In Vitro and In Vivo Assessment of the Infection Resistance and Biocompatibility of Small-Molecule-Modified Polyurethane Biomaterials.

Bacterial intracellular nucleotide second messenger signaling is involved in biofilm formation and regulates biofilm development. Interference with the bacterial nucleotide second messenger signaling provides a novel approach to control biofilm formation and limit microbial infection in medical devices. In this study, we tethered small-molecule derivatives of 4-arylazo-3,5-diamino-1H-pyrazole on polyurethane biomaterial surfaces and measured the biofilm resistance and initial biocompatibility of modified biomaterials in in vitro and in vivo settings. Results showed that small-molecule-modified surfaces significantly reduced the Staphylococcal epidermidis biofilm formation compared to unmodified surfaces and decreased the nucleotide levels of c-di-AMP in biofilm cells, suggesting that the tethered small molecules interfere with intracellular nucleotide signaling and inhibit biofilm formation. The hemocompatibility assay showed that the modified polyurethane films did not induce platelet activation or red blood cell hemolysis but significantly reduced plasma coagulation and platelet adhesion. The cytocompatibility assay with fibroblast cells showed that small-molecule-modified surfaces were noncytotoxic and cells appeared to be proliferating and growing on modified surfaces. In a 7-day subcutaneous infection rat model, the polymer samples were implanted in Wistar rats and inoculated with bacteria or PBS. Results show that modified polyurethane significantly reduced bacteria by ∼2.5 log units over unmodified films, and the modified polymers did not lead to additional irritation/toxicity to the animal tissues. Taken together, the results demonstrated that small molecules tethered on polymer surfaces remain active, and the modified polymers are biocompatible and resistant to microbial infection in vitro and in vivo.

Surfaces modified with small molecules that interfere with nucleotide signaling reduce Staphylococcus epidermidis biofilm and increase the efficacy of ciprofloxacin.

Staphylococcus epidermidis are common bacteria associated with biofilm related infections on implanted medical devices. Antibiotics are often used in combating such infections, but they may lose their efficacy in the presence of biofilms. Bacterial intracellular nucleotide second messenger signaling plays an important role in biofilm formation, and interference with the nucleotide signaling pathways provides a possible way to control biofilm formation and to increase biofilm susceptibility to antibiotic therapy. This study synthesized small molecule derivates of 4-arylazo-3,5-diamino-1 H-pyrazole (named as SP02 and SP03) and found these molecules inhibited S. epidermidis biofilm formation and induced biofilm dispersal. Analysis of bacterial nucleotide signaling molecules showed that both SP02 and SP03 significantly reduced cyclic dimeric adenosine monophosphate (c-di-AMP) levels in S. epidermidis at doses as low as 25 µM while having significant effects on multiple nucleotides signaling including cyclic dimeric guanosine monophosphate (c-di-GMP), c-di-AMP, and cyclic adenosine monophosphate (cAMP) at high doses (100 µM or greater). We then tethered these small molecules to polyurethane (PU) biomaterial surfaces and investigated biofilm formation on the modified surfaces. Results showed that the modified surfaces significantly inhibited biofilm formation during 24 h and 7-day incubations. The antibiotic ciprofloxacin was used to treat these biofilms and the efficacy of the antibiotic (2 µg/mL) was found to increase from 94.8% on unmodified PU surfaces to > 99.9% on both SP02 and SP03 modified surfaces (>3 log units). Results demonstrated the feasibility of tethering small molecules that interfere with nucleotide signaling onto polymeric biomaterial surfaces and in a way that interrupts biofilm formation and increases antibiotic efficacy for S. epidermidis infections.

Crosslinkable fluorophenoxy-substituted poly[bis(octafluoropentoxy) phosphazene] biomaterials with improved antimicrobial effect and hemocompatibility.

