FIELD OF THE INVENTION 
       [0001]    The invention relates to magnetic microspheres and methods of forming the microspheres. The microspheres have many possible applications including, but not limited to, smart drug delivery, braking and damping devices, a controllable elastomer ball array and a single spherical micro-damper. The invention also relates to a method of dispensing a liquid, especially but not necessarily a drug, using the microspheres. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the field of medicine, it is known to encapsulate a drug inside a microsphere in order to provide a method of delivery to a patient, e.g. by ingestion. In a separate development, in recent years, attention has been given to use of magnetic drug carriers for the purpose of guiding the drug to a specific location by a magnetic field, or for use in magnetic resonance imaging. 
       SUMMARY OF THE INVENTION 
       [0003]    It would be desirable to control the time at which a liquid, especially a drug, is released from a microsphere and/or to provide a mechanism for triggering the release of the liquid. Furthermore it would be desirable to control the rate at which a liquid is released. While the invention may find application to many liquids, it will be especially useful where the liquid is a drug. 
         [0004]    A first aspect of the present invention provides a microsphere comprising a shell and a core. The core comprises a liquid while the shell comprises magnetic particles. The microsphere is deformable in response to a magnetic field and may be caused to expand and/or contract by application of a magnetic field. Deformation of the microsphere causes some or all of the liquid in the core to be dispensed. 
         [0005]    Preferably the core comprises a drug in liquid form. In this way the time at which the liquid (e.g. the drug) is released may be controlled. For example, release of the drug may be initiated or accelerated by application of a magnetic field to deform the microsphere. Furthermore, the rate at which the drug is released may be controlled by varying the strength, or other characteristics, of the magnetic field. 
         [0006]    The magnetic field may be varied, for example in the form of a periodic waveform. This causes the microsphere to expand and contract sequentially, which has a pumping effect on the liquid in the core causing it to be released from the microsphere. 
         [0007]    Preferably the microsphere is elastically deformable. The shell may comprise a plurality of magnetic particles in an elastically deformable substrate. The microsphere preferably comprises magnetic particles in a matrix of organic material. The organic material may for example, be an elastomer, a silicon oil, or any suitable polymer. Preferably the shell is cross-linked. 
         [0008]    The drug is preferably water soluble. The drug may, for example, comprise aspirin or amoxicillin. The magnetic particles may comprise magnetite. In one embodiment the shell comprises magnetite particles, an aqueous solution of acetic acid and chitosan. 
         [0009]    Preferably the microsphere has a diameter of 50 to 200 microns. The diameter refers to the diameter of the microsphere in the ‘undeformed’ substantially spherical rest state. 
         [0010]    A second aspect of the invention provides a method of forming a microsphere comprising the steps of providing a first fluid, providing a second fluid comprising magnetic particles and mixing said first and second fluids such that the first fluid forms a core of the microsphere and the second fluid forms a shell of the microsphere. Preferably the first fluid comprises a drug. 
         [0011]    Preferably the shell formed by the second fluid is elastically deformable. In a preferred arrangement the first and second fluids are mixed using a fluid flow device comprising a plurality of channels. Preferably the device has at least first, second and third channels, with the second channel being provided between the first and third channels. The first and third channels join with the second channel at a first junction. The first fluid is injected into the second channel and the second fluid is injected into the first and third channels. The junction has a flow focusing function that causes the first fluid in the second channel to form a core which is enveloped by the second fluid from the first and third channels. Each channel has a flow rate and the flow rates of the channels are adjusted so as to form a co-axial jet with the first fluid forming a core enveloped by a shell of the second fluid. 
         [0012]    The fluid flow device may have fourth and fifth channels outward of the first to third channels. The fourth and fifth channels join said second channel at a second junction which is downstream of said first junction. A carrier fluid is injected into the fourth and fifth channels. The carrier fluid may be an oil, for example sunflower oil. It enables the microspheres formed by the first and second fluid to pass through the channel downstream of the second junction, without adhering to the sides of the channel. 
         [0013]    The method preferably further comprises a process for solidifying the shell formed by the second fluid. The solidifying process may comprise one or more of the following: photo-polymerization, heating, hydrolysis, condensation, de-wetting coacervation or another chemical reaction. Here “dewetting” means drawing the solvent out of a droplet&#39;s outer layer. The solidifying process may comprise adding a de-wetting agent (e.g. to draw out water) from the shell of the microsphere. The de-wetting agent may be injected into the channel downstream of the first or second junction. Suitable de-wetting agents include, but are not limited to, butanol, ethanol and propanol. The solidifying process may comprise a cross-linking reaction. For example, a cross linking re-agent may be injected into the fluid flow device, preferably downstream of the first or second junction. In one embodiment glutaraldehyde was used as cross linking re-agent, however other alternative re-agents will be apparent to a person skilled in the art. The solidifying process may comprise heating the microsphere, for example by heating the fluid flow device or a channel within the fluid flow device. The solidifying process may involve quenching in order to absorb excess de-wetting agent. For example, a quenching agent may be injected into a channel of the fluid flow device. In one embodiment, the quenching agent comprised oleic acid. 
