A fluidic device comprising: an article comprising first and second surfaces; a first microfluidic channel segment (26, 28, 30) formed in the first surface of the article, wherein the first microfluidic channel segment has a non-circular cross-section; a second microfluidic channel segment (34, 36) formed in the second surface of the article, wherein the second microfluidic channel segment has a non-circular cross-section; a first intervening channel (40, 44, 48, 50) passing through the article from the first surface to the second surface and connecting the first microfluidic channel segment and the second microfluidic channel segment, wherein the intervening channel has a circular cross-section; a first fluid reagent (85, 87) contained in the first microfluidic channel segment; a second fluid reagent (82, 84) contained in a different first microfluidic channel segment, separated by a second intervening channel (40, 44, 48, 50) passing through the article from the first surface to the second surface and connecting the different first microfluidic channel segment and the second microfluidic channel segment, wherein the second intervening channel has a circular cross-section or in the second microfluidic channel segment; an inlet in fluid communication with the first and second microfluidic channel segments; an outlet (64, 64-A) in fluid communication with the first and second microfluidic channel segments; a first cover positioned over the first microfluidic channel segment(s) so as to enclose the first microfluidic channel segment(s); and a second cover positioned over the second microfluidic channel segment so as to enclose the second microfluidic channel segment, wherein the first microfluidic channel segment(s) is/are connected to the second microfluidic channel segment to form a single continuous channel, and wherein the inlet and the outlet are sealed prior to first use of the fluidic device. an article comprising first and second surfaces; a first microfluidic channel segment (26, 28, 30) formed in the first surface of the article, wherein the first microfluidic channel segment has a non-circular cross-section; a second microfluidic channel segment (34, 36) formed in the second surface of the article, wherein the second microfluidic channel segment has a non-circular cross-section; a first intervening channel (40, 44, 48, 50) passing through the article from the first surface to the second surface and connecting the first microfluidic channel segment and the second microfluidic channel segment, wherein the intervening channel has a circular cross-section; a first fluid reagent (85, 87) contained in the first microfluidic channel segment; a second fluid reagent (82, 84) contained in a different first microfluidic channel segment, separated by a second intervening channel (40, 44, 48, 50) passing through the article from the first surface to the second surface and connecting the different first microfluidic channel segment and the second microfluidic channel segment, wherein the second intervening channel has a circular cross-section or in the second microfluidic channel segment; in a different first microfluidic channel segment, separated by a second intervening channel (40, 44, 48, 50) passing through the article from the first surface to the second surface and connecting the different first microfluidic channel segment and the second microfluidic channel segment, wherein the second intervening channel has a circular cross-section or in the second microfluidic channel segment; an inlet in fluid communication with the first and second microfluidic channel segments; an outlet (64, 64-A) in fluid communication with the first and second microfluidic channel segments; a first cover positioned over the first microfluidic channel segment(s) so as to enclose the first microfluidic channel segment(s); and a second cover positioned over the second microfluidic channel segment so as to enclose the second microfluidic channel segment, wherein the first microfluidic channel segment(s) is/are connected to the second microfluidic channel segment to form a single continuous channel, and