Controlled drug delivery in tissue engineering

Bone tissue engineering: a review in bone biomimetics and drug delivery strategies

Critical-sized defects in bone, whether induced by primary tumor resection, trauma, or selective surgery have in many cases presented insurmountable challenges to the current gold standard treatment for bone repair. The primary purpose of a tissue-engineered scaffold is to use engineering principles to incite and promote the natural healing process of bone which does not occur in critical-sized defects. A synthetic bone scaffold must be biocompatible, biodegradable to allow native tissue integration, and mimic the multidimensional hierarchical structure of native bone. In addition to being physically and chemically biomimetic, an ideal scaffold is capable of eluting bioactive molecules (e.g., BMPs, TGF-betas, etc., to accelerate extracellular matrix production and tissue integration) or drugs (e.g., antibiotics, cisplatin, etc., to prevent undesired biological response such as sepsis or cancer recurrence) in a temporally and spatially controlled manner. Various biomaterials including ceramics, metals, polymers, and composites have been investigated for their potential as bone scaffold materials. However, due to their tunable physiochemical properties, biocompatibility, and controllable biodegradability, polymers have emerged as the principal material in bone tissue engineering. This article briefly reviews the physiological and anatomical characteristics of native bone, describes key technologies in mimicking the physical and chemical environment of bone using synthetic materials, and provides an overview of local drug delivery as it pertains to bone tissue engineering is included.

Use of templates to fabricate nanoscale spherical structures for defined architectural control

Architectural design of biomaterial structures is essential to reach the full potential of the materials' chemical and biological properties. Clinically, these properties depend on the targeted applications of delivery, such as tissue regeneration, imaging, or cancer. To get an efficient material for biological applications, key properties are needed, such as degradability, low toxicity, cell specificity, relative efficiency, and capability of delivering multiple molecules. In recent years, significant progress has been made through either the design of the material itself (synthetic or natural polymers, dendrimers, crosslinking) or the fabrication technique (nozzle reactor, emulsion, and template). The combination of these materials and techniques results in a large variety of biomaterials that have varied shape and physico-chemical and biological properties. Nevertheless, these inherent properties are not sufficient and interest in discovering and developing new techniques that present these biomaterials in different light is now under focus. A useful strategy to prepare biomaterials with unique and novel architectures is through the use of templates that have defined geometrical features. This holds great promise, especially for the development of hollow structures, such as spheres. The nanoscale structural design of biomaterials via the use of templates and their potential clinical applications are discussed. In addition, the conceptual hurdles that must be overcome to produce applications that are clinically relevant are examined.

Preliminary studies on noncovalent hyperbranched polymers based on PNA and DNA building blocks

In this work, we report thermodynamic, kinetic, and microrheological studies relative to the formation of PNA- and PNA/DNA-based noncovalent polymeric systems, useful tools for biotechnological and bioengineering applications. We realized two kinds of systems: a PNA-based system formed by a self-assembling PNA tridendron, and a PNA/DNA hybrid system formed by a PNA tridendron and a DNA linker. The formation of a three-dimensional polymeric network, by means of specific Watson-Crick base pairing, was investigated by a detailed UV and CD spectroscopic study. Preliminary microrheology experiments were performed on both systems to evaluate their viscoelastic properties which resulted in agreement with the formation of soluble hyperbranched polymers that could be useful for drug/gene delivery, as well as for encapsulating organic pollutants of different shapes and sizes in environmental applications.

Tissue engineering of skin

The engineering of skin substitutes and their application on human patients has become a reality. However, cell biologists, biochemists, technical engineers, and surgeons are still struggling with the generation of complex skin substitutes that can readily be transplanted in large quantities, possibly in only one surgical intervention and without significant scarring. Constructing a dermo-epidermal substitute that rapidly vascularizes, optimally supports a stratifying epidermal graft on a biodegradable matrix, and that can be conveniently handled by the surgeon, is now the ambitious goal. After all, this goal has to be reached coping with strict safety requirements and the harsh rules of the economic market.