         [0014]    A third aspect of the present invention provides a method of dispensing a liquid comprising providing a microsphere having a liquid core and a shell comprising magnetic particles and applying a magnetic field across the microsphere in order to deform the microsphere. 
         [0015]    Preferably the magnetic field is varied and the microsphere expands and contracts as the magnetic field is varied. This has a pumping effect on the liquid causing at least a portion of the liquid to be expelled from the microsphere. The magnetic field may be varied periodically in a regular fashion (i.e. a periodic waveform). The magnetic field may be an alternating magnetic field and may be provided by an electromagnet. 
         [0016]    When the sphere is deformed, e.g. into an ellipsoidal shape, liquid from the core is leaves the microsphere through the shell. The shell is preferably permeable to the liquid in the core, at least when it is stretched. The stretching may increase the degree of permeability. Preferably the shell is elastically deformable. 
         [0017]    The core liquid preferably comprises a drug. In this way the method may be used to control delivery of a drug. In particular the time of release and the rate of release may be controlled by application of the magnetic field. The rate of release is influenced by the strength of the magnetic field, the frequency of the magnetic field and the type of waveform. A stronger magnetic field and a higher frequency both lead to an increased rate of release. The magnetic field may for example be varied in a step wave form, sinusoidal wave, or a saw wave. The more rapid the increase or decrease of the magnetic field the more rapidly the drug is released. Thus the step wave form causes a higher rate of release than the sinusoidal waveform. 
         [0018]    A fourth aspect of the present invention provides an apparatus including one or more microspheres according to the first aspect of the present invention and a device for generating a magnetic field and varying said magnetic field. The device may, for example, be an electromagnet. The device has a controller for varying the strength of the magnetic field and a controller for varying the frequency of the magnetic field. 
         [0019]    The apparatus can be used to control the release of a liquid, especially a drug, from the core of the microsphere. 
         [0020]    A fifth aspect of the present invention provides a microsphere comprising magnetic particles surrounded by elastically deformable material; the microsphere being elastically deformable in response to application of an external magnetic field. Preferably the modulus of elasticity of the elastically deformable material is at least 120 kPa (1.2×10 5  N/m 2 ), more preferably at least 200 kPa. In some arrangements the modulus of elasticity may be in the range 200−1000 kPa, or 350 to 800 kPa. Preferably the stiffness of the microsphere may also be varied in response to application of a magnetic field. 
         [0021]    The ability of the microsphere to change its shape and/or stiffness in response to a magnetic field is very useful and has many applications. For example the microsphere may be used in braking and damping devices, a controllable elastomer ball array or as a spherical micro-damper. 
         [0022]    The microsphere preferably comprises an elastically deformable organic material and magnetic particles. 
         [0023]    Preferably the organic material is a hydrocarbon oil; most preferably a silicon oil. The silicon oil may be PDMS. Alternatively the organic material may be a rubber based material. The magnetic particles are preferably nano or microsized. 
         [0024]    In one arrangement the microsphere comprises a magnetic core surrounded by an organic material shell. The magnetic core may be a magnetic colloid. In this case the organic material shell is elastically deformable. When a magnetic field is applied the magnetic core may expand or contract in response to the magnetic field and the shell deforms to accommodate this expansion or contraction. 
         [0025]    In an alternative arrangement the microsphere is a single phase structure comprising a plurality of magnetic particles dispersed in an organic substrate. The magnetic particles are preferably magnetite. The magnetic particles may be nano or micro sized. ‘Single phase structure’ means a one phase structure, rather than a core shell structure. Typically the one phase structure will be a organic material (e.g. a gel) in which a plurality of magnetic particles are dispersed. The one phase microsphere may comprise silicone oil, PDMS gel and magnetic particles, the magnetic particles being 20% by weight. 
         [0026]    A sixth aspect of the present invention provides a microsphere comprising magnetic particles in an elastically deformable matrix; the stiffness of the microsphere being variable in response to application of a magnetic field. 
         [0027]    A seventh aspect of the present invention provides a method of making the microsphere of the fifth or sixth aspect of the present invention comprising the step of providing a fluid flow device having at least first, second and third channels, said second channel being provided between said first and third channels, said first and third channels joining with said second channel at a first junction; injecting a first fluid is comprising magnetic material into the second channel and injecting a second fluid into the first and third channels; mixing the fluids at the junction such that the first fluid forms a core and the second fluid forms a shell of the microsphere. This method makes a core-shell microsphere. 
         [0028]    An eighth aspect of the present invention provides a method of making the microsphere of the fifth or sixth aspect of the present invention comprising the step of forming droplets of a fluid in a fluid flow device, the fluid comprising an organic material and magnetic particles. 
