligand chemically linked to particle  240 . (The techniques and methods for forming such linkages will be described in greater detail below). 
         [0052]    When particle  240  undergoes circular motion, a centripetal force F is exerted on the particle, as defined by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   F 
                   = 
                   
                     
                       mv 
                       2 
                     
                     R 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where F is the net centripetal force, m and v are the mass and the linear velocity of the particle, respectively, and R is the distance of the particle from rotation axis  102 . In a rotating reference frame in which orbiting particle  240  appears stationary, particle  240  experiences an inertial centrifugal force equal to F in a direction perpendicular to outer surface  140   b  and away from central axis  102  (shown by arrow  250 ). In some examples where particle  240  is a spherical bead in solution with radius r and relative density ρ, rewriting equation (1) in terms of angular velocity ω yields: 
         [0000]    
       
         
           
             
               
                 
                   F 
                   = 
                   
                     
                       4 
                        
                       πρ 
                        
                       
                           
                       
                        
                       
                         r 
                         3 
                       
                        
                       R 
                        
                       
                           
                       
                        
                       
                         ω 
                         2 
                       
                     
                     3 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0053]    When sample  140  rotates about axis  102  at a very low speed, centrifugal force F is countered by the interaction force of chemical bond  243 , allowing particle  240  to continue to adhere to surface  140   b . As the rotational speed ω rises, the increasing magnitude of centrifugal force F causes bead  240  to move with respect to surface  140   b . The characteristics of the relative motion (e.g., the root-mean-square displacement or the direction of the motion) can be monitored and analyzed to quantify certain chemical and/or mechanical properties of bond  243  (e.g., properties associated with its transitional states and conformational changes). The increasing F may also cause the rupture of chemical bond  243 , at which point, particle  240  is released from surface  140   b . The magnitude of centrifugal force F at the particle release indicates the rupture force of chemical bond  243 . 
         [0054]    During operation, the magnitude of centrifugal force F can be controlled, for example, by adjusting the angular velocity ω of rotary arm  120 . For instance, sample  140  can be subjected to multiple cycles of force application in which the centrifugal force on particle  240  is increased and/or decreased through step changes in angular velocity ω. 
         [0055]    In addition to changing the angular velocity ω, it is also possible to change the radius of rotation R either statically or dynamically by changing the sample position relative to the axis of rotation. For example, in system  100 , sample  140  may be mounted to an adjustable rotary arm with extendible length, or staged on a positioner that can be translated in a radial direction with respect to central axis  102 . 
         [0056]    In cases where particle  240  is a spherical bead, the centrifugal force F can also be varied by changing one or more of the particle characteristics ρ and r shown in equation (2). For example, microspheres are commercially available in a wide range of materials and sizes (see Table 1 below). By conjugating subjects of study (e.g., molecules or cells) to selected microspheres, the centrifugal force applied to the microspheres (and translated to the subject) can be varied based on bead properties. In addition, the degree of monodispersity of beads can control the range of forces applied for a given spin. For instance, a highly monodisperse sample (e.g., using beads of substantially the same size and properties) may cause all beads to experience the same force, while a polydisperse sample (e.g., using beads of various sizes and/or properties) would have a wide range of forces being applied. Moreover, ρ of particle  240  can also be altered by changing the density of the buffer solution. Furthermore, the geometry of the sample chamber can be varied to control the effects of fluid flow, which can add hydrodynamic forces to immobilized particles in the chamber. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 The materials and sizes of common beads 
               
             
          
           
               
                   
                 Bead Material 
                 Specific Density (g/cm 3 ) 
                 Size Range (μm) 
               
               
                   
                   
               
             
          
           
               
                   
                 Borosilicate 
                 1.5 
                   1-100+ 
               
               
                   
                 Polystyrene 
                 0.05 
                  0.05-100+ 
               
               
                   
                 Silica 
                 1.2 
                 0.01-100  
               
               
                   
                 Gold 
                 18.3 
                 0.002-0.25  
               
               
                   
                 Melamine 
                 0.51 
                 0.5-10  
               
               
                   
                 Iron Oxide 
                 4.24 
                  1-10 
               
               
                   
                   
               
             
          
         
       
