the present embodiment consists of a means for dialyzing species in microchannel devices that is based on the species &# 39 ; size . utility is achieved by polymerizing a thin porous polymer membrane across a channel intersection within the microchannel device . a membrane of about 0 . 5 μm to about 20 μm in thickness can be used for this purpose . because the shape and thickness of the membrane is controlled primarily by a uv light beam used to initiate a polymerization reaction in a solution contained within a microchannel , control of the excitation light beam focus and collimation can be used to control the membrane thickness . the thickness of the membrane is also negatively effected by photo - initiated radical diffusion , solvent - phase polymer diffusion , and bulk fluid motion within the fluid microchannel . these factors can be controlled by eliminating bulk fluid flow before initiating polymerization , and by the incorporation of polymerization inhibitors to minimize radical diffusion . in preparing the desired membrane , various monomers and solvents may be chosen to provide a polymerized membrane having a specific distribution of pore size and one which incorporates specific molecules into the membrane that impart a specific property to the membrane and therefore to the membrane pore structure . such membranes , therefore , can be adapted or “ engineered ” to pass molecules having a specific size or having a specific protein molecular weight cutoff ( as measured in dalton units ). moreover the choice of monomer / solvent combinations can be used to dictate polymer properties such as ( i ) pore size ; ( ii ) mechanical strength , which can be enhanced by using high polymer cross - linking density ( using for example , 1 % to 100 % of polyfunctional acrylates such as pentaerythritol triacrylate , polyfunctional methacrylate , such as 1 , 3 butanediol dimethacrylate , or polyfunctional acrylamide , such as methylene bisacrylamide ); ( iii ) hydrophobicity / hydrophilicity , which can be controlled through the choice of monomers , e . g ., ethylene glycol diacrylate , or zwitterionic molecules , for hydrophilicity , and alkyl - acrylates for hydrophobicity ; and ( iv ) polymer charge , which can be controlled through incorporation of charged monomers into the membrane , such as for example , [ 2 -( acryloyloxy ) ethyl ] ammonium methyl sulfate salt ( moe ) for positive charge , 2 - acrylamido - 2 - methyl - 1 - propanesulfonic acid ( amps ) for negative charge . of all of these properties , however , pore size is most common and most important . by utilizing carefully chosen appropriate combinations of monomers and solvents such as are shown in table 1 , pore sizes may be adjusted from small to large in the dialysis membrane . in particular , for a given concentration of solute , solvents that are characterized as “ strong ” with respect to the solute monomer provide for a smaller average pore size upon polymerization , while solvents characterized as “ weak ” provide for a larger average pore size . utilizing a monomer such as spe ( n , n - dimethyl - n -( 2 methacryloyl oxyethyl )- n -( 3 sulfopropyl ) ammonium betaine ) and a solvent such as water , an average pose sizes of 1 nm to 3 nm is achieved , while a monomer such as pentaerythritol triacrylate with a solvent such as 1 - propanol , the measured pore size is about 30 nm . this embodiment of the invention allows for two or more liquids ( one sample liquid and one or more perfusion liquids ) to be brought into contact on a microfluidic chip separated only by a thin ( 0 . 5 μm - 100 μm ) photopatterned porous polymer membrane ; concentration gradient - driven diffusion will cause those molecules whose size is smaller than the membrane pore size to be transported from sample through the membrane to the perfusion liquids . implementing this in a microfluidic chip format allows molecules having a size range of interest to be transported to analysis channels ( e . g ., chemical separation ), to reaction zones ( labeling , enzymatic ), or to off - chip sites for mass spectrometry . a variety of geometries may be used to implement on - chip dialysis , including co - flow and counter - flow operation , single - and multiple - membrane configuration , straight and tortuous path configuration , and both single - pass and recirculating configurations . in particular , fig5 illustrates an example of a counter - flow geometry wherein the dialysis is 1 cm in length . standard glass microchips having conventional cross - shaped channels were obtained from micralyne ; chemicals were obtained from aldrich and used as received . in order to facilitate bonding between the glass surfaces within the channels and the polymer membrane , the glass surfaces within the microchannels were first exposed to a 2 : 2 : 1 ( by volume ) mixture of water , glacial acetic acid , and 3 -( trimethoxysilylpropyl ) acrylate for a period of 30 minutes , covalently linking the silane to the wall and exposing the acrylate group for polymerization . following surface treatment , the microchannels are filled with a monomer / solvent / photo - initiator solution comprising the following formulation . a monomer mixture consisting of 95 % ( by weight ) of spe ( n , n - dimethyl - n -( 2 methacryloyl oxyethyl )- n -( 3 sulfopropyl ) ammonium betaine ) cross - linked with 5 % ( by weight ) n , n ′- methylene bisacrylamide is prepared . the monomer mixture is subsequently incorporated into a quantity of water to yield a 40 : 60 monomer : solvent solution and includes 0 %- 30 % ( by weight ) of an organic additive to help control pore size and a small amount of a buffer solution to control the ph of the solution mixture . in the present formulation , the organic additive was about 2 % ( by weight ) 2 - methoxyethanol , although c1 - c3 alcohols or acetonitrile could be used also ) and the buffer solution was about a 2 % ( by weight ) 10 mm concentration of a phosphate buffer solution to maintain the monomer / solvent solution mixture at a ph of 5 . 