Thiol-acrylate nanocomposite foams for critical size bone defect repair: A novel biomaterial

Fabrication and characterization of thiol-triacrylate polymer via Michael addition reaction for biomedical applications

Thiol-acrylate polymers have therapeutic potential as biocompatible scaffolds for bone tissue regeneration. Synthesis of a novel cyto-compatible and biodegradable polymer composed of trimethylolpropane ethoxylate triacrylate-trimethylolpropane tris (3-mercaptopropionate) (TMPeTA-TMPTMP) using a simple amine-catalyzed Michael addition reaction is reported in this study. This study explores the impact of molecular weight and crosslink density on the cyto-compatibility of human adipose derived mesenchymal stem cells. Eight groups were prepared with two different average molecular weights of trimethylolpropane ethoxylate triacrylate (TMPeTA 692 and 912) and four different concentrations of diethylamine (DEA) as catalyst. The materials were physically characterized by mechanical testing, wettability, mass loss, protein adsorption and surface topography. Cyto-compatibility of the polymeric substrates was evaluated by LIVE/DEAD staining^® and DNA content assay of cultured human adipose derived stem cells (hASCs) on the samples over over days. Surface topography studies revealed that TMPeTA (692) samples have island pattern features whereas TMPeTA (912) polymers showed pitted surfaces. Water contact angle results showed a significant difference between TMPeTA (692) and TMPeTA (912) monomers with the same DEA concentration. Decreased protein adsorption was observed on TMPeTA (912) -16% DEA compared to other groups. Fluorescent microscopy also showed distinct hASCs attachment behavior between TMPeTA (692) and TMPeTA (912), which is due to their different surface topography, protein adsorption and wettability. Our finding suggested that this thiol-acrylate based polymer is a versatile, cyto-compatible material for tissue engineering applications with tunable cell attachment property based on surface characteristics.

Hybrid Synthetic-Biological Hydrogel System for Adipose Tissue Regeneration

Hydrogels are promising scaffolds for adipose tissue regeneration. Currently, the incorporation of bioactive molecules in hydrogel system is used, which can increase the cell proliferation rate or improve adipogenic differentiation performance of stromal stem cells but often suffers from high expense or cytotoxicity because of light/thermal curing used for polymerization. In this study, decellularized adipose tissue is incorporated, at varying concentrations, with a thiol-acrylate fraction that is then polymerized to produce hydrogels via a Michael addition reaction. The results reveal that the major component of isolated adipose-derived extracellular matrix (ECM) is Collagen I. Mechanical properties of ECM polyethylene glycol (PEG) are not negatively affected by the incorporation of ECM. Additionally, human adipose-derived stem cells (hASCs) are encapsulated in ECM PEG hydrogel with ECM concentrations varying from 0% to 1%. The results indicate that hASCs maintained the highest viability and proliferation rate in 1% ECM PEG hydrogel with most lipids formation when cultured in adipogenic conditions. Furthermore, more adipose regeneration is observed in 1% ECM group with in vivo study by Day 14 compared to other ECM PEG hydrogels with lower ECM content. Taken together, these findings suggest the ECM PEG hydrogel is a promising substitute for adipose tissue regeneration applications.

Polymer Nanocomposites via Click Chemistry Reactions

The emerging areas of polymer nanocomposites, as some are already in use in industrial applications and daily commodities, have the potential of offering new technologies with all manner of prominent capabilities. The incorporation of nanomaterials into polymeric matrix provides significant improvements, such as higher mechanical, thermal or electrical properties. In these materials, interface/interphase of components play a crucial role bringing additional features on the resulting nanocomposites. Among the various preparation strategies of such materials, an appealing strategy relies on the use of click chemistry concept as a multi-purpose toolbox for both fabrication and modulation of the material characteristics. This review aims to deliver new insights to the researchers of the field by noticing effective click chemistry-based methodologies on the preparation of polymer nanocomposites and their key applications such as optic, biomedical, coatings and sensor.

The effect of a crosslinking chemical reaction on pattern formation in viscous fingering of miscible fluids in a Hele-Shaw cell

Viscous fingering can occur in fluid motion whenever a high mobility fluid displaces a low mobility fluid in a Darcy type flow. When the mobility difference is primarily attributable to viscosity (e.g., flow between the two horizontal plates of a Hele-Shaw cell), viscous fingering (VF) occurs, which is sometimes termed the Saffman-Taylor instability. Alternatively, in the presence of differences in density in a gravity field, buoyancy-driven convection can occur. These instabilities have been studied for decades, in part because of their many applications in pollutant dispersal, ocean currents, enhanced petroleum recovery, and so on. More recent interest has emerged regarding the effects of chemical reactions on fingering instabilities. As chemical reactions change the key flow parameters (densities, viscosities, and concentrations), they may have either a destabilizing or stabilizing effect on the flow. Hence, new flow patterns can emerge; moreover, one can then hope to gain some control over flow instabilities through reaction rates, flow rates, and reaction products. We report effects of chemical reactions on VF in a Hele-Shaw cell for a reactive step-growth cross-linking polymerization system. The cross-linked reaction product results in a non-monotonic viscosity profile at the interface, which affects flow stability. Furthermore, three-dimensional internal flows influence the long-term pattern that results.

