NOVELTY - Making (M1) a microstructured device, involves: i) providing first and second substrates with respective first and second bonding surfaces, where at least one of the bonding surface is formed with microstructured features; ii) exposing the bonding surfaces to solvent vapor to heal defects in surface while preserving microstructured features; iii) bringing first and second bonding surfaces into contact; and iv) applying pressure to the substrates to urge first and second bonding surfaces together to bond together the first and second substrates and form the microstructured device.    USE - For making a microstructured device (claimed) useful as tools for the analysis of a variety of fluids including chemical and biological fluids, and for sample diagnosis, and also useful in digital (droplet-based) microfluidics, molecular assays (including polymerase chain reaction (PCR) amplification chips and micro arrays for fluorescent in situ hybridization (FISH) detection of DNA/RNA sequences, liquid chromatography, protein analysis, cell separation, cell manipulation, cell culturing), microfluidic modular (bolt-on) components (e.g. pumps, valves and mixers), adaptive landscape chips to study evolutionary biology, cellular biophysics chips, optofluidic devices, acoustics based microfluidic devices, microfluidic fuel cells, cytometers, continuous flow systems, stop flow systems, multiplexed stop flow systems, flow injection analysis, segmented flow analysis, fresh water analyzers, sea water analyzers, bio-fluid analyzers and medical analyzers.    ADVANTAGE - The method heals defects in the surface of the substrate while preserving the microstructured features and reducing the surface roughness of the microstructured features; reduces surface roughness of the microfluidic channel from 200-15 nm or less. At least one of the bonding surfaces formed with microstructured features has a magnitude of surface roughness in the region of 50-250 nm prior to the exposing which reduces to less than 25 nm as a result of the exposing. Reducing the surface roughness seeks to reduce the amount of microfluidic channel surface roughness after formation from non-optical quality to optical quality. The controlled delivery and uptake of solvent to the surface containing the microstructured features is achieved by exposure to a solvent vapor atmosphere.    DETAILED DESCRIPTION - Making (M1) a microstructured device, involves: i) providing a first substrate with a first bonding surface and a second substrate with a second bonding surface, where at least one of the bonding surface is formed with microstructured features; ii) exposing at least one of the bonding surfaces to solvent vapor for a period of time to heal defects in the surface while preserving the microstructured features.; iii) bringing the first and second bonding surfaces into contact; and iv) applying pressure to the substrates to urge the first and second bonding surfaces together to bond together the first and second substrates and thereby form the microstructured device. INDEPENDENT CLAIMS are included for the following:    (1) processing (M2) a microstructured substrate to heal surface defects in it, involving: ia) providing a substrate having a surface bearing microstructured features; iia) exposing the surface to solvent vapor for a period of time to heal defects in the surface while preserving the microstructured features; and    (2) a microstructured device produced by the method (M1).