     
         [0057]    With proper parameter selection, the force applied to particle  240  can span 9 orders of magnitude, ranging from microNewtons (e.g., r=10 μm, ρ=1.5 g/cm 3 , R=500 mm, ω=100 Hz) to femtoNewtons (e.g., r=1 μm, ρ=0.05 g/cm 3 , R=250 mm, ω=2 Hz). 
         [0058]    The direction of the centrifugal force F can also be controlled. In some examples, sample  140  may be configured in an orientation perpendicular to rotational axis  102 , resulting in a centrifugal force F along surface  140   b . In other examples, sample may be configured to form a selected angle with respect to rotational axis  102  so that centrifugal force F may be applied in any given direction. For instance, a compressive (rather than tensile) force can be applied to particle  240  if the particle is positioned on inner surface  140   a  (rather than outer surface  140   b ) of the cover glass. For particular implementations, it may be desirable to place sample in a parallel position with respect to rotation axis  102  because pulling particle  240  away from surface  140   b  reduces the likelihood of the particle forming new interactions with unoccupied binding sites of molecule A on the surface  140   b.    
         [0059]    In addition to centrifugal force F, other types of forces can also be applied to particle  240  through spinning. For example, if particle  240  is contained in a chamber filled with a liquid medium, the rotation of sample  140  can generate regional flows that exert a viscous drag force D to particle  240 . The direction of the drag force depends on factors such as the geometry of the chamber and the orientation of the sample. The magnitude of the drag force depends on factors such as the viscosity and the temperature of the liquid medium, the size of the particle, and the rotational velocity and acceleration of the sample. 
       2.2 Observing Motion Characteristics 
       [0060]    In spinning force system  100 , motion of particle  240  (e.g., displacement caused by molecular folding, unfolding or rupture of bond  243 ) can be observed by video tracking methods (e.g., by taking successive images of the particle at a high temporal resolution). Because light source  130 , sample  140  and objective  150  rotate together at the same angular velocity ω, these three components appear stationary to each other in a rotating reference frame. Therefore, images of particle  240  can be formed using traditional imaging techniques, including transmitted- or reflected-light techniques and fluorescence techniques. 
         [0061]    Referring to  FIGS. 3A and 3B , images  310  and  320  are transmitted-light images of the same region of sample  140  produced at two different focal depths, respectively. In this example, sample  140  contains multiple beads (appearing, for example, as circular objects  312 ,  314  and  316  in image  310 ) near surface  140   b . The location of each bead in the images represents the lateral position of the bead (e.g., along x- and y-axes of  FIGS. 1 ,  3 A and  3 B). The image pattern of the bead (e.g., the size, the sharpness and the ring pattern of circular objects  312 ,  314  and  316 ) indicate the focal depth, namely, the distance of the bead from objective  150  (e.g., along z-axis of  FIG. 1 ). This focal depth can be used to represent the relative position of the bead above surface  140   b.    
         [0062]    By analyzing the lateral position and the image pattern of individual beads, the movement of the beads with respect to time can be characterized in a three-dimensional space. Such characterization enables the quantification of the affinity and the kinetics of chemical bond  243  that links the bead to surface  140   b . For example, before spinning, the position of objective  150  can be adjusted such that particle  240  situates in the objective&#39;s focal plane, resulting in a sharp in-focus image when the particle adheres to surface  140   b . During spinning, if the centrifugal force F is sufficiently strong to cause bond rupture, particle  240  is quickly (typically at a speed of microns/s or faster) pulled away by centrifugal force F from surface  140   b  (e.g., along z-axis of  FIG. 1 ). The escape of the particle from the objective&#39;s focal plane can be observed by changes in the characteristics of the detected image that are correlated with changes in focal depths. For example, a disappearance or blurriness of a circular pattern representing particle  240  (e.g., similar to the disappearance of object  316  from image  320  or the blurriness of object  314 ′ in image  320 ) indicates that the particle is being pulled away from the focal plane of objective  150 , e.g., as a result of bond rupture. Once bond rupture is identified, the bond force can be computed based on the magnitude of centrifugal force F, as defined in equation (1) or (2). 
         [0063]    In examples where bond rupture is not necessarily observed or desired, detecting particle movement can provide valuable information about the compliance of a molecular tether that includes bond  243 . For example, the displacement of particle  240  subjected to varying amplitude of centrifugal force F can be used to compute the elastic modulus of the molecular tether, and possibly to map the force dependency of such modulus. Also, stepwise changes in the position of particle  240  may indicate the transitions between molecular states that are associated with conformational changes of a chemical bond. For example, a bead that remains tethered but moves suddenly away from the cover glass can signify unfolding of a protein domain. 