Thiol-Ene Photo-Click Collagen-PEG Hydrogels: Impact of Water-Soluble Photoinitiators on Cell Viability, Gelation Kinetics and Rheological Properties

Thiol-ene photo-click hydrogels were prepared via step-growth polymerisation using thiol-functionalised type-I collagen and 8-arm poly(ethylene glycol) norbornene-terminated (PEG-NB), as a potential injectable regenerative device. Type-I collagen was thiol-functionalised by a ring opening reaction with 2-iminothiolane (2IT), whereby up to 80 Abs.% functionalisation and 90 RPN% triple helical preservation were recorded via 2,4,6-Trinitrobenzenesulfonic acid (TNBS) colorimetric assay and circular dichroism (CD). Type, i.e., either 2-Hydroxy-1-[4-(2-hydroxyethoxy) phenyl]-2-methyl-1-propanone (I2959) or lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP), and concentration of photoinitiator were varied to ensure minimal photoinitiator-induced cytotoxicity and to enable thiol-ene network formation of collagen-PEG mixtures. The viability of G292 cells following 24 h culture in photoinitiator-supplemented media was largely affected by the photoinitiator concentration, with I2959-supplemented media observed to induce higher toxic response (0.1 → 0.5% (w/v) I2959, cell survival: 62 → 2 Abs.%) compared to LAP-supplemented media (cell survival: 86 → 8 Abs.%). In line with the in vitro study, selected photoinitiator concentrations were used to prepare thiol-ene photo-click hydrogels. Gelation kinetics proved to be largely affected by the specific photoinitiator, with LAP-containing thiol-ene mixtures leading to significantly reduced complete gelation time (τ: 187 s) with respect to I2959-containing mixtures (τ: 1683 s). Other than the specific photoinitiator, the photoinitiator concentration was key to adjusting the hydrogel storage modulus (G'), whereby 15-fold G' increase (232 → 3360 Pa) was observed in samples prepared with 0.5% (w/v) compared to 0.1% (w/v) LAP. Further thiol-ene formulations with 0.5% (w/v) LAP and varied content of PEG-NB were tested to prepare photo-click hydrogels with porous architecture, as well as tunable storage modulus (G': 540⁻4810 Pa), gelation time (τ: 73⁻300 s) and swelling ratio (SR: 1530⁻2840 wt %). The photoinitiator-gelation-cytotoxicity relationships established in this study will be instrumental to the design of orthogonal collagen-based niches for regenerative medicine.

Bone-forming cells with pronounced spread into the third dimension in polymer scaffolds fabricated by two-photon polymerization

The main aim of this work was to stimulate bone‐forming cells to produce three‐dimensional networks of mineralized proteins such as those occurring in bones. This was achieved by a novel approach using a specific type of mesenchymal progenitor cells (i.e., primary fibroblast cells from human hair roots) seeded on to polymer scaffolds. We wrote polymer microstructures with one or more levels of quadratic pores on to a flexible substrate by means of two‐photon polymerization using a Ti‐sapphire femtosecond laser focused into a liquid acrylate‐based resin containing a photoinitiator. Progenitor cells, differentiated into an osteogenic lineage by the use of medium supplemented with biochemical stimuli, can be seeded on to the hydrophilic three‐dimensional scaffolds. Due to confinement to the microstructures and/or mechanical interaction with the scaffold, the cells are stimulated to produce high amounts of calcium‐binding proteins, such as collagen type I, and show an increased activation of the actin cytoskeleton. The best results were obtained for quadratic pore sizes of 35 µm: the pore volumes become almost filled with both cells in close contact with the walls of the structure and with extracellular matrix material produced by the cells. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 891–899, 2017.

In vitro evaluation of thermal frontally polymerized thiol-ene composites as bone augments

Because of the large number of total knee replacement (TKR) surgeries conducted per year, and with projections of increased demand to almost a million primary TKR surgeries per year by 2030 in the United States alone, there is a need to discover more efficient working materials as alternatives to current bone cements. There is a need for surgeons and hospitals to become more efficient and better control over the operative environment. One area of inefficiency is the cement steps during TKR. Currently the surgeon has very little control over cement polymerization. This leads to an increase in time, waste, and procedural inefficiencies. There is a clear need to create an extended working time, moldable, osteoconductive, and osteoinductive bone augment as a substitution for the current clinically used bone cement where the surgeon has better control over the polymerization process. This study explored several compositions of pentaerythritol-co-trimethylolpropane tris-(3-mercaptopropionate) hydroxyapatite composite materials prepared via benzoyl peroxide-initiated thermal frontal polymerization. The 4:1 acrylate to thiol ratio containing augment material shows promise with a maximal propagation temperature of 160°C ± 10°C, with mechanical strength of 3.65 MPa, and 111% cytocompatibility, relative to the positive control. This frontally polymerized material may have application as an augment with controlled polymerization supporting cemented implants. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1152-1160, 2016.

