Understanding factors affecting cell invasion influences the design ofengineered constructs for tissue regeneration. The objective of this workwas to investigate the effect of matrix stiffness on invasion of tumor cellsthrough a synthetic hydrogel with well-defined properties. A novel staracrylate-functionalized polyethylene glycol-co-lactide (SPELA) macromer wassynthesized to produce hydrogels with well-defined water content, elasticmodulus, degree of crosslinking and hydrophilicity. The hydrogel was formedby photo-polymerization of the macromer with or without integrin-bindingcell adhesive RGD peptide. Cell invasion experiments were carried out in atranswell with SPELA hydrogel as the invading matrix and 4T1 mouse breastcancer cells. The invading cells on the lower membrane side were countedwith an inverted fluorescent microscope. The concentration of SPELA macromerranged from 10-25 wt% and that of RGD ranged from 1x10-4 to 1x10-2 M. The shear modulus of the hydrogel varied from 200 Pato 25 kPa as the SPELA concentration increased from 10 to 25 wt%. Cellinvasion slightly increased with increasing RGD concentration. However, RGDconcentration >1% resulted in a significant decrease in cell migration.As the matrix stiffness increased from 0.15 to 0.4, 3, 5, 6, 14, and 25 kPathe invasion rate decreased from 18.0 to 5.5, 6, 5.7, 5.2, 1.5, and 1.0 cells/mm2/h, respectively. There was a sharp decrease ininvasion rate for matrix stiffness greater than 10 kPa. Results demonstratethat matrix stiffness plays a major role in invasion of tumor cell through agelatinous matrix.

Recombinant human bone morphogenetic protein (rhBMP-2) plays a major role in differentiation of marrow stromal cells (MSCs). Peptides based on the active domains of rhBMP- 2, like the LYLTSIASLETPVSSAKPIK (BMP peptide), have been proposed as an alternative to reduce the side effects associated with high doses of rhBMP-2. The objective of this work was to determine the osteogenic activity of the BMP peptide grafted to poly(lactide fumarate) nanoparticles (PLAF NPs). A cysteine-terminated BMP peptide was grafted to PLAF NPs by linking the cysteine residue to the fumarate groups. Groups included blank NPs, free BMP peptide, free BMP-2 protein, and BMP-grafted NPs. The decrease in cell numbers after 21 days was consistent with an increase in mineral content and decrease in proliferation with osteogenic differentiation of MSCs. Alkaline phosphatase (ALPase) activity peaked at 14 days, consistent with the start of the osteogenic cascade. A slightly higher mineral content was observed in the BMP-grafted NP group after 14 days. m-RNA and immunostaining showed that cells in the BMP-grafted NP group had a higher content of osteocalcin protein after 21 days. Results suggest that the BMP-grafted NPs have a greater affinity to BMP cell surface receptors, leading to a stronger activation of the pathways leading to osteogenesis.

In this work the microstructures of star acrylated poly(ethyleneglycol-co-lactide) (SPELA) with different LA:EG ratios in the aqueoussolution have been simulated via Dissipative Particle Dynamics (DPD)approach at the mesoscale. The system components were coarse-grained intodifferent beads (set of atoms) which moved according to the Newton’sequations of motion integrated via a modified Velocity-Verlet algorithm. Theforce acting on each bead, in a specific cutoff distance (rc),was divided into a conservative force (FC), random force (FR), dissipativeforce (FD), bond force (FS) and bond angle force (FE). The repulsionparameters of the conservative force (αij) were calculated fromthe solubility parameter of the beads, each of which were extracted from anatomistic molecular dynamics simulation (MD). Simulations showed theformation of micelles with lactide and acrylate beads occupied the core andhydrophilic ethylene oxide segments extending through the water to form thecorona. The micelles showed an increasing trend in size and decreasing trendin number density with increase in LA:EG ratio. Results showed that theacrylate density decreased from the center of the micelles to the coresurface although the overall amount of acrylates increased due to theincrease in volume. Furthermore, the running integration number ofacrylate-water beads showed decreasing accessibility of acrylates to waterwith increasing PLA volume fraction.

In this work, a novel star 4-arm poly(ethylene glycol-co-lactide) acrylate macromonomer (SPELA) is synthesized, and the effect of macromonomer concentration and architecture on modulus, swelling ratio and sol fraction is investigated. The results show that the storage modulus of the hydrogel had an increasing trend with polymer concentration. Changing the polymer architecture from linear to 4-arm increased the storage modulus by 2.2-fold. The water content depended on the hydrophilic segment density as well as the extent of crosslinking and showed a decreasing trend with macromonomer concentration. The sol fractions of the SPELA hydrogels changed from 13% to 5% when concentration increased from 10% to 25%. The star SPELA hydrogel with high modulus, fast gelation time, and low sol fraction is potential useful as a degradable carrier in cell-based therapies. Results show that the SPELA hydrogel supports viability and osteogenic differentiation of the encapsulated bone marrow stromal cells.

