Speed, S.; Gupta, K.; Peng, Y.-H.; Krieg, E.

Current methods for sequence-selective biomolecule isolation suffer from high cost, off-target effects, and limited flexibility. Here, we introduce LASSO (crossLink-Assisted Sequence-Selective isOlation), a versatile platform using programmable polymer phase separation to capture biomolecules under native conditions. LASSO relies on combinatorial crosslinker libraries--diverse mixtures of DNA strands that collectively trigger the formation of highly swollen polymer agglomerates with near-zero background binding. We demonstrate >80% pulldown efficiency for diverse targets, including DNA, SARS-CoV-2 RNA, and human thrombin. LASSO provides 8-20x higher binding capacity (4 nmol/mg polymer) than commercial microbeads. In RNA-seq workflows, LASSO depleted ribosomal RNA with 86% efficiency while yielding up to 7x fewer off-target outliers (p<0.001) versus state-of-the-art magnetic beads (riboPOOLs) and RNase H (NEBNext) kits. Thrombin was captured via switchable aptamers with 92% efficiency, and a gentle release mechanism allowed the subsequent isolation of 72% enzymatically active proteins from the polymer. LASSOs cost-effectiveness ($0.96/sample vs. $46-$51 for commercial kits), long-term stability (7+ years), simple usage, and modularity position it to transform diagnostics, transcriptomics, and bionanotechnology workflows.