NOVELTY - A microchannel device comprises: several laminae (104, 114), in which lamina(e) has front side having front side features and back side having back side features, where front side features are fluidly connected to back side features by via(s) (108, 118); and transfer layer(s) (110) interleaving the laminae.    USE - As a microchannel device, useful for transferring heat or a substance from one fluid to another (preferably dialysis of blood) (claimed), for removal of contaminants from groundwater, separation of potable water from contaminated water sources, pasteurization of water, miniaturized heat exchange, solvent dehydration, and fuel cell power generation.    ADVANTAGE - The microchannel device provides both mass transfer and/or heat transfer. The microchannel device provides improved membrane separation by including microchannels. The reduction in channel depths reduces mass transfer limitations caused by diffusion through bulk fluid. The device overcomes another problem with membrane separation by using microfluidic devices i.e. inherent volume limitations, and also mitigates risk of cross contamination. The device includes compression sealing to reduce cross contamination of fluids. The device is effectively scaled to desired volumetric capacity, by forming stack of subunits that performs unit operation. The transfer layers comprise cut-outs in areas corresponding to inlets and outlets headers of adjacent layers. Similar to gasket this prevents the layer from collapsing into headers and occluding flow. The fluid inlets and outlets define support columns for preventing adjacent laminae from sagging into inlets and outlets and occluding flow. The vias connecting both sides are orthogonal to microchannels, and intersect laminae at non-orthogonal angle, which minimizes areas of fluid stagnation or gas entrapment. The device can be configured in various ways. When subunits are identical, and are oriented with front side of first lamina facing same compression plate, with microchannels of adjacent layers within subunit parallel or orthogonal to each other; then subunits with parallel microchannels and orthogonal to adjacent subunits, maintain compression seal around headers. Adjacent subunits can also be mirror images of each other and can be oriented such that front sides of first laminae and front sides of second laminae of adjacent subunits face each other. This provides a simplified gasket design, and reduces cross contamination, as layers having same fluid are adjacent to each other. Individual laminae can also have more than one independent flow path, when both back and front sides contain inlet and outlet headers and microchannels. This reduces number of lamina required per transfer unit. The devices having through-cut microchannels ameliorate cross contamination risk while realizing efficiency of double-sided device, in which portion of microchannels of each lamina extends through entire lamina i.e. from front to back side to form through-cut microchannels; and microchannels simultaneously operate with two separation units. The device fabrication can be performed by conventional high volume, high through-put, and low cost fabrication techniques.    DETAILED DESCRIPTION - A microchannel device comprises: several laminae, in which lamina(e) (104, 114) has front side having front side features and back side having back side features, where front side features are fluidly connected to back side features by via(s) (108, 118); and transfer layer(s) (110) interleaving laminae. First and second laminae and transfer layer form subunit; and device comprises several subunits, and gaskets positioned between subunits. The front side features comprise inlet (102) and outlet headers (116); and back side features comprise microchannels (106, 112), in which inlet header is fluidly connected to microchannels by inlet via(s), and outlet header is fluidly connected to microchannels by outlet via(s). The back side features comprise a flow field defining several wall segments. The microchannels of each lamina are substantially parallel or orthogonal to microchannels of adjacent lamina(e). The microchannels of adjacent subunits are orthogonal. At least one via is non-orthogonal to front side and back side, and intersects front and back sides at non-orthogonal angle. The transfer layer is positioned between back sides of first and second laminae, where front sides of first laminae and front sides of second laminae of adjacent subunits are mirror images of each other and face each other. The back sides of first and second laminae further comprise inlet header and outlet header, and front sides of first and second laminae further comprise microchannels fluidly connected to inlet and outlet headers on back side. The transfer layers are substantially sealed to adjacent laminae between inlet and outlet headers and microchannels. A portion of microchannels extend from front side to back side to form through-cut microchannels, which are fluidly connected to inlet via(s) by first partial thickness microchannels, and to outlet via(s) by second partial thickness microchannels; and microchannels are formed by partial thickness wall segments arranged in herringbone pattern, where adjacent wall segments alternate between flush with front and back sides of lamina. Transfer layer(s) are mass transfer layer (preferably a membrane or semi-permeable membrane; especially fluid membrane comprising lamina having through-cut microchannels containing fluid, and two supports), heat transfer layer, and/or define cutouts corresponding to headers. INDEPENDENT CLAIMS are included for the following:    (1) transferring heat or a substance from one fluid to another, involving: providing a microchannel transfer device having the first and second lamina(e) interleaved with transfer layer(s), in which laminae have front and back sides, and front side has inlet header fluidly connected to several microchannels on back side by inlet via(s), microchannels fluidly connected to outlet header on front side by outlet via(s), and first layer for receiving first fluid(s) (preferably blood) and second layer for receiving second fluid(s) (preferably dialyzate); and providing first and second fluids to inlet header of first and second laminae, respectively; and    (2) making microfluidic device involving: aligning several laminae and interleaving several laminae with several transfer layers to form stack; positioning aligned laminae and transfer layers between first and second compression plates; and compressing stack.    DESCRIPTION OF DRAWING(S) - The figure shows counter current flow diagram of first and second fluids in microchannel device.    Microchannel inlet for first fluid (102)    Upper lamina for flow of first fluid (104)    Microchannel (106, 112)    Vias (108, 118)    Transfer layer (110)    Lower lamina for flow of second fluid (114)    Microchannel outlet for second fluid (116)