Biomaterial-associated microbial infection is one of the most frequent and severe complications associated with the use of biomaterials in medical devices. In previous studies, we developed new fluorinated polyphosphazenes, poly[bis(octafluoropentoxy) phosphazene] (OFP) and crosslinkable OFP (X-OFP), and demonstrated the inhibition of bacterial adhesion and biofilm formation, thereby controlling microbial infection. In this study, two additional fluorinated polyphosphazenes (PPs, defined as LS02 and LS03) with fluorophenoxy-substituted side groups, 4-fluorophenoxy and 4-(trifluoromethyl)phenoxy, respectively, based on X-OFP general structure, were synthesized and applied as coatings on stainless steel. The linkage of fluorophenoxy groups to the P-N backbone of PPs was found to increase the surface stiffness and significantly reduced Staphylococcus bacterial adhesion and inhibited biofilm formation. It also significantly reduced microbial infection compared to OFP, our prior X-OFPs or poly[bis(trifluoroethoxy) phosphazene] (TFE). The biofilm experiments show that the newly synthesized PPs LS02 and LS03 are biofilm free up to 28 days. Plasma coagulation and platelet adhesion/activation experiments also demonstrated that new PPs containing fluorophenoxy side groups are hemocompatible. The development of new crosslinkable fluorinated PPs containing fluorophenoxy-substituted side groups provides a new generation of polyphosphazene materials for medical devices with improved resistance to microbial infections and thrombosis formation.

In Vivo Evaluation of the Regenerative Capability of Glycylglycine Ethyl Ester-Substituted Polyphosphazene and Poly(lactic-co-glycolic acid) Blends: A Rabbit Critical-Sized Bone Defect Model.

In an effort to understand the biological capability of polyphosphazene-based polymers, three-dimensional biomimetic bone scaffolds were fabricated using the blends of poly[(glycine ethylglycinato)75(phenylphenoxy)25]phosphazene (PNGEGPhPh) and poly(lactic-co-glycolic acid) (PLGA), and an in vivo evaluation was performed in a rabbit critical-sized bone defect model. The matrices constructed from PNGEGPhPh-PLGA blends were surgically implanted into 15 mm critical-sized radial defects of the rabbits as structural templates for bone tissue regeneration. PLGA, which is the most commonly used synthetic bone graft substitute, was used as a control in this study. Radiological and histological analyses demonstrated that PNGEGPhPh-PLGA blends exhibited favorable in vivo biocompatibility and osteoconductivity, as the newly designed matrices allowed new bone formation to occur without adverse immunoreactions. The X-ray images of the blends showed higher levels of radiodensity than that of the pristine PLGA, indicating higher rates of new bone formation and regeneration. Micro-computed tomography quantification revealed that new bone volume fractions were significantly higher for the PNGEGPhPh-PLGA blends than for the PLGA controls after 4 weeks. The new bone volume increased linearly with increasing time points, with the new tissues observed throughout the defect area for the blend and only at the implant site's extremes for the PLGA control. Histologically, the polyphosphazene system appeared to show tissue responses and bone ingrowths superior to PLGA. By the end of the study, the defects with PNGEGPhPh-PLGA scaffolds exhibited evidence of effective bone tissue ingrowth and minimal inflammatory responses. Thus, polyphosphazene-containing biomaterials have excellent translational potential for use in bone regenerative engineering applications.

Inhibition of bacterial adhesion and biofilm formation by a textured fluorinated alkoxyphosphazene surface.

The utilization of biomaterials in implanted blood-contacting medical devices often induces a persistent problem of microbial infection, which results from bacterial adhesion and biofilm formation on the surface of biomaterials. In this research, we developed new fluorinated alkoxyphosphazene materials, specifically poly[bis(octafluoropentoxy) phosphazene] (OFP) and crosslinkable OFP (X-OFP), with improved mechanical properties, and further modified the surface topography with ordered pillars to improve the antibacterial properties. Three X-OFP materials, X-OFP3.3, X-OFP8.1, X-OFP13.6, with different crosslinking densities were synthesized, and textured films with patterns of 500/500/600 nm (diameter/spacing/height) were fabricated via a two stage soft lithography molding process. Experiments with 3 bacterial strains: Staphylococcal epidermidis, Staphylococcal aureus, and Pseudomonas aeruginosa showed that bacterial adhesion coefficients were significantly lower on OFP and X-OFP smooth surfaces than on the polyurethane biomaterial, and surface texturing further reduced bacterial adhesion due to the reduction in accessible surface contact area. Furthermore the anti-bacterial adhesion effect shows a positive relationship with the crosslinking degree. Biofilm formation on the substrates was examined using a CDC biofilm reactor for 7 days and no biofilm formation was observed on textured X-OFP biomaterials. The results suggested that the combination of fluorocarbon chemistry and submicron topography modification in textured X-OFP materials may provide a practical approach to improve the biocompatibility of current biomaterials with significant reduction in risk of pathogenic infection.

Polyphosphazenes: Phosphorus in Inorganic-Organic Polymers.

