A chemical assay device includes a hydrophilic substrate and one or more hydrophobic structures that extend from a first side of the hydrophilic substrate to a second side of the hydrophilic substrate. A hydrophobic structure in the hydrophilic substrate forms a fluid barrier wall that extends from the first side of the hydrophilic substrate to the second side of the hydrophilic substrate with a deviation of less than 20° from a perpendicular axis between the first side and the second side. The hydrophobic structure is formed from a wax or a phase change ink.