         [0064]    The transmitted light imaging technique described above can easily provide a spatial resolution along the optical axis of about 100 nm for observation. In certain implementations where a higher resolution is desirable, more sophisticated imaging and image processing techniques can also be applied. For example, by using advanced techniques such as using transmitted or reflected light interference patterns of individual beads, the position of a bead relative to surface  140   b  can be determined with sub-nanometer accuracy. 
         [0065]    Referring to  FIGS. 4A-4C , the interference patterns of a single particle imaged by a reflection interference contrast microscope (RICM) at three different heights are shown, respectively. In these figures, the alternation of dark and bright fringes as well as the intensity and the size of each fringe can be decoded to reconstruct the height profile of the particle with a sub-nanometer axial resolution. (The white bar shown in these figures represents 10 μm). An example of particle tracking using the RICM technique is described by Heinrich et al., in Fast Three-Dimensional Tracking of Laser-Confined Carrier Particles, published in  Langmuir,  24(4):1194-1203, 2008, the contents of which are incorporated herein by reference. 
         [0066]    In addition to the aforementioned imaging techniques, fluorescence techniques can also be implemented alone or together with transmitted/reflected light techniques to enhance resolution and to enable visualization of subtle molecular transitions during experiment. 
       3 Small-Scale Spinning Force System 
       [0067]    Referring to  FIG. 14 , in an alternate embodiment, a small-scale spinning force system  1400  uses a standard laboratory centrifuge  1402  to provide rotational force for the study of molecular interactions. Spinning force system  1400  includes optical path components  1404 , including an LED light source  1406  (e.g., an LED525E available from Thorlabs), a sample chamber  1408 , a lens  1410  used for magnification, and a detector  1412 , such as a consumer-grade 5 Megapixel CCD camera or a CMOS (complementary metal-oxide-silicon) camera. Detector  1412  is, for instance, a VholdR (Seattle, Wash.) Contour HD CCD camera. Lens  1410  may be an aspheric lens (such as lens C230TME-A available from Thorlabs) that is sufficiently aberration-free to allow only a single lens to be used; alternatively, lens  1401  may be a multi-lens microscope objective. In some cases, optical path components  1404  act as a roughly 10× microscope. In some embodiments, a lens is not used and the detector  1412  receives light directly from sample chamber  1408 . 
         [0068]    Optical path components  1404  are aligned within a lens tube  1414 , such as a ½ inch diameter SM05 lens tube (Thorlabs), placed inside of a centrifuge receptacle. In some embodiments, the centrifuge receptacle is a standard centrifuge tube  1416 , such as a plastic centrifuge tube having a volume of, e.g., 50 mL, 15 mL, 1.5 mL, or another volume. A cone-shaped base  1417  formed from molded epoxy fits snugly at the end of centrifuge tube  1416 . A hole is drilled in the center of epoxy base  1417  and the LED end of the lens tube  1414  is inserted into the hole in the base  1417 , stabilizing the position of the optical components  1404  within centrifuge tube  1416 . In some cases, such as that shown in  FIG. 14 , centrifuge tube  1416  is placed within a swing bucket  1430  of centrifuge  1402 ; in other cases, centrifuge  1402  is configured to accept centrifuge tube  1416  in a centrifuge tube holder (not shown). In an alternate embodiment, the centrifuge receptacle is itself a standard centrifuge bucket, e.g., a bucket configured to hold multiple centrifuge tubes. In still other embodiments, the centrifuge receptacle may be a modified centrifuge rotor. 
         [0069]    Referring to  FIG. 15 , in an alternative embodiment, for a more compact setup, the optical path of optical path components  1404  is bent by two 45 degree mirrors  1500   a ,  1500   b.    
         [0070]    Referring again to  FIG. 14 , a memory device, such as a solid-state secure digital (SD) card  1418 , records video signals received from CCD camera  1412  representative of images of the sample formed by lens  1410 . The video images recorded on SD card  1418  can be analyzed by an external computer  1419  after an experiment is completed. Both camera  1412  and LED  1406  are powered by a small battery  1420 , such as a lithium polymer battery. In some embodiments, SD card  1418  and battery  1420  are placed in swing bucket  1430  along with centrifuge tube  1416 , with electrical connections between SD card  1418  and camera  1412  and between battery  1420  and both LED  1406  and camera  1412 . Electronic components  1424  (e.g., a circuit board) that control camera  1412  are also positioned in swing bucket  1430 . In this configuration, SD card  1418 , battery  1420 , and electronic components  1424  rotate along with the optical path components  1404 , but are not subject to size constraints that would be imposed by assembly within centrifuge tube  1416 . In other embodiments, SD card  1418 , battery  1420 , and electronic components  1424  are placed in another centrifuge tube (not shown), which may be positioned within the swing bucket  1430  along with centrifuge tube  1416 , or which may be placed in another centrifuge tube holder. In still other embodiments, SD card  1418 , battery  1420 , and electronic components  1424  are small enough to be positioned within centrifuge tube  1416 . A port  1432 , such as a Universal Series Bus (USB) port, is included in centrifuge  1402  to enable computer control of the rotation. 