         [0029]    Preferably the fluid flow device comprises first and second channels which join together to form a third channel, the junction having a flow focusing effect. The fluid may comprise PDMS gel, silicone oil and magnetic particles at a concentration of 20% by weight. This method makes a single phase microsphere. 
         [0030]    Features of any of the fifth to eighth aspects of the invention may be combined together. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0031]    Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:— 
           [0032]      FIG. 1  shows a microsphere comprising a liquid drug core and a shell with magnetic particles; 
           [0033]      FIGS. 2  ( a ) to  2  ( e ) show the reaction of the microspheres to a magnetic field; 
           [0034]      FIG. 2  ( f ) shows deformation of the microspheres with increasing magnetic field; 
           [0035]      FIG. 3  ( a ) shows drug release over time for different strengths of magnetic field; 
           [0036]      FIG. 3  ( b ) shows drug release over time for different frequencies of magnetic field; 
           [0037]      FIG. 3  ( c ) shows drug release over time for a step waveform magnetic field; 
           [0038]      FIG. 3  ( d ) shows drug release over time for a sinusoidal magnetic field; 
           [0039]      FIG. 4  shows an apparatus for dispensing a drug from microspheres; 
           [0040]      FIG. 5  ( a ) shows a fluid flow device for forming a magnetic microsphere; 
           [0041]      FIGS. 5  ( b ) to  5  ( e ) are diagrams showing steps in the process of forming the microsphere; 
           [0042]      FIGS. 6  ( a ) to  6  ( e ) show steps in solidifying the shell of the microsphere; 
           [0043]      FIG. 6  ( f ) is a FITR spectrogram showing the presence of chitosan and cross-linked chitosan in the microsphere; 
           [0044]      FIG. 7  ( a ) is a SEM cross section of core shell structure; 
           [0045]      FIG. 7  ( b ) is a SEM cross section of a solid structure; 
           [0046]      FIG. 8  shows the deformation of a core shell microsphere under application of a magnetic field; 
           [0047]      FIG. 9  ( a ) shows a microfluidic chip for forming the microspheres; 
           [0048]      FIG. 9  ( b ) is a schematic diagram of some channels of the microfluidic chip of  FIG. 9  ( a ) 
           [0049]      FIG. 10  is a schematic diagram of an apparatus for measuring the deformation and deformation force when a magnetic field is applied to the microsphere; 
           [0050]      FIG. 11  ( a ) is a graph showing the variation in deformation force with magnetic field for solid and core-shell microspheres; and 
           [0051]      FIG. 11  ( b ) is a graph showing the variation in deformation force for various degrees of mechanically applied deformation for both solid and core-shell microspheres in the presence of a magnetic field and in the absence of a magnetic field. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0052]      FIG. 1  shows a microsphere comprising a core  110  and a shell  120 . The core comprises a drug in liquid form, for example aspirin in an aqueous solution. The core comprises magnetic particles in an organic material substrate or matrix. In a preferred embodiment the shell comprises magnetite particles in a chitosan substrate. The shell  120  is preferably cross-linked, e.g. the chitosan may be cross-linked to form a solid skin for the microsphere. 
         [0053]    The microsphere is deformable by application of an external magnetic field.  FIG. 2  ( a ) shows the microsphere when the magnetic field is zero. It has a diameter of d.  FIG. 2  ( b ) shows the microsphere when the magnetic field is increased and the microsphere is stretched along the direction of the field into an ellipsoidal shape. This is due to the magnetic particles forming chains aligned along the field direction, due to the dipole-dipole interaction between the particles. When stretched the microsphere has a diameter of d+Δd along its longest axis. When the magnetic field strength reached 3000 G, the observed deformation          d/d of a core-shell microsphere was 6.3%. 
         [0054]    The external field also tends to align randomly dispersed microspheres into a line along the direction of the field. This is shown in  FIGS. 2  ( c ) to  2  ( e ). 
         [0055]    This stretching phenomena also occurred when the magnetic field was periodically varied as an alternating magnetic field (e.g. a field produced by an AC electromagnet).  FIG. 2  ( f ) shows the elongation of the microspheres under 100 G, 200 G and 300 G magnetic fields when frequency of the field was 5 Hz. The x axis of the graph represents the magnetic field strength, while the y axis of the graph represents the aspect ratio (d+Δd)/d. As can be seen from the graph, the degree of extension could be adjusted by varying the strength of the magnetic field. It was observed that when the microspheres changed their shapes from spherical to ellipsoidal, the drug solution was released through the cross-linked shell. This effect is discussed in more detail below. 
         [0056]    To study the effect of magnetically responsive behavior on drug release rate, we measured the aspirin release under magnetic field. Aspirin-release characteristics were determined by UV/VIS spectra carried out with Perkin Elmer Lambda 20 UV/VIS spectroscopy, where the aspirin concentration indication line was chosen at the maximum absorption wavelength of 296 nm. For each measurement, microspheres (2 g) were stored in a dialysis bag (4×4 cm). The aspirin release efficiency was accessed in 3 ml PBS. In order to study the aspirin release activated under an applied magnetic field, an electromagnet with two parallel poles (PHEWY Products, Germany) was used to generate a homogeneous magnetic field, in which the dialysis bag was placed. A PM 5139 function generator (PHILIPS) and stereo power amplifier 216 (THX, LUCASFILM Co.) were used to create pulsed signals. Absorbance measurement was performed on a ND-1000 spectrophotometer at λ ex =495 nm (DIAMed). 
         [0057]    Magnetic fields with different strength, frequency and time variation profile were applied to the sample. We used a dialysis technique to measure the release and the percentage of cumulatively released aspirin C was defined as 
         [0000]    
       
