Ultrasonic monitoring of early-stage biofilm growth on polymeric surfaces

Biofilm growth on polymeric surfaces was monitored using ultrasonic frequency-domain reflectometry (UFDR). The materials utilized for this study included nonporous polycarbonate (PC) sheets, polyamide (PA) nanofiltration composite membranes and porous polyvinylidene fluoride (PVDF) microfiltration membranes (nominal pore size: 0.65 μm). Coupons of each material were placed in a biologically active annular reactor for up to 300 days, and subjected to a constant shear field (0.12 N m^{- 2}), which induced sessile microbial growth from acetate amended municipal tap water. Acoustic monitoring was non-destructively executed by traversing coupons in a constant temperature water bath using a spherically focused 20- MHz immersion transducer. This semi-automated system was configured to obtain reflections from 50 regions (c.a. 120 × 10³ μm²) distributed evenly near the centerline of each coupon. The resulting reflected power distributions were compared with standard biochemical and microscopic assays that described surface associated biofilms. When compared to clean (virgin) conditions, biofilms growing on coupons induced consistent attenuations in reflection amplitude, which caused statistically significant shifts in reflected power (p < 0.01). Using exocellular polysaccharides as a surrogate measure of total biofilm mass, UFDR was able to detect biofilms developing on any of the materials tested at surface-averaged masses ≤ 150 μg cm^{- 2}. Above these threshold levels, increasing amounts of exocellular polysaccharides correlated with significant decreases in total reflected power (TRP). The distribution of biomass on the coupon surfaces determined by acoustic spectra was consistent with that observed using environmental scanning electron microscopy (ESEM). These results suggest that UFDR may be used as a non-destructive tool to monitor biofouling in a wide variety of applications.