         [0071]    In some embodiments, an onboard computer  1426  (e.g., a small computer available from gumstix, Portola Valley, Calif.) is placed in the centrifuge tube  1416  or in the modified swing bucket  1422 . The onboard computer is capable of locally analyzing the video received from CCD camera  1412 ; the results of the analysis are stored on SD card  1418 . In other embodiments, a wireless communication device  1428  is used in addition to or in place of SD card  1418  so that the video feed from the detector can be transmitted in real time and analyzed with a remote computer. Alternatively, the wireless communication device communicates with the onboard computer such that analyzed results can be transmitted in real time. 
         [0072]    Referring to  FIG. 16 , in still other embodiments, a fiber optic rotary joint  1600  integrated into centrifuge  1402  enables high speed communications between the standard laboratory environment (i.e., computer  1419 ) and the rotating sample. Data is transmitted from detector  1412  to an Ethernet-to-fiber media converter  1604  and then to external computer  1419  via fiber optic cable  1602  passing through fiber optic rotary joint  1600 . Power for the electronic components in centrifuge tube  1416  may also be transmitted in a similar fashion, allowing dynamic control of these components during rotation of the small-scale system. 
         [0073]    The principle of operation of small-scale spinning force system  1400  is similar to the operating principle of the spinning force system  100  described above. In spinning force system  1400 , sample chamber  1408  is loaded with functionalized microspheres (e.g., 3 μm latex beads coated with molecule “A”) and a functionalized cover slip (e.g., a glass cover slip coated with molecule “B”). The “A”-functionalized beads are brought into contact with the “B”-functionalized cover slip for a short time, for instance by turning centrifuge tube  1416  upside down. The lens tube  1414  and other components, such as SD card  1418 , battery  1420 , and electronic components  1424 , are secured within centrifuge tube  1416  and/or swing bucket  1422 . CCD camera  1412  is enabled such that video recording to the SD card  1418  is initiated. The centrifuge is properly balanced, if necessary, and rotation is started. Once the centrifuge is spinning, the “A”-functionalized particles will experience a force away from the “B”-functionalized cover slip. As described above, some particles bound to the cover slip via A-B molecular complexes will separate from the cover slip at various times; this time signature reveals the force-dependent dissociation of the A-B complex. Video analysis of the resulting data is performed after centrifugation is completed using particle detection techniques described herein. 
       4 Applications 
       [0074]    Using the centrifugal force (and possibly other forces) of a spinning sample, spinning force system  100  is capable of performing single-molecule experiments for application in a wide range of areas, including receptor-ligand interactions, DNA mechanics, the kinetics of motor proteins, and the dynamics of intramolecular transitions such as protein folding and unfolding. In addition, spinning force system  100  can also be used for high-throughput molecular screening in which thousands of single-molecule experiments of the same or different types can be performed in parallel and/or in series. This can be particularly useful for drug discovery and screening. The following section provides several scenarios in which spinning force system  100  is useful. 
       4.1 Example I 
       [0075]    One application of spinning force system  100  relates to studying the molecular interactions between two or more interacting molecules or molecular complexes, including, for example, measuring the association rate K on  and dissociation rate K off  of the interaction, identifying metastable states, and determining the transition rates between such states. Examples of interacting molecules suitable as subject of study include receptor-ligand pairs, such as biotin-streptavidin, antibody-antigen, enzyme-substrate, and DNA-polymerase. 
         [0076]    Referring to  FIGS. 5A-5D , one procedure of preparing a sample containing two interacting molecules A (e.g., biotin) and B (e.g., streptavidin) for measurement is illustrated. 
         [0077]    For example, in  FIG. 5A , a cover glass  510  is coated with a dispersed layer of molecule A  512  using proper functionalization techniques such as physisorption or covalent linkage. 
         [0078]    In  FIG. 5B , a solution of glass beads  520  is then added onto cover glass  510 . Each bead is pre-coated with one or more molecules B  522 , again using proper functionalization techniques. 
         [0079]    In  FIG. 5C , cover glass  510  is incubated with glass beads  520  to enable formation of interaction  530  between molecules A and B. Consequently, some beads  520  become attached to the surface of cover glass  510  through newly formed interactions  530 . Unattached beads can be removed from the surface by rinsing with an inactive solution, or alternatively, removed by a centrifugal force applied during measurement. In some examples, the amount of interactions  530  formed during the incubation can be controlled, for example, by the temperature and time duration of the incubation, the concentration of pre-coated bead solution, and the density of molecule A  512  seeded on cover glass  510 . 