         
           
             
               
                 
                   C 
                   = 
                   
                     
                       
                         C 
                         i 
                       
                       
                         C 
                         a 
                       
                     
                     × 
                     100. 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Here C i  is a cumulative amount of released aspirin for each measurement, while C a  is the total amount of aspirin—the encapsulated amount. We defined the total amount of aspirin as that in a 1 ml syringe which is 10 mg, since for each measurement a 1 ml syringe of aspirin solution was used to make one dialysis bag. Each measurement was repeated three times and the average value was calculated. 
         [0058]      FIG. 3  ( a ) is a graph showing the cumulative release of aspirin over time for microspheres to which magnetic fields of between 0 and 300 G were applied. The frequency of the magnetic field was fixed at 5 Hz. It can be seen that the applied magnetic field enhanced the rate of release. Increasing the strength of the magnetic field increased the rate of release. 
         [0059]      FIG. 3  ( b ) is a graph showing the cumulative release of aspirin over time for microspheres to which magnetic fields of frequencies between 0 and 20 Hz were applied. The field strength was fixed at 300 G. A higher frequency increased the aspirin release rate/efficiency as the frequency was varied from 0 Hz to 20 Hz. 
         [0060]    It can be seen from the insets in  FIGS. 3  ( a ) and  3  ( b ) that over a period of 6 hours, the rate of release of the aspirin could be enhanced by up to 9% by increasing the field strength from 0 to 300 G. Meanwhile, over 6 hours the amount of released aspirin could be enhanced by up to 26% by varying frequency. Tables 1 and 2 list the amount of aspirin released under different magnetic field strengths and frequencies. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 The amount of aspirin released from the microspheres under different 
               
               
                 magnetic field when frequency is 5 Hz. 
               
             
          
           
               
                 Time (h) 
                 0 G 
                 50 G 
                 100 G 
                 150 G 
                 200 G 
                 250 G 
                 300 G 
               
               
                   
               
               
                 1 
                 2.51 
                 2.56 
                 2.60 
                 2.62 
                 2.64 
                 2.74 
                 2.84 
               
               
                 2 
                 3.47 
                 3.56 
                 3.65 
                 3.69 
                 3.73 
                 3.89 
                 4.06 
               
               
                 3 
                 4.18 
                 4.27 
                 4.37 
                 4.44 
                 4.50 
                 4.64 
                 4.78 
               
               
                 4 
                 4.64 
                 4.71 
                 4.79 
                 4.88 
                 4.96 
                 5.08 
                 5.20 
               
               
                 5 
                 5.05 
                 5.12 
                 5.18 
                 5.27 
                 5.36 
                 5.53 
                 5.70 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 The amount of aspirin released from the microspheres under different 
               
               
                 frequency when magnetic field is 300 G. 
               
             
          
           
               
                   
                 Time (h) 
                 0 Hz 
                 5 Hz 
                 10 Hz 
                 15 Hz 
                 20 Hz 
               
               
                   
                   
               
               
                   
                 1 
                 2.52 
                 2.66 
                 2.81 
                 2.94 
                 3.07 
               
               
                   
                 2 
                 3.47 
                 3.76 
                 4.03 
                 4.25 
                 4.46 
               
               
                   
                 3 
                 4.18 
                 4.54 
                 4.91 
                 5.23 
                 5.57 
               
               
                   
                 4 
                 4.65 
                 5.20 
                 5.50 
                 5.92 
                 6.37 
               
               
                   
                 5 
                 5.05 
                 5.70 
                 6.00 
                 6.51 
                 7.06 
               
               
                   
                 6 
                 5.38 
                 5.98 
                 6.45 
                 6.98 
                 7.56 
               