Although the best-known examples of synthetic polymers are derived from carbon-based monomers, there exists another large and growing family of macromolecules based on the chemistry of phosphorus. These are the poly(organophosphazenes): polymers with a backbone of alternating phosphorus and nitrogen atoms and with two organic side groups attached to each phosphorus. The methods of synthesis of these polymers allow access to property combinations not found in all-organic counterparts, and this provides pathways to new materials that are important in biomedical research, energy generation and storage, aerospace materials, and numerous other specialized applications.

A Regenerative Polymer Blend Composed of Glycylglycine ethyl ester-substituted Polyphosphazene and Poly (lactic-co-glycolic acid).

In the pursuit of continuous improvement in the area of biomaterial design, blends of mixed-substituent polyphosphazenes and poly (lactic acid-glycolic acid) (PLGA) were prepared, and their morphology of phase distributions for the first time was studied. The degradation mechanism and osteocompatibility of the blends were also evaluated for their use as regenerative materials. Poly [(ethyl phenylalanato)25(glycine ethyl glycinato)75phosphazene](PNEPAGEG) and poly [(glycine ethyl glycinato)75(phenylphenoxy)25phosphazene](PNGEGPhPh) were blended with PLGA at various weight ratios to yield different blends, namely PNEPAGEG-PLGA 25:75, PNEPAGEG-PLGA 50:50, PNGEGPhPh-PLGA 25:75, and PNGEGPhPh-PLGA 50:50. The molecular interactions, domain sizes, and phase distributions of the blends were confirmed by atomic force microscopy (AFM) as the PNEPAGEG-PLGA and PNGEGPhPh-PLGA blends showed different domain sizes and phase distributions. Due to the extensive hydrogen bonding within the two polymer components, PNEPAGEG-PLGA exhibited small-sized domains and well-distributed morphology. While for the PNGEGPhPh-PLGA blends, the presence of phenylphenol (PhPh) caused the formation of PLGA large-sized domains as the PLGA formed a continuous phase and PNGEGPhPh constituted a dispersed phase. In addition to AFM results, scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and Fourier transform infrared spectroscopy (FTIR) results demonstrated the miscibility of the blends. The PNEPAGEG-PLGA and PNGEGPhPh-PLGA blends presented in situ 3D interconnected porous structures upon degradation in phosphate-buffered saline (PBS) media at 37°C. However, the blends showed different mechanistic pathways to the formations of the pores. The difference in the erosion patterns could be attributed to the nature of molecular attractions that exist in the blends. Furthermore, the novel blends were able to support cell growth as compared to PLGA, and accommodate cell infiltrations, which ultimately augmented surface area for better cell-material interactions.

New cross-linkable poly[bis(octafluoropentoxy) phosphazene] biomaterials: Synthesis, surface characterization, bacterial adhesion, and plasma coagulation responses.

Biomaterial-associated microbial infection and thrombosis represent major issues to the success of long-term use of implantable blood-contacting medical devices. The development of new poly[bis(octafluoropentoxy) phosphazene (OFP) biomaterials provides new routes for combatting microbial infection and thrombosis. However, the limited mechanical properties of OFP to date render them unsuitable for application in medical devices and inhibit any attempts at subsequent surface topography modification. In this study, we synthesized cross-linkable OFPs (X-OFPs) with the different degrees of cross-linking in an effort to improve the mechanical properties. The results showed that the surface chemistry and surface topography of X-OFPs do not change significantly, but the surface mechanical stiffness increased after cross-linking. Atomic force microscopic phase images showed that the polymer phase separation structures changed due to cross-linking. Experiments with three bacterial strains: Staphylococcal epidermidis, Staphylococcal aureus, and Pseudomonas aeruginosa showed that bacterial adhesion was significantly decreased on the OFP and X-OFPs for both the pre-cross-linked and cross-linked as compared to polyurethane biomaterials. Furthermore, bacterial adhesions were lower on X-OFP surfaces than on pre-cross-linked materials, suggesting that mechanical stiffness is an important parameter influencing bacterial adhesion. Blood plasma coagulation responses revealed longer coagulation times for OFP and X-OFP materials than on polyurethanes, indicating that the new cross-linked OFPs are resistant to plasma coagulation compared to currently used polyurethane biomaterials.

Polyphosphazene polymers: The next generation of biomaterials for regenerative engineering and therapeutic drug delivery.