Recent Advances in Hydroxyapatite Scaffolds Containing Mesenchymal Stem Cells

Modern day tissue engineering and cellular therapies have gravitated toward using stem cells with scaffolds as a dynamic modality to aid in differentiation and tissue regeneration. Mesenchymal stem cells (MSCs) are one of the most studied stem cells used in combination with scaffolds. These cells differentiate along the osteogenic lineage when seeded on hydroxyapatite containing scaffolds and can be used as a therapeutic option to regenerate various tissues. In recent years, the combination of hydroxyapatite and natural or synthetic polymers has been studied extensively. Due to the interest in these scaffolds, this review will cover the wide range of hydroxyapatite containing scaffolds used with MSCs for in vitro and in vivo experiments. Further, in order to maintain a progressive scope of the field this review article will only focus on literature utilizing adult human derived MSCs (hMSCs) published in the last three years.

In vitro characterization of polyesters of aconitic acid, glycerol, and cinnamic acid for bone tissue engineering

In this study, a novel class of polyesters of glycerol, aconitic acid, and cinnamic acid were synthesized along with their hydroxyapatite (HA) composites, and studied for their potential application in bone defect repair. An osteogenic study was conducted with human adipose derived mesenchymal stem cells (hASCs) to determine the osteoinductive ability of aconitic acid-glycerol (AG) polyesters, AG:HA (80:20), aconitic acid-glycerol-cinnamic acid (AGC) polyesters, and AGC:HA (80:20) to serve as bone scaffolds. The results indicate that AGC scaffolds have the highest mechanical strength in comparison to AG, AG:HA (80:20), and AGC:HA (80:20) scaffolds due to its low porosity. It was determined by cytotoxicity and osteogenesis experiments that hASCs cultured for 21 days on AG:HA (80:20) scaffolds in stromal medium exhibited a greater number of live cells than control PCL:HA composites. Moreover, hASCs cultured on foamed AG:HA (80:20) scaffolds resulted in the highest levels of mineralization, increased alkaline phosphatase (ALP) expression, and the greatest osteocalcin (OCN) expression after 21 days. Overall, AG:HA (100:0 and 80:20) scaffolds had higher mechanical strength and cytocompatibility than the PCL:HA control. In vitro osteogenic study demonstrated that AG:HA (100:0 and 80:20) synthesized using sugarcane industry by-products hold potential as scaffolds for bone tissue engineering applications.

In vitro and in vivo characterization of pentaerythritol triacrylate-co-trimethylolpropane nanocomposite scaffolds as potential bone augments and grafts

A thiol-acrylate-based copolymer synthesized via an amine-catalyzed Michael addition was studied in vitro and in vivo to assess its potential as an in situ polymerizing graft or augment in bone defect repair. The blends of hydroxyapatite (HA) with pentaerythritol triacrylate-co-trimethylolpropane (PETA), cast as solids or gas foamed as porous scaffolds, were evaluated in an effort to create a biodegradable osteogenic material for use as a bone-void-filling augment. Osteogenesis experiments were conducted with human adipose-derived mesenchymal stromal cells (hASCs) to determine the ability of the material to serve as an osteoinductive substrate. Poly(ɛ-caprolactone) (PCL) composites PCL:HA (80:20) (wt/wt%) served as the control scaffold, while the experimental scaffolds included PETA:HA (100:0), (85:15), (80:20), and (75:25) composites (wt/wt%). The results indicate that PETA:HA (80:20) foam composites had higher mechanical strength than the corresponding porous PCL:HA (80:20) scaffolds made by thermo-precipitation method, and in the case of foamed composites, increasing HA content directly correlated with increased yield strength. For cytotoxicity and osteogenesis experiments, hASCs cultured for 21 days on PETA:HA scaffolds in stromal medium displayed the greatest number of live cells compared with PCL:HA composites. Moreover, hASCs cultured on foamed PETA:HA (80:20) scaffolds resulted in the greatest mineralization, increased alkaline phosphatase (ALP) expression, and the highest osteocalcin (OCN) expression after 21 days. Overall, the PETA:HA (80:20) and PETA:HA (85:15) scaffolds, with 66.38% and 72.02% porosity, respectively, had higher mechanical strength and cytocompatibility compared with the PCL:HA control. The results of the 6-week in vivo biocompatibility study using a posterior lumbar spinal fusion model demonstrate that PETA:HA can be foamed in vivo without serious adverse effects at the surgical site. Additionally, it was demonstrated that cells migrate into the interconnected pore volume and are found within centers of ossification.

High strength biocompatible PEG single-network hydrogels

A single-chain PEG hydrogel with extremely high strength was prepared via precise design and control over the molecular topology of the polymeric network.