Flow cell surface patterning

A method, comprising:

• applying a polymeric hydrogel to a surface of a substrate including:
• a first region that is covered by an ultraviolet light blocking layer;
• a second region that is adjacent to the first region and that is free of the ultraviolet light blocking layer;
• and
• depressions separated by interstitial regions, each depression defined within at least a portion of the first region and within at least a portion of the second region;
• before or after applying the polymeric hydrogel, grafting a primer set to the polymeric hydrogel to form a grafted layer, the primer set including 3′ blocked uncleavable first pre-primers and 3′ blocked uncleavable second pre-primers;
• removing the polymeric hydrogel or the grafted layer from the interstitial regions;
• at a first predetermined region of the grafted layer within the depression and within the second region:
• altering 3′ blocked uncleavable first pre-primers to introduce cleavable first primers;
• and
• altering 3′ blocked uncleavable second pre-primers to introduce uncleavable second primers;
• and
• at a second predetermined region of the grafted layer within the depression and within the first region:
• altering 3′ blocked uncleavable first pre-primers to introduce uncleavable first primers;
• and
• altering 3′ blocked uncleavable second pre-primers to introduce cleavable second primers.