Development of aliphatic biodegradable photoluminescent polymers.

None of the current biodegradable polymers can function as both implant materials and fluorescent imaging probes. The objective of this study was to develop aliphatic biodegradable photoluminescent polymers (BPLPs) and their associated cross-linked variants (CBPLPs) for biomedical applications. BPLPs are degradable oligomers synthesized from biocompatible monomers including citric acid, aliphatic diols, and various amino acids via a convenient and cost-effective polycondensation reaction. BPLPs can be further cross-linked into elastomeric cross-linked polymers, CBPLPs. We have shown representatively that BPLP-cysteine (BPLP-Cys) and BPLP-serine (BPLP-Ser) offer advantages over the traditional fluorescent organic dyes and quantum dots because of their preliminarily demonstrated cytocompatibility in vitro, minimal chronic inflammatory responses in vivo, controlled degradability and high quantum yields (up to 62.33%), tunable fluorescence emission (up to 725 nm), and photostability. The tensile strength of CBPLP-Cys film ranged from 3.25 +/- 0.13 MPa to 6.5 +/- 0.8 MPa and the initial Modulus was in a range of 3.34 +/- 0.15 MPa to 7.02 +/- 1.40 MPa. Elastic CBPLP-Cys could be elongated up to 240 +/- 36%. The compressive modulus of BPLP-Cys (0.6) (1:1:0.6 OD:CA:Cys) porous scaffold was 39.60 +/- 5.90 KPa confirming the soft nature of the scaffolds. BPLPs also possess great processability for micro/nano-fabrication. We demonstrate the feasibility of using BPLP-Ser nanoparticles ("biodegradable quantum dots") for in vitro cellular labeling and noninvasive in vivo imaging of tissue engineering scaffolds. The development of BPLPs and CBPLPs represents a new direction in developing fluorescent biomaterials and could impact tissue engineering, drug delivery, bioimaging.

Hexamethyldisiloxane-based nanoprobes for (1) H MRI oximetry.

Quantitative in vivo oximetry has been reported using (19) F MRI in conjunction with reporter molecules, such as perfluorocarbons, for tissue oxygenation (pO(2) ). Recently, hexamethyldisiloxane (HMDSO) has been proposed as a promising alternative reporter molecule for (1) H MRI-based measurement of pO(2) . To aid biocompatibility for potential systemic administration, we prepared various nanoemulsion formulations using a wide range of HMDSO volume fractions and HMDSO to surfactant ratios. Calibration curves (R(1) versus pO(2) ) for all emulsion formulations were found to be linear and similar to neat HMDSO for low surfactant concentrations (<10% v/v). A small temperature dependence in the calibration curves was observed, similar to previous reports on neat HMDSO, and was characterized to be approximately 1 Torr/ °C under hypoxic conditions. To demonstrate application in vivo, 100 µL of this nanoemulsion was administered to healthy rat thigh muscle (Fisher 344, n=6). Dynamic changes in mean thigh tissue pO(2) were measured using the PISTOL (proton imaging of siloxanes to map tissue oxygenation levels) technique in response to oxygen challenge. Changing the inhaled gas to oxygen for 30 min increased the mean pO(2) significantly (p<0.001) from 39 ± 7 to 275 ± 27 Torr. When the breathing gas was switched back to air, the tissue pO(2) decreased to a mean value of 45 ± 6 Torr, not significantly different from baseline (p>0.05), in 25 min. A first-order exponential fit to this part of the pO(2) data (i.e. after oxygen challenge) yielded an oxygen consumption-related kinetic parameter k=0.21 ± 0.04 min(-1) . These results demonstrate the feasibility of using HMDSO nanoemulsions as nanoprobes of pO(2) and their utility to assess oxygen dynamics in vivo, further developing quantitative (1) H MRI oximetry.

Physical principles of quantitative nuclear magnetic resonance oximetry.