The demand for new biomaterials in several biomedical applications, such as regenerative engineering and drug delivery, has increased over the past two decades due to emerging technological advances in biomedicine. Degradable polymeric biomaterials continue to play a significant role as scaffolding materials and drug devices. Polyphosphazene platform is a subject of broad interest, as it presents an avenue for attaining versatile polymeric materials with excellent structure and property tunability, and high functional diversity. Macromolecular substitution enables the facile attachment of different organic groups and drug molecules to the polyphosphazene backbone for the development of a broad class of materials. These materials are more biocompatible than traditional biomaterials, mixable with other clinically relevant polymers to obtain new materials and exhibit unique erosion with near-neutral degradation products. Hence, polyphosphazene represents the next generation of biomaterials. In this review, the authors systematically discuss the synthetic design, structure-property relationships, and the promising potentials of polyphosphazenes in regenerative engineering and drug delivery.

Biomedical applications of polyphosphazenes.

Ever since the pioneering research efforts on their utility in biomedicine, polyphosphazene polymers have witnessed enormous growth and expansion in several biomedical applications due to their unique properties. The development of this exceptional biodegradable system with extraordinary design flexibility, property tunability and neutral bioactivity could stimulate an unprecedented paradigm in biomaterial design. Thus, polyphosphazenes are, undoubtedly, the next-generation biomaterials. This editorial provides a brief perspective on the promising prospects of polyphosphazene-based biomaterials for medical device technology, focusing mainly on the authors' work on this particular polymeric system.

Crystal structures of three hexakis-(fluoroar-yloxy)cyclo-triphosphazenes.

The syntheses and crystal structures of three cyclo-triphosphazenes, all with fluorinated ar-yloxy side groups that generate different steric characteristics, viz. hexa-kis-(penta-fluoro-phen-oxy)cyclo-triphosphazene, N3P3(OC6F5)6, 1, hexa-kis-[4-(tri-fluoro-methyl)-phen-oxy]cyclo-triphosphazene, N3P3[OC6H4(CF3)]6, 2 and hexa-kis-[3,5-bis(-tri-fluoro-methyl)-phen-oxy]cyclo-triphosphazene, N3P3[OC6H3(CF3)2]6 3, are reported. Specifically, each phospho-rus atom bears either two penta-fluoro-phen-oxy, 4-tri-fluoro-methyl-phen-oxy, or 3,5-tri-fluoro-methyl-phen-oxy groups. The central six-membered phosphazene rings display envelope pucker conformations in each case, albeit to varying degrees. The maximum displacement of the 'flap atom' from the plane through the other ring atoms [0.308 (5) Å] is seen in 1, in a mol-ecule that is devoid of hydrogen atoms and which exhibits a 'wind-swept' look with all the aromatic rings displaced in the same direction. In 3 an intra-molecular C-H(aromatic)⋯F inter-action is observed. All the -CF3 groups in 2 and 3 exhibit positional disorder over two rotated orientations in close to statistical ratios. The extended structures of 2 and 3 are consolidated by C-H⋯F inter-actions of two kinds: (a) linear chains, and (b) cyclic between mol-ecules related by inversion centers. In both 1 and 3, one of the six substituted phenyl rings has a parallel-displaced aromatic π-π stacking inter-action with its respective symmetry mate with slippage values of 2.2 Šin 1 and 1.0 Šin 3. None of the structures reported here have solvent voids that could lead to clathrate formation.

Generational Biodegradable and Regenerative Polyphosphazene Polymers and their Blends with Poly (lactic-co-glycolic acid).