               
                   
                   
               
             
          
         
       
     
         [0061]    In both Tables 1 and 2 the amount of aspirin released is in units of mg. So, for example, in Table 1 at 0 G of magnetic field 2.51 mg of aspirin where released after 1 hour. 
         [0062]    Further experimentation showed that the time variation profile of the magnetic field also had an effect on the aspirin release rate. Both a step function magnetic field and a sinusoidal magnetic field were tested.  FIG. 3   c  shows the variation in cumulative aspirin release for step function and sinusoidal magnetic fields of different strengths, with the frequency fixed at 20 Hz. It can be seen that as the field strength was raised at a fixed frequency (20 Hz), both the step function and sinusoidal time profiles resulted in a linear enhancement of the cumulative aspirin release. However, is the step function time profile shows a steeper slope than the sinusoidal one. 
         [0063]      FIG. 3  ( d ) shows the variation in cumulative aspirin release for step function and sinusoidal magnetic fields of different frequencies, with the field strength fixed at 300 G. The two different time profiles yield almost the same aspirin release percentages for frequencies up to 5 Hz, but beyond that the step function time profile has a clear advantage in enhancing the aspirin release rate. Indeed for frequencies above about 10 Hz the rate of release for the sinusoidal time profile started to decline. 
         [0064]    In general terms, the aspirin release rate is sensitive to sudden changes in the magnitude of the applied field. Thus the step function field leads to a larger rate of release than the sinusoidal field. It is believed that a saw wave function would be between these two extremes, having a greater rate of release than the sinusoidal function but less than the step function 
         [0065]    While reference has been made above to aspirin as the drug it is to be understood that any drug in liquid form could be substituted for the aspirin. 
         [0066]      FIG. 4  shows an apparatus for dispensing a liquid, e.g. a drug in liquid form, from the core of a microsphere. The apparatus comprises an electromagnet  210  for generating a varying magnetic field. The electromagnet  210  is powered by an AC current. Controller  220  controls the frequency, strength and time profile (e.g. step function, sinusoidal etc) of the magnetic field. The apparatus further comprises a container  200  containing a plurality of microspheres. The microspheres have a drug in liquid form in the core and a shell comprising magnetic particles in an organic matrix. The microspheres are elastically deformable in response to application of a magnetic field from the electromagnet  210 . The container has an outlet  230  for to dispensing the drug released from the microspheres. The rate of release of the drug may be controlled by controlling the magnetic field as discussed above. 
         [0067]    In other cases, the microspheres may be ingested by the patient and the patent may carry a device for generating an appropriate magnetic field (e.g. an electromagnet is which may be put in a pocket or worn on a strap. 
         [0068]    Methods and apparatus for manufacturing the microspheres will now be described. 
         [0069]    A preferred method of forming the microspheres is to use a microfluidic flow device.  FIG. 5  ( a ) shows a schematic diagram of an microfluidic flow (MFF) device, also known as a microfluidic chip. The device has first  10 , second  20 , third  30 , fourth  40  and fifth  50  channels, each with an inlet tube for injecting fluid into the channel. The first  10 , second  20  and third  30  channels join at a first junction  15  and combine to form a single sixth channel  60  (the ‘main channel’). The fourth  40  and fifth  50  channels join with this sixth channel  60  at a second junction  25  downstream of the first junction  15 . The second channel  20  is in the middle between the first and third channels. A seventh channel  70  joins the main channel  60  from the side and may be used to inject a de-wetting solution. Finally the MFF device has an outlet  80  at the end of the sixth channel  60 . The first to fifth channels are preferably 200 μm in both depth and width. The main channel  60  is preferably 900 μm in width and in depth. However, the channels narrow towards the junctions in order to focus the flow of the fluids. 
         [0070]    The MFF chip can be fabricated by first fixing a plexiglass channel mold on a plexiglass substrate. The mold is generated by laser cutting the substrate (e.g. laser cutting machine MT-MCSERIES). Polydimethylsiloxane (PDMS) gel is poured on the mold and placed in vacuum for 20 min to ensure layer uniformity. After curing, the PDMS layer with embedded channels was peeled off. This PDMS layer is then bonded to another half-cured PDMS layer to form sealed channels. Alternatively the MFF may be fabricated using soft-lithography. 
         [0071]    The MFF device is used to make microspheres with a liquid core and a shell comprising magnetic particles. A first fluid which is a drug in liquid form (e.g. an aqueous solution of aspirin or amoxicillin) is injected into the second channel  20 . Meanwhile, a second fluid which contains magnetic particles, is injected into the first and third channels. The first junction  15  has the effect of focusing the flow so that the liquid drug from the second channel is enveloped by a shell of the magnetic particle containing fluid from the first and third channels. At the second junction  25 , the fluid is from the second channel is sheared to form droplets which are carried along down the main channel  60  by the carrier fluid from the fourth and fifth channels. This process is shown in  FIGS. 5  ( b ) to  1  ( e ). 