         [0080]    In  FIG. 5D , once the sample is ready for measurement, it is loaded onto spinning force system  100 . In this example, cover glass  510  is configured to be parallel to rotational axis  102  of system  100 . The rotation of the sample therefore results in a centrifugal force F on bead  520 , pulling it away in a direction perpendicular to the surface of cover glass  510 . The movement of bead  520  can be monitored using one or more of the imaging techniques described above. When the magnitude of centrifugal force F is sufficiently high to rupture interaction  530 , bead  520  is released from cover glass  510 . 
         [0081]    In this example, hundreds or thousands of molecular interactions  530  can be formed and observed in one experiment, allowing statistical analysis of such observations to be performed in a highly efficient manner. 
       4.2 Example II 
       [0082]    Another application of spinning force system  100  relates to studying the intramolecular dynamics of a single molecule or a molecular tether (e.g., a protein or a DNA tether), including, for example, measuring the folding, unfolding, stretching, and relaxation of a molecular strand. 
         [0083]      FIGS. 6A-6D  illustrate use of system  100  in a procedure for preparing a sample containing a molecular tether to be measured. 
         [0084]    In  FIG. 6A , a cover glass  610  is coated with a dispersed layer of molecule A  612  using proper functionalization techniques such as physisorption. 
         [0085]    In  FIG. 6B , a solution of glass beads  620  and a solution of molecular tethers  630  (e.g., DNA tethers as shown in  FIGS. 6B-6D  for illustrative purposes) are added onto cover glass  610 . Each bead  620  is pre-coated with one or more molecule A  612 , and each molecular tether  630  is functionalized with one or more molecule B  632  capable of forming interactions with molecule A  612 . Examples of commonly used functionalization techniques and procedures include silanization for glass substrate, formation of a peptide bond between a carboxylated surface and a free amine on a protein, and formation of disulfide bonds or S—C bonds for cysteine residues. 
         [0086]    In  FIG. 6C , cover glass  610  is incubated with glass beads  620  and molecular tether  630  to enable formation of interaction  640  between molecules A and B. Consequently, some beads  620  become attached to the surface of cover glass  610  through a newly formed interaction  640  between bead  620  and molecular tether  630  and another newly formed interaction  640 ′ between molecular tether  630  and cover glass  610 . Unattached beads can be removed from the surface by rinsing with an inactive solution, or alternatively, removed by a centrifugal force during measurement. In some examples, the amount of interactions  640  formed during incubation can be controlled, for example, by the temperature and time duration of incubation, the concentration of pre-coated bead solution and molecular tether solution, and the density of molecule A  612  seeded on cover glass  610 . 
         [0087]    In  FIG. 6D , once the sample is ready for measurement, it is loaded onto spinning force system  100 . For illustrative purposes, cover glass  610  is configured to be parallel to rotational axis  102  of system  100 . The rotation of the sample therefore results in a centrifugal force F on bead  620 , pulling it away in a direction perpendicular to the surface of cover glass  610 . The movement of bead  620  indicates the force-induced conformational changes of molecular tether  630  and can be used to measure variables characterizing the folding, unfolding, stretching and relaxation of the molecular tether in response to an external force. When the magnitude of F is varied (without exceeding the bond rupture force to break interaction  634 ), the force dependent dynamics of DNAs can also be quantified. 
         [0088]    Again, in this example, hundreds or thousands of molecular tethers  630  can be studied in a single experiment, allowing statistical analysis of the results to be performed efficiently. 
       4.3 Example III 
       [0089]    Another application of spinning force system  100  relates to studying the characteristics of molecules or molecular interactions in controlled chemical environments, including, for example, quantifying molecular dynamics at various temperatures, pH conditions, and/or salt concentrations, as well as in the presence of various kinds of surfactants and/or enzymes. 
         [0090]      FIGS. 7A-7D  illustrate use of system  100  in a procedure for preparing a sample containing a molecular tether  730  that can be modified by a restriction enzyme  760 . 
         [0091]    For example, in  FIG. 7A , molecular tether  730  (e.g., a DNA strand) is attached to a cover glass  710  and a particle  720  using preparation techniques described above with reference to  FIGS. 6A-6C . 
         [0092]    In  FIG. 7B , a solution of restriction enzymes  760  is added onto cover glass  710 . Examples of restriction enzyme  760  include an enzyme that cuts double-stranded or single-stranded DNA at specific recognition nucleotide sequences (known as restriction sites). Restriction enzymes can be used for manipulating DNA in various applications such as DNA digestion and gene insertion. 
         [0093]    In  FIG. 7C , molecular tether  730  is incubated in the solution of restriction enzymes  760  to allow one or more enzyme molecules to bind to the restriction sites of tether  730 . 