Current biomaterials as a scaffold for bone regeneration are limited by the extent of degradation concurrent with bone formation, mechanical strength, and the extent of osteogenic differentiation of marrow stromal cells migrating from the surrounding tissues. In this project, a novel laminated nanocomposite scaffold is fabricated, consisting of poly (L-lactide ethylene oxide fumarate) (PLEOF) hydrogel reinforced with poly (L-lactide acid) (PLLA) electrospun nanofibers and hydroxyapatite (HA) nanoparticles. The laminated nanocomposites were fabricated by dry-hand lay up technique followed by compression molding and thermal crosslinking. The laminated nanocomposites were evaluated with respect to mechanical strength and osteogenic differentiation of marrow stromal (BMS) cells. Laminates showed modulus values much higher than that of hydrogel or fiber. The effect of laminated nanocomposites on osteogenic differentiation of BMS cells was determined in terms of ALPase activity and calcium content. Our results demonstrate that grafting RGD peptide to a PLEOF/HA hydrogel reinforced with PLLA nanofibers synergistically enhances osteogenic differentiation of BMS cells.

Nanoparticles (NPs) are being used extensively for tumor drug delivery. These devices offer the advantage of their size, protection of the encapsulated species, and biodegradability to prevent accumulation in the interstitium. Our laboratory has synthesized polylactide fumarate (PLAF, PLGF) macromers capable of self-assembling into biodegradable and biocompatible NPs with average particle size of 280 nm. To assess the possibility of further decreasing the particle size and distribution of PLGF NPs, the polymers were conjugated with the peptide sequence Cys-Val-Val-Val-Val-Val-Val-Lys-Lys (CV6K2), which is known to self-assemble in aqueous solution into vesicles of about 50-60 nm in size. The results indicate that the PLGF-CV6K2 conjugates are capable of self-assembling into NPs of 100 nm in diameter. The NPs are proposed as having a bilayer structure, with peptide chains facing the aqueous environment and the polymer chains compacted in an internal hydrophobic layer. Degradation kinetics and release profiles show that the NPs could effectively retain and release Paclitaxel up to 30 days until completely degraded. The NPs act as reservoirs for sustained release of the active agent by diffusion and degradation of the matrix when taken up by tumor cells.

Stromal derived factor-1α (SDF-1α) is an important chemokine in stem cell trafficking and plays a critical role in the homing, osteogenesis as well as angiogenesis of bone marrow stromal (BMS) cells. The objective of this work was to investigate the release characteristics of SDF-1α from the degradable poly(lactide ethylene oxide fumarate) (PLEOF) hydrogels and to determine the effect of sustained release of SDF-1α on migration of BMS cells. Three PLEOF macromers with PLA content of 6, 9, and 24 by weight were synthesized by condensation polymerization. The cumulative amount of biologically-active SDF-1α released from the PLEOF hydrogels after 3 weeks was between 20-25% of the initial loading and was independent of PLA/PEG ratio in the hydrogel. The migration of BMS cells in response to the time-release SDF-1α from PLEOF hydrogels closely followed the release kinetics of SDF-1α from the hydrogels. Results demonstrate that migration of BMS cells was significantly increased by the sustained release of SDF-1α from PLEOF hydrogels.

A molecular model is proposed for the dynamics of polymer chains in dilute polymer solutions containing well-dispersed spherical particles. In the presence of short range energetic affinity between the monomers and filler surface, the equilibrium structure of the adsorbed polymer layer is determined by a scaling theory. The viscoelastic response of the suspension is studied by a Maxwell model. It is shown that the solid-like properties of polymer nanocomposites in low frequency regimes could be attributed to the slowdown of the relaxation process of polymer chains. This process is controlled by the monomer-particle frictional interactions, density of the adsorbed polymer chains on the particles surface (controlled by monomer-particle adsorption energy), and volume fraction of the interfacial layer which can be enhanced by reduction of filler size or increasing the filler concentration.

An analytical model is developed for the effect of surface gradient in ligand density on the adhesion kinetics of a curved elastic membrane with mobile receptors. The displacement and speed of spreading at the edge of the adhesion zone as well as the density profile of receptors along the membrane are predicted as a function of time. According to results, in the diffusion-controlled regime, the front edge displacement of adhesion zone and the rate of membrane spreading decreased with increasing ligand density in a certain direction. Furthermore, the displacement of the edge of the adhesion zone did not scale with the square root of time, as observed on substrates with uniform ligand density.

Bone is a composite material consisting of aqueous gel and mineral phases. The aqueous gel phase gives bone its form and contributes to its ability to resist tension, while the mineral component resists compression. The combination of a hard inorganic phase and an elastic gel network provides bone with unique mechanical properties as well as a medium for diffusion and release of biologically active agents and it also facilitates communication with the cellular environment. A tissue engineered synthetic biomaterial as a scaffold for bone regeneration should provide temporary structural support to the reconstructed region and a medium for solubilization, diffusion, release of nutrients and growth factors, and their interactions with cells. In this work, the material and biologic properties of a novel synthetic matrix metalloproteinase (MMP) degradable hydrogel/apatite nanocomposite is investigated for its usefulness as a model matrix to mimic the gel and mineral components of the bone matrix and to fabricate aqueous-based scaffolds for bone regeneration. The gel phase is made from poly(lactide-ethylene oxide-fumarate), hereafter designated as PLEOF, terpolymer in which the water content can be adjusted by changing the ratio of the hydrophobic (lactide) to hydrophilic (ethylene oxide) oligomers. The hydrogel and apatite phases are crosslinked using an MMP degradable peptide crosslinker to modulate the matrix degradation kinetics with the migration of bone marrow stromal (BMS) cells. The results demonstrate that MMP degradable scaffolds fabricated from the PLEOF hydrogel and apatite nanoparticles are biocompatible and support cell attachment and migration.