In vivo Imaging of the Peri-Implant pH of dissolving Mg(OH)2 as Novel Scaffold Material in Regenerative Medicine

Introduction The peri-implant pH value is assumed to change after the implantation of biodegradable magnesium implants or the dissolution of its corrosion products [1]. However, the periimplant pH needs to be controlled to obtain an appropriate foreign body reaction, since the local pH value can influence the healing phase [2]. Mg(OH)2 seems to have high potential for scaffolds in regenerative bone reconstruction [3], while its influence on the local tissue pH during its dissolution is not clear. Common approaches to measure the pH in vivo are using wired techniques [4], which are limited by the use of wires. Therefore, this study investigates the pH changes around Mg(OH)2 in vivo using a pH sensitive fluorescent dye which emission spectra is changing according to the environmental pH. The emission spectra are recorded using an in vivo imaging system which is capable to analyze the spectral information. Materials and Methods The pH dependent shift in the emission spectra of 5-(and-6)-carboxy SNARF-1 (Ex: 488530 nm) results in two peaks which can be correlated to an acidic (600 nm) or a basic milieu (650 nm). Therefore, ratiometric measurements (Em: 600 / 650 nm) can be correlated to a pH value in a certain range. This range was investigated in vitro and in vivo. For in vitro evaluation, well plates with different concentrations of SNARF-1 and standard buffers (pH3-10) were measured using the Maestro System, CRi. For in vivo evaluation, adult female hairless but immunocompetent mice (Crl:SKH1-hr) were used and SNARF-1 was administrated intravenously. The pH standardization was used to determine the pH value around a subcutaneous Mg(OH)2 implant. Results The ratiometric measurements of 5-(and-6)carboxy SNARF-1 can distinguish between acidic (pH 3-6), neutral (pH 7) and alkaline environments (pH 8-10) in vitro as well as in vivo (Table 1). The environment around a compressed Mg(OH)2 tablet was determined as alkaline (>pH 8, Fig.1) for the initial implantation period, while the pH seem to return to neutral values after a few days and only peak into the alkaline range when the Mg(OH)2 partly dissolves. Table 1: Ratiometric values (n>3) obtained from emission spectra measured from pH standards in vitro and in vivo.