         [0072]    A side channel  70  injects a de-wetting solution and cross-linking reagent to the main channel, which causes the shell of the droplets to solidify. Thus microspheres with a solid shell and liquid core are formed. The microspheres are carried along by the carrier fluid and deposited from the outlet  80 . 
         [0073]    An example of how the microspheres are formed and the chemical processes involved is described in more detail below. 
         [0074]    Magnetite nanoparticles for the second fluid were prepared by a co-precipitation method. An aqueous solution containing 2 M hydrochloric acid and ferrous chloride was first mixed with 1 M ferrite chloride. 0.7 M ammonia solution was then added. The mixture was stirred for 30 mins and centrifuged at a speed of 10000 rpm; the supernatant solution was removed by decantation. The precipitates were rinsed with deionized water until pH=7 and then freeze-dried. The magnetic nanoparticles were dispersed by ultrasonic treatment for 3 hours in aqueous solution (100 ml) comprising low molecular weight chitosan (1.5 g) and 2 w/v % acetic acid. The suspension was subsequently centrifuged and freeze-dried to obtain the modified nanoparticles. 
         [0075]    To prepare drug loaded microspheres, we used 1 w/v % aqueous solution of aspirin or amoxicillin as model drugs which were infused through the second channel  20 . The model drugs injected was enveloped by streams composed of 2.5 g modified magnetite particles in 150 ml aqueous solution of 2 w/v % acetic acid and 1.5 w/v % of high molecular weight chitosan from the first and third channels  10 ,  30 . The chemical hexadecan, with 0.5 w/v % span-80 was used as the carrying fluid and injected through the outermost channels (fourth channel  40  and fifth channel  50 ). With proper control of the relative flow rates of the streams in different channels, the drug solution in the second channel becomes unstable and breaks into uniform droplets. A core-shell double emulsion comprising an inner core of drug in liquid form (e.g. aspirin solution) and an outer shell of organic material and magnetic particles (e.g. is high molecular weight chitosan, embedded with magnetic nanoparticles) was thus formed. We employed a “dewetting” effect and cross-linking reaction to solidify the shell of the microsphere. 
         [0076]    The seventh channel  70  was used to inject a dewetting solvent, n-butanol, and a cross-linking reagent, glutaraldehyde (comprising 10 w/v % glutaraldehyde in butanol). The solubility of n-butanol in water is 9.1 ml/100 ml at room temperature. When the core-shell droplets (shown in  FIG. 6  ( a )) flowed through the mixture of glutaraldehyde and butanol, water in the droplets was gradually drawn out by butanol, resulting in a thin outer layer (see  FIGS. 6  ( b ) and ( c )). We also tried ethanol, 1-propanol (both are fully miscible in water) and 1-pentanol (3.3 ml/100 ml water) as alternative “dewetting” reagents. The time required to fully dehydrate droplets varied. For ethanol and 1-propanol, the dehydrating time was less than 5 s. For n-butanol and 1-pentanol, the times were 20 s and 60 s respectively. In this experiment, the calculated residence time of droplets flowing in channel was 5-10 s, so we used n-butanol to partially absorb the water. The resulting particles were then baked at 60° C. for 2 hr to facilitate Schiff&#39;s base reaction between glutaraldehyde and chitosan (see  FIG. 6  ( e )). By this reaction, a cross-linked layer was eventually formed (see  FIGS. 6  ( d ), ( e ) and ( f )). We used Fourier transform infrared spectroscopy (FTIR) to confirm the primary amide bands of chitosan at 1655 cm −1  split into peaks at 1635 cm −1  and 1650 cm −1  which are assigned to the C═N imines absorption. The increased intensity of C—H stretching vibration frequency at 2936 cm −1  can also reflect the contribution of the glutaraldehyde molecules in the cross-linking chain (see  FIG. 6(   f )). However, if the residence time is longer than dehydrating time, over-permeation of water can occur and lead to the complete shrinkage of the core-shell microspheres. To prevent over-dehydration of the droplets, a quenching process was preferably used to absorb the de-wetting agent. For example, where the de-wetting agent was butanol it could be absorbed by adding a solution of oleic acid in hexane (30 v/v %). We used three syringe pumps to control flow rates of the different liquids. The droplet sizes were adjustable in the range between 40 μm to 200 μm by varying the relative flow rates. 
       Magnetic Microspheres as Elastomers 
       [0077]    Another embodiment of the invention will now be described. In this embodiment the microspheres do not have a liquid drug in the core. However, the microspheres are deformable by a magnetic field and have a stiffness which may be varied by application of a magnetic field. They may be used as magnetic elastomers in braking, shock absorption, as actuators or in other applications. 
