Showing papers by "Jason A. Burdick published in 2012"

Shear-thinning hydrogels for biomedical applications

Abstract: Injectable hydrogels are becoming increasingly important in the fields of tissue engineering and drug delivery due to their tunable properties, controllable degradation, high water content, and the ability to deliver them in a minimally invasive manner. Shear-thinning is one promising technique for the application of injectable hydrogels, where preformed hydrogels can be injected by application of shear stress (during injection) and quickly self-heal after removal of shear. Importantly, these gels can be used to deliver biological molecules and cells during the injection process. This review aims to highlight the range of injectable shear-thinning hydrogel systems being developed, with a focus on the various mechanisms of formation and shear-thinning and their use in biomedical applications.

Stiffening hydrogels to probe short- and long-term cellular responses to dynamic mechanics

Abstract: Biological processes are dynamic in nature, and growing evidence suggests that matrix stiffening is particularly decisive during development, wound healing and disease; yet, nearly all in vitro models are static. Here we introduce a step-wise approach, addition then light-mediated crosslinking, to fabricate hydrogels that stiffen (for example, ~3-30 kPa) in the presence of cells, and investigated the short-term (minutes-to-hours) and long-term (days-to-weeks) cell response to dynamic stiffening. When substrates are stiffened, adhered human mesenchymal stem cells increase their area from ~500 to 3,000 μm(2) and exhibit greater traction from ~1 to 10 kPa over a timescale of hours. For longer cultures up to 14 days, human mesenchymal stem cells selectively differentiate based on the period of culture, before or after stiffening, such that adipogenic differentiation is favoured for later stiffening, whereas osteogenic differentiation is favoured for earlier stiffening.

Moving from static to dynamic complexity in hydrogel design

Abstract: Hydrogels are water-swollen polymer networks that have found a range of applications from biological scaffolds to contact lenses. Historically, their design has consisted primarily of static systems and those that exhibit simple degradation. However, advances in polymer synthesis and processing have led to a new generation of dynamic systems that are capable of responding to artificial triggers and biological signals with spatial precision. These systems will open up new possibilities for the use of hydrogels as model biological structures and in tissue regeneration.

Sacrificial nanofibrous composites provide instruction without impediment and enable functional tissue formation

Abstract: The fibrous tissues prevalent throughout the body possess an ordered structure that underlies their refined and robust mechanical properties. Engineered replacements will require recapitulation of this exquisite architecture in three dimensions. Aligned nanofibrous scaffolds can dictate cell and matrix organization; however, their widespread application has been hindered by poor cell infiltration due to the tight packing of fibers during fabrication. Here, we develop and validate an enabling technology in which tunable composite nanofibrous scaffolds are produced to provide instruction without impediment. Composites were formed containing two distinct fiber fractions: slow-degrading poly(e-caprolactone) and water-soluble, sacrificial poly(ethylene oxide), which can be selectively removed to increase pore size. Increasing the initial fraction of sacrificial poly(ethylene oxide) fibers enhanced cell infiltration and improved matrix distribution. Despite the removal of >50% of the initial fibers, the remaining scaffold provided sufficient instruction to align cells and direct the formation of a highly organized ECM across multiple length scales, which in turn led to pronounced increases in the tensile properties of the engineered constructs (nearly matching native tissue). This approach transforms what is an interesting surface phenomenon (cells on top of nanofibrous mats) into a method by which functional, 3D tissues (>1 mm thick) can be formed, both in vitro and in vivo. As such, this work represents a marked advance in the engineering of load-bearing fibrous tissues, and will find widespread applications in regenerative medicine.

