TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Methods: The step of forming the perimeter barrier comprises making a structural form that prevents reactant from one plenum from moving into the other plenum, or forming a perimeter barrier comprises depositing a barrier material on a portion of at least one of the process layers.. The method includes making a thin flat fuel cell layer, thin curvilinear fuel cell layer, or volume enclosing fuel cell layer. The step of forming a perimeter barrier comprises forming a barrier from barrier material and then joining the formed barrier material to portion(s) of process layer(s). The forming of process layers includes creating process layers with a rectangle, a square, a triangle, an annular ring, an arc, an irregular shape, and other prismatic shapes. The step of forming the process layers is by depositing a precursor material on a substrate. The method includes (partially) filling low aspect ratio cavities with material(s) to facilitate the transport of reactant material. The step of (partially) filling the low aspect ratio cavity or cavities comprises forming a micro-structure in the low aspect ratio cavity or with porous media and a catalyst layer. The step of forming of the barrier material comprises ablating, etching, stamping, printing, milling, die cutting, molding, casting, water jetting, the barrier material or depositing of additional material on a substrate. The step of creating intermediate assemblies comprises joining 2-100,000 intermediate assemblies. The method of claim 1, wherein the step of forming of the barrier material comprises ablating, etching, stamping, printing, milling, die cutting, molding, casting, water jetting, the barrier material or depositing of additional material on a substrate. The method step of forming of the barrier material comprises ablating, etching, stamping, printing, milling, die cutting, molding, casting, water jetting the barrier material, or depositing of additional material on a substrate. Preferred Components: The structural form can be a micro-structure or a three-dimensional structure with a tortuous path. The material comprises a catalyst, a porous material, and/or electrical conductor. Each process layer comprises an electrolyte, an ion exchange membrane, a proton exchange membrane, a filtration membrane, a separation membrane, a micro structure diffusion mixer, a heater, a catalyst, an electrical conductor, a thermal conductor, a micro-structure of a polymer, a filled micro structure of a polymer, a filled epoxy composite, a filled graphite composite, a filled metal composite, and/or plastic. The substrate comprises a release layer. The release layer comprises a polyamide, a polyethylene, or polytetrafluorethylene. The substrate is one of the low aspect ratio process layers, a unit reactor (12, 16,18, 20, 22, 24, 26, 28, 30), or a portion of a unit reactor. The substrate is (partially) filled low aspect ratio cavity. The substrate is a fuel cell or electrolyzer. Each perimeter barrier comprises a metal, a silicone, a rubber, a polyamide, a synthetic rubber, an epoxy, polytetrafluorethylene, polyvinyldifluoride, an ion exchange material, a proton exchange material, their composites, their laminates, and/or their alloy. The structural form can be a micro structure or a three dimensional structure with a tortuous path. Preferred Properties: The high aspect ratio cavities is a ratio of the length to width. The high aspect ratio is less than 1 cm/cm.