         [0078]      FIG. 7  ( a ) is a cross sectional SEM (scanning electron microscope) image of a core-shell microsphere  200 . The core  210  of the microsphere is a magnetic colloid. The shell  220  of the microsphere is elastically deformable and comprises PDMS. In other embodiments the shell may be a different elastically deformable material. The modulus of elasticity of the shell should be at least 120 kPa (1.2×10 5  N/m 2 ), more preferably at least 200 kPa. In some arrangements the modulus of elasticity may be in the range 200−1000 kPa, or 350 to 800 kPa. 
         [0079]      FIG. 7  ( b ) is a cross sectional (scanning electron microscope) image of a one phase microsphere  300 . The microsphere  300  comprises magnetic particles  310  in an organic material matrix  320 . The magnetic particles are randomly dispersed in the microsphere. The organic material matrix is elastically deformable, it may for example comprise PDMS or a rubber material. The organic material matrix should have a modulus of elasticity of at least 120 kPa (1.2×10 5  N/m 2 ), more preferably at least 200 kPa. In some arrangements the modulus of elasticity may be in the range 200−1000 kPa, or 350 to 800 kPa. 
         [0080]    Both the core-shell microsphere and the one phase microsphere can be deformed by application of a magnetic field. The core-shell arrangement of  FIG. 7(   a ) is less deformable (e.g. less highly compressible) than the one phase arrangement of  FIG. 7(   b ).  FIG. 8  shows how the core shell microsphere is deformed by application of a magnetic field B. As can be seen, the magnetic particles in the core align with each other and stretch out along the field lines causing the shell to deform (stretch). The deformation is discussed in more detail below. However, first the method of forming the microspheres will be described. 
         [0081]    The microspheres are preferably fabricated by a microfluidic flow device (MFFD), e.g. as shown in  FIGS. 9  ( a ) and  9  ( b ). The device has first  410 , second  420 , third  430 , fourth  440  channels and fifth  450  channels, each with an inlet tube for injecting fluid into the channel. The first  410 , second  420  and third  430  channels join at a first junction  415  and combine to form a single sixth channel  460  (the ‘main channel’). The fourth  440  and fifth  450  channels join with this sixth channel  460  at a second junction  425  downstream of the first junction  415 . The second channel  420  is in the middle between the first and third channels. The MFF device has an outlet  480  at the end of the sixth channel  460 . The first to fifth channels are preferably 200 μm in both depth and width. The main channel  460  is preferably 900 μm in width and in depth. However, the channels narrow towards the junctions in order to focus the flow of the fluids. Two glass capillaries (e.g. 150 mm in inner diameter) may be inserted at the two junctions for this purpose. In that case, the gaps between the glass capillaries and the inner channel walls may be filled with PDMS gel to prevent stream branching. 
         [0082]    The MFF chip can be fabricated by first fixing a plexiglass channel mold on a plexiglass substrate. The mold is generated by laser cutting the substrate (e.g. laser cutting machine MT-MCSERIES). Polydimethylsiloxane (PDMS) gel is poured on the mold and placed in vacuum for 20 min to ensure layer uniformity. After curing, the PDMS layer with embedded channels was peeled off. This PDMS layer is then bonded to another half-cured PDMS layer to form sealed channels. Alternatively the MFF may be fabricated using soft-lithography. 
         [0083]    A method of making the core-shell microspheres of  FIG. 7  ( a ) will now be described. A first fluid which comprises magnetic particles (e.g a magnetic colloid) is injected into the second channel  420 . Meanwhile, a second fluid which is a PDMS gel is injected into the first and third channels. By adjusting the flow rates of the five channels appropriately, the first junction  415  has the effect of focusing the flow so that the magnetic colloid from the second channel is enveloped by a shell of the PDMS fluid from the first and third channels. At the second junction  425 , the fluid from the to second channel breaks up to form droplets which are carried along down the main channel  460  by the carrier fluid from the fourth and fifth channels. The droplets quickly form a spherical shape due to interfacial tension. Thus microspheres with a solid shell and liquid core are formed. The carrier fluid from the fourth and fifth helps to prevent the PDMS gel from touching the main channel walls. 
         [0084]    The microspheres are carried along by the carrier fluid and deposited from the outlet  80 . The core-shell microspheres were solidified by heating in a beaker  490  containing sunflower oil at 120° C. Under this relatively high temperature, the PDMS gel was rapidly cured. Solid and monodispersed microspheres were obtained by continuously stirring the oil during the solidification process. 
         [0085]    In one example, the first fluid was a magnetic colloid having a viscosity of 140 mPa and consisting of magnetic nanoparticles Fe3O4_dispersed in sunflower oil at a particle concentration of 30% by weight of Fe3O4. The second fluid was PDMS gel diluted by silicone oils at a concentration of 77% by weight PDMS, having a viscosity of 600 mPa. The carrier fluid was sunflower oil, in particular Soon Hup Co. Ltd., sunflower oil with a viscosity of 60 mPa, which was found particularly suitable due to its nonwetting characteristics. 