New fields such as regenerative engineering have driven the design of advanced biomaterials with a wide range of properties. Regenerative engineering is a multidisciplinary approach that integrates the fields of advanced materials science and engineering, stem cell science, physics, developmental biology, and clinical translation for the regeneration of complex tissues. The complexity and demands of this innovative approach have motivated the synthesis of new polymeric materials that can be customized to meet application-specific needs. Polyphosphazene polymers represent this fundamental change and are gaining renewed interest as biomaterials due to their outstanding synthetic flexibility, neutral bioactivity (buffering degradation products), and tunable properties across the range. Polyphosphazenes are a unique class of polymers composed of an inorganic backbone with alternating phosphorus and nitrogen atoms. Each phosphorus atom bears two substituents, with a wide variety of side groups available for property optimization. Polyphosphazenes have been investigated as potential biomaterials for regenerative engineering. Polyphosphazenes for use in regenerative applications have evolved as a class to include different generations of degradable polymers. The first generation of polyphosphazenes for tissue regeneration entailed the use of hydrolytically active side groups such as imidazole, lactate, glycolate, glucosyl, or glyceryl groups. These side groups were selected based on their ability to sensitize the polymer backbone to hydrolysis, which allowed them to break down into non-toxic small molecules that could be metabolized or excreted. The second generation of degradable polyphosphazenes developed consisted of polymers with amino acid ester side groups. When blended with poly (lactic acid-co-glycolic acid) (PLGA), the feasibility of neutralizing acidic degradation products of PLGA was demonstrated. The blends formed were mostly partially miscible. The desire to improve miscibility led to the design of the third generation of degradable polyphosphazenes by incorporating dipeptide side groups which impart significant hydrogen bonding capability to the polymer for the formation of completely miscible polyphosphazene-PLGA blends. Blend system of the dipeptide-based polyphosphazene and PLGA exhibit a unique degradation behavior that allows the formation of interconnected porous structures upon degradation. These inherent pore-forming properties have distinguished degradable polyphosphazenes as a potentially important class of biomaterials for further study. The design considerations and strategies for the different generations of degradable polyphosphazenes and future directions are discussed.

Synthesis, Physicochemical Analysis, and Side Group Optimization of Degradable Dipeptide-Based Polyphosphazenes as Potential Regenerative Biomaterials.

We report the synthesis and physicochemical analysis of mixed-substituent dipeptide-based polyphosphazene polymers, poly[(glycineethylglycinato) x (phenylphenoxy) y phosphazene] (PNGEG x PhPh y ) and poly[(ethylphenylalanato) x (glycineethylglycinato) y phosphazene] (PNEPA x GEG y ), using glycylglycine ethyl ester (GEG) as the primary substituent side group and cosubstituting with phenylphenol (PhPh) and phenylalanine ethyl ester (EPA), respectively. The suitability of the cosubstituted polyphosphazenes to regenerative engineering was evaluated. The physicochemical evaluation revealed that the molecular weights, glass transition temperatures, hydrophilicity, and mechanical properties could be modulated by varying the compositions of the side groups to obtain a variety of properties. The PNEPA25GEG75 and PNGEG75PhPh25 polymers exhibited the most promising physicochemical properties. These two polymers were further subjected to in vitro hydrolysis and cell proliferation studies using poly(lactic-co-glycolic acid) (PLAGA) as a control. The hydrolysis experiments revealed that the two polymers hydrolyzed to near-neutral pH media (~5.3 to 7.0) in a relatively slow fashion, whereas a pH value as low as 2.2 was obtained for the PLAGA media over 12 weeks of degradation study. Furthermore, the two polymers showed continuous MC3T3 cell proliferation and growth in comparison to PLAGA over a 21-day culture period. These findings establish that cosubstitution of different side groups of polyphosphazenes and exploitation of the hydrogen-bonding capacity of peptide bonds in GEG offer a flexible tool that can be employed to make new and fascinating polymeric biomaterials with different and tailored properties that can suit different regenerative needs.

A new textured polyphosphazene biomaterial with improved blood coagulation and microbial infection responses.

A new poly[bis(octafluoropentoxy) phosphazene] (OFP) was synthesized for the purpose of blood contacting medical devices. OFP was further either developed into crosslinkable polyphosphazene (X-OFP) or blended with polyurethane (PU) as the mixture (OFP/PU) for improvement of mechanical property of polyphosphazene polymers. All the materials were fabricated as smooth films or further textured with submicron pillars for the assay of antimicrobial and antithrombotic properties. Results showed that crosslinkable OFP (X-OFP) and blends of OFP/PU successfully improved the mechanical strength of OFP and fewer defects of pillars were found on the textured polyphosphazene surfaces. The antithrombotic experiments showed that polyphosphazene OFP materials reduced human Factor XII activation and platelet adhesion, thereby being resistant to plasma coagulation and thrombosis. The bacterial adhesion and biofilm experiments demonstrated that OFP materials inhibited staphylococcal bacterial adhesion and biofilm formation. The surface texturing further reduced the platelet adhesion and bacterial adhesion, and inhibited biofilm formation up to 23 days. The data suggested that textured OFP materials may provide a practical approach to improve the biocompatibility of current biomaterials in the application of blood contacting medical devices with significant reduction in risk of pathogenic infection and thrombosis. STATEMENT OF SIGNIFICANCE: The thromboembolic events and microbial infection have been the significant barriers for the long term use of biomaterials in blood-contacting medical devices. The development of new materials with multiple functions including anti-thrombosis and antibacterial surfaces is a high research priority. This study synthesized new biostable and biocompatible polyphosphazene polymers, poly[bis(octafluoropentoxy)phosphazene] (OFP) and crosslinkable OFP, and successfully improved the mechanical strength of polyphosphazenes. Polymers were fabricated into textured films with submicron pillars on the surfaces. The antimicrobial and antithrombotic assays demonstrated that new materials combined with surface physical modification have significant reduction in risk of pathogenic infection and thrombosis, and improve the biocompatibility of current biomaterials in the application of blood-contacting medical devices. It would be interest to biomaterials and bioengineering related communities.