5 . lastly , a small quantity of a commercial grade photo - initiator is added to the monomer / solvent solution mixture to render the solution sensitive to uv light exposure . in the present case , the photo - initiator was 2 , 2 ′- azobis ( 2 - methylpropionamide ) dihydrochloride , purchased from wako chemicals usa , inc ., a division of wako pure chemical industries , ltd ., osaka , japan , under the trade name of v - 50 ®. this material is added to the monomer / solvent solution in concentrations of generally about 10 mg / ml of the monomer solution and complete the polymerizable solution formulation used to create the dialysis membrane of the present invention . the other monomer / solvent solution mixture formulations are , of course , possible , including each of those listed in table 1 . other photo - initiators are also possible , particularly [ 2 , 2 ′- azobis - isobutyronitrile ], also known as aibn or v - 40 ®, again purchased from wako chemicals usa , inc . however , the formulation recited above is preferred for practicing dialysis as described herein . after preparing the interior surfaces of the microchannel system and filling it with the single phase monomer / solvent solution the intersection region of the microchannels was then exposed to a focused , collimated beam of uv laser light , shown in fig2 . as this beam of light interacts with the single phase solution a phase - separation polymerization reaction is initiated ( and catalyzed by the presence of the photo - initiator ) within the cross - sectional region of the microchannel into which the laser light is imaged . the polymerization reaction eventually produces the desired porous membrane within the microchannel as shown schematically in fig1 . actual images of operational membranes are shown in fig3 b and 3c as well as fig4 b - 4 e . as shown in fig1 and 2 , a thin ( 4 μm - 14 μm ) porous polymer membrane is fabricated in - situ in glass micro channels by projection lithography ; shaping and focusing the 355 nm output of a 12 khz , 800 ps - pulse , 160 nj - pulse , frequency - tripled nd : yag laser into a 1 - 2 μm sheet and using this sheet to generate photo - initiated phase separation polymerization in the irradiated region . the thickness of the laser sheet was minimized by spatially filtering the focused laser output beam with a 2 μm slit and imaging the resulting diffraction pattern at ˜ 0 . 5 magnification onto the desired channel location into which the membrane is to be formed . as noted above , a related photolithography technique is described in commonly owned u . s . patent ser . no . 10 / 141 , 906 . however , this reference recites a contact photolithographic process that is inoperable in the present case . because the imaging light beam must propagate through roughly a millimeter of glass covering the embedded microchannel in which the membrane is to be formed , the incoming light is subject to degradation due to the effects of diffraction and dispersion . in order to overcome these problems the applicants have adapted projection photolithography techniques for focusing an image of the desired structure cross - section into the region of the microchannel and thus avoiding the problems of image integrity in the former technique as applied to the present embodiment . the process is described in greater detail in “ voltage - addressable on / off microvalves for high - pressure microchip separations ”, ( j . chromatography a ; 979 , pp . 147 - 154 , 2002 ), herein incorporated by reference . the final thickness of the membrane , however , is determined by factors that include more than just the optical properties of the incident laser beam sheet . the membrane thickness is also effected by diffusion of radical species , by solved - phase polymer diffusion , and by bulk fluid motion . effects of radical diffusion are reduced by retaining the natural polymerization inhibitors present in the system ( 15 ppm hydroquinone monomethyl ether , solved o 2 ); this effectively decreases the chemical lifetime and diffusion length of the radical products of photo - dissociation . laser excitation was terminated upon the onset of phase separation . phase separation was inferred from light scattering from the membrane - fluid interface . following polymerization , the system was flushed thoroughly with 1 - propanol and water to remove residual polymer / monomer / solvent material and then filled with aqueous solutions for testing . the nominal pore size of the present embodiment of porous polymer was established to be about 1 nm to about 3 nm as measured with mercury porosimetry , bet , and with sem . fig3 a - c illustrate one embodiment of the present invention . fig3 a shows a schematic of the channel configuration . the operation of the porous membrane is shown in fig3 c by filling the channel assembly on one side of the polymerized membrane with an aqueous solution of fluorescein ( mw = 0 . 