Microsphere-based drug releasing scaffolds for inducing osteogenesis of human mesenchymal stem cells in vitro

In this study, in vitro osteogenesis was successfully achieved in human mesenchymal stem cells (hMSCs) by controlled release of the osteogenesis-inducing drugs dexamethasone, ascorbic acid (AA) and beta-glycerophosphate (GP) from poly(lactic-co-glycolic acid) (PLGA) sintered microsphere scaffolds (SMS). We investigated the osteogenesis of human MSCs (hMSCs) on dexamethasone laden PLGA-SMS (PLGA-Dex-SMS), and dexamethasone, AA and GP laden PLGA-SMS (PLGA-Com-SMS). hMSCs cultured on the microsphere systems, which act as drug release vehicles and also promote cell growth/tissue formation-displayed a strong osteogenic commitment locally. The osteogenic commitment of hMSCs on the scaffolds were verified by alkaline phosphatase (ALP) activity assay, calcium secretion assay, real-time PCR and immunohistochemistry analysis. The results indicated hMSCs cultured on PLGA-Com-SMS exhibited superior osteogenic differentiation owing to significantly high phenotypic expression of typical osteogenic genes-osteocalcin (OC), type I collagen, alkaline phosphatase (ALP), and Runx-2/Cbfa-1, and protein secretion of bone-relevant markers such as osteoclast and type I collagen when compared with PLGA-Dex-SMS. In conclusion, by promoting osteogenic development of hMSCs in vitro, this newly designed controlled release system opens a new door to bone reparation and regeneration.

Osmosis based method drives the self-assembly of polymeric chains into micro- and nanostructures

Polymers derived from monomers with a variety of functionalities provide materials with a vast range of properties and applications. Worldwide research has recently developed a wide number of methods suitable for the preparation of polymeric materials of nanometric dimensions, in view of the fact that, at the nanoscale level, new and unexpected properties emerge and lead to innovative applications. In this framework, we have exploited an easy method for the generation of nanostructures, regardless of the chemical structure of the pristine amorphous polymers, that is, biopolymers (e.g., polysaccharides) and synthetic, functional, and structural polymers (i.e, polystyrene, polymethylmethacrylates, polyacetylenes, and polymetallaynes). The nanostructure of these macromolecules, considered as the prototypes of various classes of polymeric materials, was achieved by using a simple and versatile procedure based on an osmotic method (OBM). Depending on the choice of solvent/nonsolvent pairs, the dialysis membrane molecular weight cutoff (MWCO), temperature, and polymer concentration, different morphologies can be obtained (e.g., spheres, sponges, disks, and fibers); also, a tuning of the nanoparticle dimensions ranging from the micro- to nanoscale has been obtained.

Bioactivated collagen-based scaffolds embedding protein-releasing biodegradable microspheres: tuning of protein release kinetics