Biodegradable Polyphosphazene-Based Blends for Regenerative Engineering.

The occurrence of musculoskeletal tissue injury or disease and the subsequent functional impairment is at an alarming rate. It continues to be one of the most challenging problems in the human health care. Regenerative engineering offers a promising transdisciplinary strategy for tissues regeneration based on the convergence of tissue engineering, advanced materials science, stem cell science, developmental biology and clinical translation. Biomaterials are emerging as extracellular-mimicking matrices designed to provide instructive cues to control cell behavior and ultimately, be applied as therapies to regenerate damaged tissues. Biodegradable polymers constitute an attractive class of biomaterials for the development of scaffolds due to their flexibility in chemistry and the ability to be excreted or resorbed by the body. Herein, the focus will be on biodegradable polyphosphazene-based blend systems. The synthetic flexibility of polyphosphazene, combined with the unique inorganic backbone, has provided a springboard for more research and subsequent development of numerous novel materials that are capable of forming miscible blends with poly (lactide-co-glycolide) (PLAGA). Laurencin and co-workers has demonstrated the exploitation of the synthetic flexibility of Polyphosphazene that will allow the design of novel polymers, which can form miscible blends with PLAGA for biomedical applications. These novel blends, due to their well-tuned biodegradability, and mechanical and biological properties coupled with the buffering capacity of the degradation products, constitute ideal materials for regeneration of various musculoskeletal tissues. LAY SUMMARY: Regenerative engineering aims to regenerate complex tissues to address the clinical challenge of organ damage. Tissue engineering has largely focused on the restoration and repair of individual tissues and organs, but over the past 25 years, scientific, engineering, and medical advances have led to the introduction of this new approach which involves the regeneration of complex tissues and biological systems such as a knee or a whole limb. While a number of excellent advanced biomaterials have been developed, the choice of biomaterials, however, has increased over the past years to include polymers that can be designed with a range of mechanical properties, degradation rates, and chemical functionality. The polyphosphazenes are one good example. Their chemical versatility and hydrogen bonding capability encourages blending with other biologically relevant polymers. The further development of Polyphosphazene-based blends will present a wide spectrum of advanced biomaterials that can be used as scaffolds for regenerative engineering and as well as other biomedical applications.

Engineered stem cell niche matrices for rotator cuff tendon regenerative engineering.

Rotator cuff (RC) tears represent a large proportion of musculoskeletal injuries attended to at the clinic and thereby make RC repair surgeries one of the most widely performed musculoskeletal procedures. Despite the high incidence rate of RC tears, operative treatments have provided minimal functional gains and suffer from high re-tear rates. The hypocellular nature of tendon tissue poses a limited capacity for regeneration. In recent years, great strides have been made in the area of tendonogenesis and differentiation towards tendon cells due to a greater understanding of the tendon stem cell niche, development of advanced materials, improved scaffold fabrication techniques, and delineation of the phenotype development process. Though in vitro models for tendonogenesis have shown promising results, in vivo models have been less successful. The present work investigates structured matrices mimicking the tendon microenvironment as cell delivery vehicles in a rat RC tear model. RC injuries augmented with a matrix delivering rat mesenchymal stem cells (rMSCs) showed enhanced regeneration over suture repair alone or repair with augmentation, at 6 and 12-weeks post-surgery. The local delivery of rMSCs led to increased mechanical properties and improved tissue morphology. We hypothesize that the mesenchymal stem cells function to modulate the local immune and bioactivity environment through autocrine/paracrine and/or cell homing mechanisms. This study provides evidence for improved tendon healing with biomimetic matrices and delivered MSCs with the potential for translation to larger, clinical animal models. The enhanced regenerative healing response with stem cell delivering biomimetic matrices may represent a new treatment paradigm for massive RC tendon tears.