33 kda , ø ≈ 1 nm ); or as shown in fig3 b with an aqueous suspension containing 200 nm , carboxylate - modified , fluorescein - impregnated latex spheres ( molecular probes ®), while filling the opposite side of each of these channel assemblies with water . both solutions were allowed to come to rest and the extent of species migration ( fluorescein or latex spheres ) across the membrane observed over a period of several minutes using 488 nm light to excite fluorescence in the fluorescein . as can be seen in fig3 c , fluorescein readily diffuses across the membrane while in fig3 b the 200 nm latex spheres do not , suggesting that the pore size cutoff for this membrane is below 200 nm since fluorescein molecules ( having a “ diameter ” that is about 1 nm ) pass freely through the membrane while the latex spheres are blocked . this observation is corroborated with sem , hg porosimetry , and bet porosimetry . a second embodiment is shown in fig4 a - f wherein the membrane , shown as element 40 diagonally separating intersecting fluid channels 41 and 42 , is subjected to a similar test as is illustrates in fig3 b and 3c . in the present case , however , the test was modified to improve the granularity of the attempt to determine the molecular weight cut - off of the spe membrane . in this case , the microchannel system was exposed to free dye ( rhodamine 560 , mw = 0 . 37 kda , ø ≈ 1 nm ) and a solution containing fitc - labeled proteins with different molecular weights . in particular , the response of insulin ( mw = 5 . 7 kda ), lactalbumin ( mw = 14 kda , ø ≈ 5 - 6 nm ), bovine serum albumin ( mw = 66 kda ), and anti - biotin ( mw = 150 kda ) in their ability to diffuse through the membrane was tested . fig4 a and 4b show the rapid permeation of the rhodamine dye through the membrane . as seen in fig4 b , at 20 seconds after its introduction the rhodamine dye has already migrated well into both arms of the fluid channels to the right of the membrane 40 . however , fig4 c and 4d show that insulin ( 5 . 7 kda ) experiences only barely measurable diffusion through the membrane , and fig4 e and 4f show that lactalbumin presents virtually no measurable diffusion across the membrane even after a residence time of over 12 minutes . the larger species , i . e ., those having mw & gt ; 14 kda , also show no diffusion and for brevity are not shown . these preliminary results , therefore , demonstrate that control of molecular weight cutoff through these porous polymer membranes is achievable by precisely engineering the constitution of water / 2 - methoxyethanol solutions . finally , because combinations of monomers and solvents may be chosen to provide specific pore size distributions ( as noted above ), those skilled in the art will realize that a dialysis device may be provided having a plurality of membranes each exhibiting a unique specific pore size which would allow for isolating particles in any specific size range for any specific application . moreover , the method described herein is applicable to many different geometries . fig2 and 3a illustrate a simple variation of the present technique wherein the membrane diagonally separates a junction made by two intersecting channels and is an example of cross - flow dialysis . fig5 illustrates a counter - flow geometry wherein the membrane divides a single channel that connects two widely separated channel junctions by interconnecting a series of intermediate spaced support posts . the geometry of fig5 has been successfully fabricated with membranes lengths of up to 1 cm . fig6 a - 6 d illustrate additional embodiments of the counter - flow geometry shown in fig5 wherein the membrane divides the separation channel 60 once , in the case of fig6 a or twice , as in the case of fig6 b . as before the dialysis structure is fabricated by interconnecting a series of intermediate spaced posts 62 which bisect fluid channel 61 with short segments 63 of the polymer membrane . it is also possible to construct a separation channel capable of selecting species having a graded series of molecular weights ( sizes ). as shown in fig6 c , wherein channel network 60 contains groups 67 and 68 of membrane segments 63 spaced out along the length of polymer membrane 69 . two groups are shown but it is obvious that more groups could be used . the structure achieves its utility for selecting particles having more than one range of molecular weights when each of the segments of a particular group of segments is fabricated with a polymer material that has a different average molecular cut - off pore size and when the groups are arranged in a logical order ( ascending or descending ) for its intended use . the particular configuration shown in fig6 c allows for molecular species with increasing molecular size to pass from the sample stream as the stream passes along the length of the membrane . while two sections are shown in fig6 c in principle any number of sections are possible . finally , as shown in fig6 d the length of the separation network of fig6 a can be increased by convoluting the fluid channel . this allows for compact structures while still allowing for sufficient dialysis length to achieve the intended separation result . it is , therefore , apparent that due to the flexibility of the present process other geometries are possible and are limited only by the routineer &# 39 ; s ability to provide the necessary lithographic tools .