In tissue engineering, the recapitulation of natural sequences of signaling molecules, such as growth factors, as occurring in the native extracellular matrix (ECM), is fundamental to support the stepwise process of tissue regeneration. Among the manifold of tissue engineering strategies, a promising one is based on the creation of the chrono-programmed presentation of different signaling proteins. This approach is based upon the integration of biodegradable microspheres, loaded with suitable protein molecules, within scaffolds made of collagen and, in case, hyaluronic acid, which are two of the fundamental ECM constituents. However, for the design of bioactivated gel-like scaffolds the determination of release kinetics must be performed directly within the tissue engineering template. In this work, biodegradable poly(lactic-co-glycolic)acid (PLGA) microspheres were produced by the multiple emulsion-solvent evaporation technique and loaded with rhodamine-labelled bovine serum albumin (BSA-Rhod), a fluorescent model protein. The microdevices were dispersed in collagen gels and collagen-hyaluronic acid (HA) semi-interpenetrating networks (semi-IPNs). BSA-Rhod release kinetics were studied directly on single microspheres through confocal laser scanning microscopy (CLSM). To thoroughly investigate the mechanisms governing protein release from PLGA microspheres in gels, BSA-Rhod diffusion in gels was determined by fluorescence correlation spectroscopy (FCS), and water transport through the microsphere bulk was determined by dynamic vapor sorption (DVS). Moreover, the decrease of PLGA molecular weight and glass transition temperature (T(g)) were determined by gel permeation chromatography (GPC) and differential scanning calorimetry (DSC), respectively. Results indicate that protein release kinetics and delivery onset strongly depend on the complex interplay between protein transport through the PLGA matrix and in the collagen-based release media, and water sequestration within the scaffolds, related to the scaffold hydrophilicity, which is dictated by HA content. The proper manipulation of all these features may thus allow the obtainment of a fine control over protein sequential delivery and release kinetics within tissue-engineering scaffolds.

Controlled growth factor delivery for tissue engineering

Growth factors play a crucial role in information transfer between cells and their microenvironment in tissue engineering and regeneration. They initiate their action by binding to specific receptors on the surface of target cells and the chemical identity, concentration, duration, and context of these growth factors contain information that dictates cell fate. Hence, the importance of exogenous delivery of these molecules in tissue engineering is unsurprising, considering their importance for tissue regeneration. However, the short half-lives of growth factors, their relatively large size, slow tissue penetration, and their potential toxicity at high systemic levels, suggest that conventional routes of administration are unlikely to be effective. In this review, we provide an overview of the design criteria for growth factor delivery vehicles with respect to the growth factor itself and the microenvironment for delivery. We discuss various methodologies that could be adopted to achieve this localized delivery, and strategies using polymers as delivery vehicles in particular.

In vitro cellular responses to scaffolds containing two microencapulated growth factors

Growth factors play an important role in the complex cascade of tissue events in periodontal regeneration, although optimal methods of delivery remain to be identified. We hypothesize that multiple delivery of growth factors, particularly via a microparticle-containing scaffold, will enhance cellular events leading to periodontal regeneration. In this study, cellular responses of periodontal ligament fibroblasts (PDLFs) in scaffolds containing microparticles (MPs) loaded with either bone morphogenetic protein (BMP)-2, insulin-like growth factor (IGF)-1, or a mixture of both MPs were evaluated, and the dual-MP-containing scaffold exhibited the release of different proteins in a sustained and independent fashion. When PDLF-seeded scaffolds were cultured in a flow perfusion bioreactor, cell metabolism and proliferation of PDLFs were significantly increased within 3 days in all IGF-1-containing scaffolds compared with those in groups lacking IGF-1 and particulate delivery enhanced these effects between 3 and 7 days. The dual-MP-containing group showed the most positive results. Both the BMP-2-in-MP and IGF-1-in-MP groups showed greater effects of alkaline phosphatase activity, more osteocalcin and osteopontin production, and more calcium deposition compared with matched GF-adsorbed groups. All osteoblastic markers were at their highest in the dual-MP-containing group at all detected time points. The combined results suggest that our dual-MP-containing scaffold can be used as a cell vehicle to positively affect cell behavior, thus exhibiting the potential to be a candidate scaffold for future periodontal tissue engineering.

Mild immobilization of diverse macromolecular bioactive agents onto multifunctional fibrous membranes prepared by coaxial electrospinning