Dynamic Compressive Loading Enhances Cartilage Matrix Synthesis and Distribution and Suppresses Hypertrophy in hMSC-Laden Hyaluronic Acid Hydrogels

Abstract: Mesenchymal stem cells (MSCs) are being recognized as a viable cell source for cartilage repair, and there is growing evidence that mechanical signals play a critical role in the regulation of stem cell chondrogenesis and in cartilage development. In this study we investigated the effect of dynamic compressive loading on chondrogenesis, the production and distribution of cartilage specific matrix, and the hypertrophic differentiation of human MSCs encapsulated in hyaluronic acid (HA) hydrogels during long term culture. After 70 days of culture, dynamic compressive loading increased the mechanical properties, as well as the glycosaminoglycan (GAG) and collagen contents of HA hydrogel constructs in a seeding density dependent manner. The impact of loading on HA hydrogel construct properties was delayed when applied to lower density (20 million MSCs/ml) compared to higher seeding density (60 million MSCs/ml) constructs. Furthermore, loading promoted a more uniform spatial distribution of cartilage matrix in ...

Harnessing Interfacial Phenomena to Program the Release Properties of Hollow Microcapsules

Abstract: The generation of near-infrared (NIR)-sensitive microcapsules is presented and it is demonstrated that the release properties of these microcapsules can be tailored by controlling their morphology. A biocompatible polymer, poly(DL-lactic-co-glycolic)acid (PLGA) is used to form hollow microcapsules from monodisperse water-in-oil-in-water (W/O/W) double emulsions. Both the composition of PLGA and the oil phase of W/O/W double emulsions significantly affect the morphology of the subsequently formed microcapsules. PLGA microcapsules with vastly different morphologies, from spherical to “snowman-like” capsules, are obtained due to changes in the solvent quality of the oil phase during solvent removal. The adhesiveness of the PLGA-laden interface plays a critical role in the formation of snowman-like microcapsules. NIR-sensitive PLGA microcapsules are designed to have responsive properties by incorporating Au nanorods into the microcapsule shell, which enables the triggered release of encapsulated materials. The effect of capsule morphology on the NIR responsiveness and release properties of PLGA microcapsules is demonstrated.

Improved cartilage repair via in vitro pre-maturation of MSC-seeded hyaluronic acid hydrogels

Abstract: Functional repair of focal cartilage defects requires filling the space with neotissue that has compressive properties comparable to native tissue and integration with adjacent host cartilage. While poor integration is a common complication with current clinical treatments, reports of tissue engineering advances in the development of functional compressive properties rarely include analyses of their potential for integration. Our objective was thus to assess both the maturation and integration of mesenchymal stem cell (MSC)-laden hyaluronic acid (HA) hydrogels in an in vitro cartilage defect model. Furthermore, we considered the effects of an initial period of pre-maturation as well as various material formulations to maximize both construct compressive properties and integration strength. MSCs were encapsulated in 1%, 3% and 5% methacrylated HA (MeHA) or 2% agarose (Ag) and gelled directly (in situ) within an in vitro cartilage defect or were formed and then pre-cultured for 4 weeks before implantation. Results showed that the integration strength of pre-cultured repair constructs was equal to (1% MeHA) or greater than (2% Ag) the integration of in situ repaired cartilage. Moreover, MSC chondrogenesis and maturation was restricted by the in situ repair environment with constructs maturing to a much lesser extent than pre-matured constructs. These results indicate that construct pre-maturation may be an essential element of functional cartilage repair.

Magnitude and presentation of mechanical signals influence adult stem cell behavior in 3-dimensional macroporous hydrogels