         [0086]    The microfluidic device of  FIG. 9  ( a ) was also used to fabricate solid one phase microspheres consisting of magnetic particles embedded in PDMS matrix. This was done by replacing the PDMS gel in the first and third channels with PDMS-based mixtures having a viscosity of 830 mPa and consisting of PDMS gel, silicone oil, and magnetic particles at a particle concentration of 20%_by weight of magnetic particles. The central second channel was not used. 
         [0087]    To gain a more quantitative understanding of the mechanical properties of both the core-shell microspheres and solid (one phase) microspheres, we studied their magnetostrictive effect using the setup illustrated in  FIG. 10 . The set comprised a horseshoe electromagnet  500  with a 5 mm gap between its ends. The electromagnet  500  was driven by a direct current amplifier. A microsphere was placed in the gap and the deformation process of the microsphere was monitored by a charge coupled device camera and recorded with a video recorder. A plastic rod of 1 mm in diameter and a microforce meter  510  were used to measure the force as the microsphere was is deformed by the magnetic field. The plastic rod was positioned with a first end linked to a microforce meter  510  and the second end passing through a hole drilled on an opened arm. The second end of the plastic rod just touched the surface of the microsphere  520 . 
         [0088]    The plastic rod was adjusted to ensure that it did not touch the wall of the drilled hole. The microforce meter was modified by a precision weighting balance _OHAUS, ANALYTICAL Plus with an accuracy of 0.01 mg. The exact position of the setup could me moved up and down by a step motor with a step size of 5 μm. The force acting on the microsphere was measured by the micro-force meter  510 . The force was reset to zero when the second end of the plastic rod just contacted the microsphere in the absence of a magnetic field. 
         [0089]      FIG. 11  ( a ) shows the measured force as a function of the applied magnetic field. The application of a magnetic field led to an elongation of the microsphere along the field direction. The resulting push against the rod caused a measurable deformation force represented by the y-axis of  FIG. 11  ( a ).  FIG. 11  ( a ) shows the measured deformation force for both a core-shell microsphere and a solid (one phase) microsphere. The results for the core shell microsphere are shown as circles, while the results for the solid microsphere are shown as squares. For both types of microsphere, the deformation force increased monotonically as a function of applied magnetic field. The deformation force tended to saturation when the field strength was above 1500 G. 
         [0090]    In  FIG. 11  ( b ) the measured deformation force is plotted as a function of Δd/d. Where d is the diameter of the microsphere and Δd is the change of diameter. Deformation of the microsphere was caused by using the step motor to drive the stage upward in measured amounts. As the stage moved upward the plastic rod compressed the microsphere causing its horizontal diameter to expand. The reaction force felt by the rod was measured by a microforce meter. For a fixed Δd/d the graph shows two values for the force; firstly the force when a constant magnetic field of 3000 G was applied and secondly the force when no magnetic field was applied. For both solid and core-shell microspheres, there was a clear increase in the measured force by approximately 5 mdyn at a fixed value of Δd/d. In other words presence of a is magnetic field caused the microsphere to be harder and more resistant to deformation from an external force. In other words the application of an external magnetic field increased the deformation modulus of the microsphere. PDMS is magnetically inactive and therefore this increase in stiffness is thought to be due to a combination of interaction of the magnetic particles with the applied magnetic field and interaction between magnetic particles. 
         [0091]    In summary, for both core-shell microspheres and solid microspheres, the deformation (stretch along the direction of the magnetic field) and the stiffness of the microsphere increases with the applied magnetic field. However, for the core-shell microsphere the stretching deformation tends to saturate earlier when the magnetic field reaches a certain critical value (about 1500 G in  FIG. 11  ( a )). Further, as can be seen in  FIG. 11  ( b ), the core-shell microsphere is less compressible than the solid microsphere. 
         [0092]    Although the above description exemplifies a few embodiments of the present invention, it should be understood that various omissions, substitutions, and changes in the form of the detail of the apparatus, system, and/or method as illustrated as well as the uses thereof, may be made by those skilled in the art, without departing from the spirit of the present invention. Consequently, the scope of the present invention should not be limited to the disclosed embodiments.
The microspheres have many possible applications including smart drug delivery, breaking and damper devices. In one arrangement, a microsphere comprises a shell 120 and a core 110. The core comprises a liquid, which may be a drug, while the shell comprises magnetic particles. The microsphere is deformable in response to to application of an external magnetic field. Also disclose is an arrangement in which a microsphere has a magnetic core and a PDMS shell.