Over the years many techniques have been devised for the measurement of tissue oxygenation (oximetry). Oximetry using polarographic needle electrodes has long been considered a gold standard. Nuclear Magnetic Resonance (NMR) based oximetry uses exogenously administered reporter molecules such as perfluorocarbons to quantitatively interrogate oxygen tension (pO2). This technique has been successfully used in vivo in the preclinical setting and shows promise for clinical applications. NMR pO2 reporter molecules display a linear dependence of the spin lattice relaxation rate on pO2, which forms the basis of this technique. Physical principles of spin lattice relaxation of pO2 reporter molecules and the pO2 dependence of relaxation rate are discussed in this review. Practical considerations for choice of reporter molecules for in vivo measurements, general methodology and new developments are also described.

Development of intrinsically photoluminescent and photostable polylactones.

A method of introducing intrinsically photo luminescent properties to biodegradable polymer is introduced, exemplified by the synthesis of intrinsically photoluminescent polylactones that enable non-invasive monitoring and tracking of material degradation in vivo in realtime, as well as the formation of theranostic nanoparticles for cancer imaging and drug delivery.

Proton imaging of siloxanes to map tissue oxygenation levels (PISTOL): a tool for quantitative tissue oximetry.

Hexamethyldisiloxane (HMDSO) has been identified as a sensitive proton NMR indicator of tissue oxygenation (pO(2)) based on spectroscopic spin-lattice relaxometry. A rapid MRI approach has now been designed, implemented, and tested. The technique, proton imaging of siloxanes to map tissue oxygenation levels (PISTOL), utilizes frequency-selective excitation of the HMDSO resonance and chemical-shift selective suppression of residual water signal to effectively eliminate water and fat signals and pulse-burst saturation recovery (1)H echo planar imaging to map T(1) of HMDSO and hence pO(2). PISTOL was used here to obtain maps of pO(2) in rat thigh muscle and Dunning prostate R3327 MAT-Lu tumor-implanted rats. Measurements were repeated to assess baseline stability and response to breathing of hyperoxic gas. Each pO(2) map was obtained in 3(1/2) min, facilitating dynamic measurements of response to oxygen intervention. Altering the inhaled gas to oxygen produced a significant increase in mean pO(2) from 55 Torr to 238 Torr in thigh muscle and a smaller, but significant, increase in mean pO(2) from 17 Torr to 78 Torr in MAT-Lu tumors. Thus, PISTOL enabled mapping of tissue pO(2) at multiple locations and dynamic changes in pO(2) in response to intervention. This new method offers a potentially valuable new tool to image pO(2) in vivo for any healthy or diseased state by (1)H MRI.

Novel 1H NMR approach to quantitative tissue oximetry using hexamethyldisiloxane.

19F NMR spin-lattice relaxometry of hexafluorobenzene (HFB) has been shown to be a highly sensitive indicator of tumor oxygenation. In this study hexamethyldisiloxane (HMDSO) was identified as a proton NMR analog, and its potential as a probe for investigating dynamic changes in tissue oxygen tension (pO2) was evaluated. HMDSO has a single proton resonance (delta= -0.3 ppm) and the spin-lattice relaxation rate, Rl (= 1/T1) exhibits a linear dependence on pO2: R1 (s(-1)) = 0.1126 + 0.0013* pO2 (torr) at 37 degrees C. To demonstrate application in vivo, HMDSO was administered into healthy rat thigh muscle (100 microl) and tumors (50 microl). Local pO2 was determined by using pulse-burst saturation recovery (PBSR) 1H NMR spectroscopy to assess R1. Water and fat signals were effectively suppressed by frequency-selective excitation of the HMDSO resonance. Rat thigh muscle had a mean baseline pO2 of 35 +/- 11 torr, with a typical stability of +/-3 torr over 20 min, when the rats breathed air. Altering the inhaled gas to oxygen produced a significant increase in pO2 to 100-200 torr. In tumors, altering the inspired gas also produced significant (albeit generally smaller) changes. This new pO2 reporter molecule offers a potentially valuable new tool for investigating pO2 in vivo.