         [0094]    In  FIG. 7D , upon binding with tether  730 , restriction enzyme  760  makes incisions at selected locations in tether  730 , cutting the tether into two or more disconnected pieces. As a result, particle  720  is released from the surface of cover glass  710 . Particle release can be observed using the imaging techniques described above. 
         [0095]    Some or all of the procedures shown in  FIG. 7A-7D  may be performed during spinning. For example, the binding/interaction between tether  730  and restriction enzyme  760  may be monitored under the influence of a controlled centrifugal force applied to particle  720  as shown in  FIG. 6D . 
       4.4 Example IV 
       [0096]    Referring to  FIGS. 11A-11D , in a specific example, spinning force system  100  is used to perform massively parallel single-molecule experiments for the characterization of the force-dependent unbinding kinetics of an antibody-antigen interaction. 
         [0097]    Referring to  FIG. 11A , a cover glass  1110  is coated with a dispersed layer of digoxigenin (molecule A  1112 ). Functionalized cover glass  1110  was prepared by base washing a glass coverslip (immersed for 5 minutes in a boiling solution of 1 part 30% H 2 O 2 , 4 parts NH 4 OH, and 19 parts d-H 2 O), followed by adsorption of monoclonal anti-digoxigenin (11094400, mouse monoclonal, available from Roche, Boulder, Colo.). The cover glass was then washed with a blocking buffer composed of phosphate buffered saline with 0.1% Tween 20 and 1 mg/mL dephosphorylated alpha-casein (C8032, Sigma Aldrich, St. Louis, Mo.). The experiments described below were performed in the same blocking buffer to decrease non-specific adhesion of beads to the surface of cover glass  1110 . 
         [0098]    In  FIG. 11B , digoxigenin (molecule B  1132 ) was tethered to glass beads  1120  with a diameter of 2.8 microns by molecular tethers, e.g., DNA tethers  1130 . Monoclonal anti-digoxigenin (molecule A  1112 ) is capable of forming a bond with digoxigenin (molecule B  1132 ). More specifically, diogoxigenin labeled DNA was prepared by labeling the cohesive ends of 48 kB lambda phage DNA (N3013S, available from New England Biolabs, Ipswich, Mass.) with biotin (biotin-14-dATP and biotin-14-dCPT, available from Invitrogen) using Klenow polymerase (M0212S, available from New England Biolabs), follows by purification using, for instance, QiaQuick purification kit available from Qiagen (Valencia, Calif.). The biotinylated DNA was then cut in half with the XbaI restriction enzyme (R014SS, New England Biolabs) and re-purified. The overhanging ends of the resulting 24 kB DNA were filled in and labeled with a single digoxigenin (dig-11-dUTP, available from Roche) or plain nucleotides for dig-labeled or unlabeled DNA. Finally, the DNA was re-purified and the dig-DNA was mixed with unlabeled DNA in a 1:4 ratio. The mixture of dig-DNA and unlabeled DNA was reacted with streptavidin labeled beads (Dynabeads M-270, available from Invitrogen, Carlsbad, Calif.) for use in the experiment. 
         [0099]    In  FIG. 11C , cover glass  1110  is incubated with a solution of the DNA-functionalized glass beads  1120  to enable the formation of bonds between molecules A and B. Consequently, some beads  1120  (e.g., beads 1 and 2) become attached to the surface of cover glass  1110  through a newly formed bond between molecules A on cover glass  1110  and molecules B attached to beads 1 and 2. Other beads, such as bead 3, are non-specifically bound to the surface of cover glass  1110 , for instance by hydrogen bonding. In some examples, the amount of bonds formed during incubation can be controlled, for example, by the temperature and time duration of incubation, the concentration of pre-coated bead solution, and the density of molecule A  1112  seeded on cover glass  1110 . 
         [0100]    In  FIG. 11D , once the sample is ready for measurement, it is loaded onto spinning force system  100  and rotated at a constant velocity in order to apply a uniform force field to all of the beads in a direction perpendicular to the surface of cover glass  1110 . Some singly-tethered beads (e.g., bead 1) responded by moving away from cover glass  1110  by a distance x consistent with the compliance of the double-stranded DNA tether  1130 . Other singly-tethered beads (e.g., bead 2) detached from cover glass  1110  as a result of the rupture of the bond between molecules A and B. Non-specifically bound beads (e.g., bead 3) remained at or near the surface of cover glass  1110 . 
         [0101]    Referring to  FIGS. 12A-12D , time lapse images show the progress of the bond rupture experiment illustrated in  FIG. 11D . The images are obtained from videos recorded at 10 frames per second. At the start of the experiment ( FIG. 12A ), thousands of anti-digoxigenin functionalized beads  1122  are bound to cover glass  1110 . Prior to beginning the experiment, the objective lens ( 150  in  FIG. 1 ) of spinning force system  100  is focused at a distance of one tether-length (x) away from the surface of cover glass  1110  ( FIG. 12B ). Since all the beads are resting on the surface of the cover glass at the start of the experiment, the beads are not in the focal plane of the objective lens and thus appear blurry. When a centrifugal force is generated that pulls the beads away from cover glass  1110 , beads tethered by a single DNA molecule move into focus ( FIG. 12C ). As single receptor-ligand bonds (i.e., digoxigenin-anti-digoxigenin bonds) rupture, beads detach from cover glass  1110 , resulting in fewer visible beads over time ( FIG. 12D ). 