Coaxial electrospinning was proved to be a facile method to produce multifunctional fibrous matrices which could essentially emulate certain features of native extracellular matrix. In order to further confer capability of immobilizing diverse macromolecular bioactive agents to the fibers, composite membranes composed of cationized gelatin-coated polycaprolactone (PCL) fibers were prepared by coaxial electrospinning. Gelatin was cationized by derivation with N,N-dimethylethylenediamine. The cationized gelatin (CG) was used as a shell material for constructing a core-shell fibrous membrane. PCL formed the core section of the core-shell fibers thereby improving the mechanical properties of nanofibrous CG hydrogel. The outer CG layer was crosslinked by exposing the membranes in glutaraldehyde vapor. The adsorption behaviors of FITC-labeled bovine serum albumin (FITC-BSA) or FITC-heparin onto the fibers were investigated. The core-shell fibers could effectively immobilize the two types of agents under mild conditions. The adsorption amount could reach about 12 microg of BSA per mg of membrane and 23 microg mg(-1) for heparin. Furthermore, vascular endothelial growth factor (VEGF) could be conveniently impregnated into the fibers through specific interactions with the adsorbed heparin in the outer CG layer. Sustained release of bioactive VEGF could be achieved for more than 15 days.

Composite glycidyl methacrylated dextran (Dex-GMA)/gelatin nanoparticles for localized protein delivery

AIM

Localized delivery of growth factors has significant potential as a future therapeutic strategy in tissue engineering and regenerative medicine. A nanoparticle vehicle was created and evaluated in this study with the intent to deliver growth factors for periodontal regeneration.

METHODS

Novel composite nanoparticles based on glycidyl methacrylate derivatized dextrans (Dex-GMA) and gelatin were fabricated by a facile method without using any organic solvents. The configurations of the resultant nanoparticles were evaluated by transmission electron microscopy, scanning electron microscopy, and atomic force microscope. Their surfaces were characterized by zeta-potential measurements, after which their properties including swelling, degradation, drug release, and cytotoxicity were also investigated using in vitro models.

RESULTS

The particle size of Dex-GMA/gelatin nanoparticles (DG-NPs) ranged from 20 to 100 nm and showed a mono-disperse size distribution (mean diameter 53.7 nm) and a strongly negative surface zeta potential (-20 mV). The DG-NPs were characterized by good swelling and degradation properties in media including dextranase. The in vitro drug release studies showed that the efficient bone morphogenetic protein (BMP) release from DG-NPs was maintained for more than 12 d under degradation conditions, where more than 90% of the loaded BMP was released. No any relevant cell damage caused by DG-NPs was found in the cytotoxicity tests for a period of 24 h.

CONCLUSION

These combined results demonstrate that DG-NPs fulfill the basic prerequisites for growth factor delivery. With further in vivo studies, those nanoparticles may offer a promising vehicle for the delivery of active drugs to the periodontium.

Hybrid multicomponent hydrogels for tissue engineering

Artificial ECMs that not only closely mimic the hybrid nature of the natural ECM but also provide tunable material properties and enhanced biological functions are attractive candidates for tissue engineering applications. This review summarizes recent advances in developing multicomponent hybrid hydrogels by integrating modular and heterogeneous building blocks into well-defined, multifunctional hydrogel composites. The individual building blocks can be chemically, morphologically, and functionally diverse, and the hybridization can occur at molecular level or microscopic scale. The modular nature of the designs, combined with the potential synergistic effects of the hybrid systems, has resulted in novel hydrogel matrices with robust structure and defined functions.

Functional immobilization of signaling proteins enables control of stem cell fate

The mode of ligand presentation has a fundamental role in organizing cell fate throughout development. We report a rapid and simple approach for immobilizing signaling ligands to maleic anhydride copolymer thin-film coatings, enabling stable signaling ligand presentation at interfaces at defined concentrations. We demonstrate the utility of this platform technology using leukemia inhibitory factor (LIF) and stem cell factor (SCF). Immobilized LIF supported mouse embryonic stem cell (mESC) pluripotency for at least 2 weeks in the absence of added diffusible LIF. Immobilized LIF activated signal transducer and activator of transcription 3 (STAT3) and mitogen-activated protein kinase (MAPK) signaling in a dose-dependent manner. The introduced method allows for the robust investigation of cell fate responses from interface-immobilized ligands.