Abstract: The mechanical properties of the microenvironment are being recognized as a key contributor to stem cell behaviour, whether in the context of tissues or when designing biomaterials. While there has been considerable evidence demonstrating the effect of 2-D static mechanics on stem cells, few systems exist to investigate the influence of the 3-D presentation of mechanical signals. In this study, methacrylated hyaluronic acid (MeHA) was processed into porous (crosslinking around spherical templates) 3-D hydrogels with tunable elastic moduli ranging from ∼1.5 kPa to 12.4 kPa. Porous hydrogels were fabricated with a sequential crosslinking process (addition crosslinking of methacrylates with dithiols, followed by UV photopolymerization) where the hydrogel mechanics are controlled by the extent of UV exposure and are subsequently seeded with human mesenchymal stem cells (hMSCs). hMSCs spread within the pores and proliferated in a mechanically dependent manner as cells within the softest 1.5 kPa hydrogels were less spread initially and showed little proliferation with time, while hMSCs in stiffer hydrogels (>1.5 kPa) had higher initial spreading and a roughly two-fold increase in cell number after 14 days. In growth media, porous hydrogels supported a slight upregulation (two to eight-fold) in chondrogenic genes across all mechanics, while there was only modest upregulation in a few conditions (and in many cases downregulation) for myogenic, osteogenic, and adipogenic genes. The secretion of various cytokines and angiogenic molecules was found to be mechanically dependent in the porous system with greater secretion at day 2 in the stiffer hydrogels (3.8 kPa and 7.4 kPa). However, by day 14 there was greater secretion in the softer hydrogels (1.5 kPa and 2.6 kPa). Finally, when mechanics were temporally increased during culture (from ∼2.6 kPa to 12.4 kPa), there was a noticeable decrease in the secretion of 15 angiogenic and cytokine proteins. Thus, the influence of mechanics on stem cells within hydrogel structures appears to be dependent on the magnitude and timing of presentation.

Soft biodegradable polymersomes from caprolactone-derived polymers

Abstract: Polymersomes have promise for advanced theranostic delivery. We report here the design and characterization of a series of block copolymers that assemble into soft, bioresorbable, and non-toxic vesicles. The polymers are based on poly(ethylene glycol)–poly(caprolactone), but the caprolactone (CL) is copolymerized with a second monomer, 1,4,8-trioxaspiro-[4,6]-9-undecanone (TOSUO). Because TOSUO polymers have no crystalline character, copolymerizing TOSUO with CL should reduce the crystallinity of the polymersomes. After synthesizing polymers with different ratios of CL to TOSUO, we found that all copolymers assemble into both micron and nano-metric vesicles. Increasing the TOSUO content of the copolymer reduces the polymer crystalline melting temperature and the area expansion modulus of vesicle membranes. Membranes with partial crystalline structure exhibit hysteresis in the tension versus strain curve during aspiration. Vesicles are not cytotoxic and exhibit first-order release of encapsulated gemcitabine. These materials are promising for the development of deformable, biodegradable polymersomes for biomedical applications.

Controlling the Cell-Adhesion Properties of Poly(acrylic acid)/Polyacrylamide Hydrogen-Bonded Multilayers

Abstract: A thorough understanding of thermal effects on the physicochemical properties of layer-by-layer (LbL) assembled multilayers is crucial in utilizing these films in a variety of applications. In this work, we investigate the effect of thermal treatment on cross-linking and swelling of a hydrogen-bonded multilayer film made of poly(acrylic acid) (PAA) and polyacrylamide (PAAm), which has been shown to exhibit excellent long-term cell adhesion resistance. We observe that the apparent swelling of PAA/PAAm multilayers treated at 90 and 180 °C in a physiologically relevant condition is similar; however, these two multilayers with different thermal history exhibit completely different cell adhesion properties when assessed with human mesenchymal stem cells (hMSCs). While the 90 °C treated samples show excellent cell adhesion resistance, those treated at 180 °C are highly cell adhesive. A combination of characterization techniques including thermogravimetric analysis (TGA) and Fourier-transform infrared (FT-IR) sp...

Aligned fibrous materials with spatially varying fiber orientation and related methods

Abstract: Provided are materials comprising layers of anisotropically aligned fibers, the alignment of which fibers may be adjusted so as to give rise to circumferentially-aligned fibers that replicate the fiber alignment of native fibrous tissue, such as the meniscus or the annulus fibrosis. Also provided are laminates formed from the disclosed materials, as well as methods of fabricating the disclosed materials and laminates.