         [0102]    Force clamps ranging from hundreds of femtoNewtons to several picoNewtons were applied using spinning force system  100 . Analysis of videos of the experiment was performed by identifying locations of fully tethered beads at a frame near the beginning of the movie (once full rotational speed was reached) and analyzing small regions of interest at each location in subsequent frames to determine the time of bond rupture. Bead locations were determined by performing a background subtraction, making a binary image, and finding the center positions and the variance of a region of interest around each bead. The image variance is important because non-specifically adsorbed (“stuck”) beads can be distinguished from tethered beads by their variance. Stuck beads were excluded from the analysis. The rupture time for each bead was identified by a dramatic drop in the measured variance to near zero, corresponding to a grey, bead-free image. In rare cases, where multiple drops in the variance were observed for a particular bead, that bead was excluded form analysis, as this behavior indicated the possibility of a multiple tether. 
         [0103]    Referring to  FIG. 13 , a histogram of the bond rupture time obtained using spinning force system  100 , with a 20 second bin width (10 seconds for the highest optical trap force), was fit with a decaying exponential curve with no offset. The resulting time constants and associated fitting errors were used to determine the force-dependent off-rate k off (f)=k 0  exp(f/f β ) for the interaction of digoxigenin and its antibody using an Arrhenius approach with force-dependence modeled by a single, sharp barrier. 
         [0104]    The force imparted on the beads by spinning force system  100  was calculated as F=mω 2 R, where m=6.9×10 −12  g is the mass of the bead (calculated using the manufacturer&#39;s bead specifications of 2.8 mm diameter and 1.6 g/cm 3  density), R=385.5 mm is the distance of the bead from the axis of rotation, and w is the magnitude of the angular velocity of the system. A small additional force from the weight of the bead was included using the Pythagorean theorem. The data plotted in  FIG. 13B  includes data obtained using spinning force system  100  (filled triangles) and using an optical trap system (open triangles). Each spinning force system data point was obtained from a single experiment lasting a few minutes. Plotted error bars result from the uncertainty in a least squares fit to the data. A stress-free off-rate of k 0 =0.015±0.002 s −1  and a force scale of f β =4.6±1.3 pN were found. These results are in good agreement with previously determined optical trap data and fall within the margin of error of previous AFM experiments. 
       4.5 Example V 
       [0105]    Another application of spinning force system  100  relates to studying different types of molecular interactions and/or molecular dynamics in parallel. This application allows a direct comparison of molecular events of similar or different nature, such as the mobility of two different motor proteins, the compliance of various pieces of DNA strands, and the affinity of various receptor-ligand pairs. To distinguish the observations of motion characteristics resulted from distinct molecular events, each type of event can be uniquely labeled, for example, using fluorescent tracers or other kinds of markers. For example, in cases where two types of cytoskeletal proteins (e.g., microtubules and actin filaments) are studied at once, each type can be labeled by fluorescent beads of distinct emission wavelengths. By using selected optical filters to alternate observation of photons of different colors, the motion characteristics of each type of cytoskeletal proteins can be distinguished based on the resulting fluorescent images. 
       4.6 Example VI 
       [0106]    Although the previous examples are provided primarily in the context of measuring molecular dynamics or interactions, spinning force system  100  is also useful in measuring characteristics of interactions that occur on a cellular and/or tissue level. One example is to study the adhesion strength between adherent cells (e.g., endothelial or epithelial cells) and the underlying substrate or matrix to which the cells adhere. A monolayer of adherent cells can be seeded on a receptor-modified (e.g., fibronectin-coated) cover glass to form adhesion complexes (e.g., clusters of adhesion molecules such as integrins, syndecans, and cadherins). The cover glass is then mounted to rotary arm  120  and cells are monitored during rotation. The magnitude of centrifugal force that detaches cells from the cover glass indicates the maximum adhesion strength of the adhesion complexes. 
       4.7 Example VII 
       [0107]    Another application of spinning force system  100  relates to using system  100  in conjunction with other detection or measurement techniques (e.g., various kinds of single-molecule force probes or fluidic systems) to obtain force-related characteristics of the sample. For example, system  100  can be combined with a magnetic system to apply both magnetic and centrifugal forces to manipulate a test subject. A biotin labeled magnetic bead can be first pulled by a magnetic force toward a cell membrane (or a streptavidin-treated surface) to form a biotin-streptavidin attachment. The strength of this attachment can then be tested by applying a centrifugal force that pulls the bead away from the membrane. 
         [0108]    System  100  can also be combined in use with devices that apply various types of chemical stimuli (e.g., chemoattractants and enzymes) and mechanical stimuli (e.g., compression, extension, and shearing) to a test subject, simulating the native environment of the subject during observation. For example, when system  100  is used for measuring the mechanical properties (e.g., elasticity or viscoelasticity) of endothelial cells, during spinning, a laminar or cyclic shear stress may also be provided to mimic the conditions of the endothelial cells in vivo. Additionally, cellular response to chemical/mechanical stimuli may also be observed. 