Tunable, biodegradable gold nanoparticles as contrast agents for computed tomography and photoacoustic imaging.

Gold nanoparticles (AuNP) have been proposed for many applications in medicine. Although large AuNP (>5.5 nm) are desirable for their longer blood circulation and accumulation in diseased tissues, small AuNP (<5.5 nm) are required for excretion via the kidneys. We present a novel platform where small, excretable AuNP are encapsulated into biodegradable poly di(carboxylatophenoxy)phosphazene (PCPP) nanospheres. These larger nanoparticles (Au-PCPP) can perform their function as contrast agents, then subsequently break down into harmless byproducts and release the AuNP for swift excretion. Homogeneous Au-PCPP were synthesized using a microfluidic device. The size of the Au-PCPP can be controlled by the amount of polyethylene glycol-polylysine (PEG-PLL) block co-polymer in the formulation. Synthesis of Au-PCPP nanoparticles and encapsulation of AuNP in PCPP were evaluated using transmission electron microscopy and their biocompatibility and biodegradability confirmed in vitro. The Au-PCPP nanoparticles were found to produce strong computed tomography contrast. The UV-Vis absorption peak of Au-PCPP can be tuned into the near infrared region via inclusion of varying amounts of AuNP and controlling the nanoparticle size. In vitro and in vivo experiments demonstrated the potential of Au-PCPP as contrast agents for photoacoustic imaging. Therefore, Au-PCPP nanoparticles have high potency as contrast agents for two imaging modalities, as well as being biocompatible and biodegradable, and thus represent a platform with potential for translation into the clinic.

The expanding field of polyphosphazene high polymers.

The importance of phosphorus in polymer chemistry is illustrated by the growth of the broad field of polyphosphazene science. Several hundred high polymers are now known with a phosphorus-nitrogen backbone and combinations or more than 250 different organic side groups. The properties of these polymers depend on both the character of the inorganic backbone and the structure of the organic side groups. This summary reviews the synthesis pathways to these materials, the often-unique structure-property relationships, and challenges for the future expansion of this field.

Polyphosphazenes with Immobilized Dyes as Potential Color Filter Materials.

Red, green, and blue dye molecules were linked covalently to polyphosphazenes to generate soluble, film-forming materials appropriate for the formation of patterned tricolor filters for possible use in liquid crystalline and other display devices or in camera sensors. The monofunctional dyes (a red 1-[(E)-(4-nitrophenyl)diazenyl]-2-naphthol, a green tetraphenylporphyrin [5-(4-hydroxyphenyl)-10,15,20-tetraphenylporphyrin], and a toluidine blue dye) were employed as representative chromophores. The cosubstituents employed included 2,2,2-trifluoroethoxy with and without aryloxy groups or cyclopentanoxy groups. The optical densities were varied by adopting several dye-to-cosubstituent side group ratios. These dyes are models for a wide range of different chromophores that can be linked to polyphosphazene chains.

Nanodisco balls: control over surface versus core loading of diagnostically active nanocrystals into polymer nanoparticles.

Nanoparticles of complex architectures can have unique properties. Self-assembly of spherical nanocrystals is a high yielding route to such systems. In this study, we report the self-assembly of a polymer and nanocrystals into aggregates, where the location of the nanocrystals can be controlled to be either at the surface or in the core. These nanospheres, when surface decorated with nanocrystals, resemble disco balls, thus the term nanodisco balls. We studied the mechanism of this surface loading phenomenon and found it to be Ca(2+) dependent. We also investigated whether excess phospholipids could prevent nanocrystal adherence. We found surface loading to occur with a variety of nanocrystal types including iron oxide nanoparticles, quantum dots, and nanophosphors, as well as sizes (10-30 nm) and shapes. Additionally, surface loading occurred over a range of polymer molecular weights (∼30-3000 kDa) and phospholipid carbon tail length. We also show that nanocrystals remain diagnostically active after loading onto the polymer nanospheres, i.e., providing contrast in the case of magnetic resonance imaging for iron oxide nanoparticles and fluorescence for quantum dots. Last, we demonstrated that a fluorescently labeled protein model drug can be delivered by surface loaded nanospheres. We present a platform for contrast media delivery, with the unusual feature that the payload can be controllably localized to the core or the surface.