Optimal artificial urine formulations for in vitro urolithiasis models

To develop and characterize standardized in vitro artificial urine (AU) models for three major stone-forming environments-calcium oxalate (CaOx), uric acid (UA), and infection-related struvite-and to identify assay operating conditions that maximize stent-associated surface deposition while limiting excessive bulk precipitation.

Homemade AU models were formulated from a physiologic base composition and modified to create disease-specific lithogenic conditions. The CaOx model used sodium oxalate fixed at 80 mg/L with stepwise calcium chloride escalation to 600%. The UA model was tested at uric acid concentrations of 500-1000 mg/L under acidic pH. The bacterial infection model was tested as two independent live-bacterial arms preset at bulk pH 6.5 or 9.0 before immersion to capture urease-driven alkalinization and surface colonization, and a non-bacterial ammonia-alkalinized control was also evaluated. Stents of three commercial types were immersed for 48 h under CDC biofilm-reactor conditions, and surface deposition was quantified as weight change per unit length (mg/mm).

In the CaOx model, surface-associated weight gain increased with calcium concentration up to 200% (0.98 g/L) but declined at higher levels owing to competing bulk precipitation. The UA model produced minimal deposition, with maximal weight gain at 600 mg/L. In the bacterial infection assay, weight gain was numerically highest in the arm preset at bulk pH 6.5 before immersion and remained substantial in the arm preset at pH 9.0, whereas the non-bacterial alkalinization model resulted in negligible deposits.

These stone type-specific AU models provide defined, lab-preparable lithogenic screening conditions. Within this 48-h reactor assay, CaCl₂ 200% + oxalate 80 mg/L, uric acid 600 mg/L, and the bacterial infection model initiated at preset bulk pH 6.5 yielded the highest observed stent-associated deposition. The platform may serve as an accelerated screening tool for evaluating stent materials, coatings, and anti-encrustation strategies.