       4.8 Other Examples 
       [0109]    In addition to detecting characteristics of biomolecular or chemical interactions, system  100  is also suitable for use with a wide range of general testing tasks. Examples include 1) measuring the mechanical properties of polymer networks or aggregates, where the overall behavior of the network may be of interest; 2) testing the strength and other characteristics of physical bonds formed between a subject and a surface; and 3) observing the behavioral changes of test subjects effected by chemical agents (e.g., an enzyme slicing a single DNA between two surfaces or urea denaturing a protein). 
         [0110]    The use of a fiber optic rotary joint with small-scale system  1400  is generally useful for communicating with rotating samples, for instance for applications related to monitoring blood sorting. 
       5 Alternative Embodiments 
       [0111]    Many variations of spinning force system  100  are possible. 
         [0112]    Referring to  FIG. 8 , an alternative embodiment of spinning force system  800  includes a coaxial detection assembly  820  mechanically coupled to a rotary stage  810 . Detection assembly  820  includes a LED lamp  830 , a sample holder  840 , an objective lens  850 , and a CCD camera  860 , each being coaxially aligned with optical axis  870 . LED lamp  830  is integrated into sample holder  840 , which can be translated (e.g., along optical axis  870 ) through a focus knob for coarse adjustment. Fine adjustment of the position of sample holder  840  can be performed using a piezoelectric stage coupled to the holder (not shown). When rotary stage  810  operates in circular motion at angular velocity ω, the entire detection assembly  820  rotates at ω. 
         [0113]    In some implementations, detection assembly  820  may be miniaturized to fit inside a standard centrifuge, eliminating the need for rotary stage  810  and reducing system cost. 
         [0114]    Referring to  FIG. 9 , another alternative embodiment of spinning force system  900  is shown. Unlike detector  160  in system  100 , detector  960  in this example is not mounted on rotary arm  920 . Rather, detector  960  remains immobile during operation of system  900 . Light from objective  950  are directed by a rotating mirror  970  to detector  960  through light paths  952  and  954  subsequently. Rotating mirror  970  is not necessarily mounted on rotary arm  920 . For example, mirror  970  can be coupled to a separate rotor (not shown) or directly coupled to rotary stage  910  in order to reflect light from the rotating objective  950  to the stationary detector  960 . 
         [0115]    In the above examples of  FIGS. 1 ,  8 , and  9 , the samples are detected using trans-illumination imaging techniques. More specifically, in  FIG. 1 , light  132  emitted from LED  130  transmits (penetrates) through sample  140  prior to being received by objective  150 . In other examples, epi-illumination imaging techniques can also be conveniently used. 
         [0116]    Referring to  FIG. 10 , a further alternative embodiment of spinning force system  1000  with epi-illumination is shown. Here, a Dichroic mirror  1070  is positioned between detector  1060  and objective  1050 , serving as a beam splitter that directs light beams of different characteristics (e.g., different wavelengths) onto different paths. For example, light beam  1032  from LED  1030  is first deflected by Dichroic mirror  1070  towards objective  1050  (along light path  1032 ′) to illuminate a selected region of sample  1040 . A light beam  1034  is reflected from sample  1040  as a result of the illumination, travelling in a direction opposite to light beam  1032 ′. After passing through objective  1050  and Dicroic Mirror  1070 , light beam  1034  is received by detector  1060  to produce an image of the illuminated region of sample  1040 . In this example, objective  1050  serves as both a light condenser and an imaging lens. 
         [0117]    In some embodiments, a vertical swinging or radially movable arm may be integrated to system  100  to enhance the flexibility of force control. System  100  may also be multiplexed to have many arms, each configured to carry a unique experiment. Each arm may also have a different and possibly adjustable length to facilitate force control. 
         [0118]    In another embodiment, the samples are detected using surface plasmon resonance techniques. For instance, referring to the example of  FIGS. 5A-5D , a baseline surface plasmon resonance signal is detected for the initial configuration of  FIG. 5A , in which only molecules A  512  are present on the surface of the sample. As glass beads  520  are attached to the surface of the sample via interactions between molecules A  512  and B  522 , the surface plasmon resonance signal increases. When the sample is rotated in spinning force system  100 , a centrifugal force F is applied to bead  520 . The rupture of interaction  530 , which releases bead  520  from the surface of the sample, causes a decrease in the surface plasmon resonance signal. The detection of the surface plasmon resonance allows for the study of properties such as the binding constant between molecule A and molecule B. 
         [0119]    It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the following claims.