Influence of Chondrocyte Zone on Co-Cultures With Mesenchymal Stem Cells in HA Hydrogels for Cartilage Tissue Engineering

Abstract: Mesenchymal stem cells (MSCs) are an attractive cell type for cartilage tissue engineering in that they can undergo chondrogenesis in a variety of 3D contexts [1] Focused efforts in MSC-based cartilage tissue engineering have recently culminated in the formation of biologic materials possessing biochemical and functional mechanical properties that match that of the native tissue [2] These approaches generally involve the continuous or intermittent application of pro-chondrogenic growth factors during in vitro culture For example, in one recent study, we showed robust construct maturation in MSC-seeded hyaluronic acid (HA) hydrogels transiently exposed to high levels of TGF-β3 [3] Despite the promise of this approach, MSCs are a multipotent cell type and retain a predilection towards hypertrophic phenotypic conversion (ie, bone formation) when removed from a pro-chondrogenic environment (eg, in vivo implantation) Indeed, even in a chondrogenic environment, many MSC-based cultures express pre-hypertrophic markers, including type X collagen, MMP13, and alkaline phosphatase [4] To address this issue, recent studies have investigated co-culture of human articular chondrocytes and MSCs in both pellet and hydrogel environments Chondrocytes appear to enhance the initial efficiency of MSC chondrogenic conversion, as well as limit hypertrophic changes in some instances (potentially via secretion of PTHrP and/or other factors) [5–7] While these findings are intriguing, articular cartilage has a unique depth-dependent morphology including zonal differences in chondrocyte identity Ng et al showed that zonal chondrocytes seeded in a bi-layered agarose hydrogel construct can recreate depth-dependent cellular and mechanical heterogeneity, suggesting that these identities are retained with transfer to 3D culture systems [8] Further, Cheng et al showed that differences in matrix accumulation and hypertrophy in zonal chondrocytes was controlled by bone morphogenic protein [9] To determine whether differences in zonal chondrocyte identity influences MSC fate decisions, we evaluated functional properties and phenotypic stability in photocrosslinked hyaluronic acid (HA) hydrogels using distinct, zonal chondrocyte cell fractions co-cultured with bone marrow derived MSCsCopyright © 2012 by ASME

Optimization of macromer density in human MSC-laden hyaluronic acid (HA) hydrogels

Abstract: Mesenchymal stem cells are attractive cell type and can undergo chondrogenesis in various 3D platforms. Hyaluronic acid (HA) hydrogel, a natural constituent of the cartilage extracellular matrix, provides a biologically relevant interface for encapsulated cells. While MSC-laden HA constructs can produce native mechanical properties using cells from animal sources, clinical repair will depend on successful translation of these findings to human MSCs (hMSCs). To optimize chondrogenesis, we assessed the ability of hMSCs to undergo chondrogenesis in varying macromer concentration HA gels. Variation in this parameter influenced construct mechanical and biochemical properties. In 1% methacrylated HA (MeHA), equilibrium modulus and GAG content were higher (86kPa (E Y ) and 2.16%) than in 2% MeHA constructs (50 kPa, 1.62% GAG). However, greater contractility occurred in 1% MeHA (−36.25%/−24.25%; Thickness/diameter) compared to 2% MeHA (−20.57%/1.02%) constructs. This study provides new insight into optimized macromer densities for hMSC-based cartilage tissue engineering using HA hydrogels.

9.22 – Photopolymerizable Systems

Abstract: Photopolymerization has become a common technology for a wide variety of applications including coatings and adhesives and recently for biotechnology. The fast polymerization kinetics under physiological conditions, the chemical versatility, and the spatial and temporal control has led to photopolymerization becoming an important processing method for the fabrication of biomaterials. In this chapter, we discuss the basics of photopolymerization reactions, specific polymer systems that are photopolymerized, the characterization of photopolymerization reactions, some of the important parameters that influence photopolymerization, and general applications with special emphasis on dental materials, tissue engineering, and drug delivery.

Infarction treatment compositions and methods

Abstract: Provided are methods of treating cardiac infarction by using an injectable material to influence cardiac structure and remodeling after infarction. Also provided are kits that comprise an injectable material to influence cardiac structure.