Biodegradable fibrous scaffolds with diverse properties by electrospinning candidates from a combinatorial macromer library

Net anionic poly(β-amino ester)s: synthesis, pH-dependent behavior, and complexation with cationic cargo

As hydrolytically-labile, traditionally-cationic polymers, poly(β-amino ester)s (PBAEs) adeptly complex anionic compounds such as nucleic acids, and release their cargo as the polymer degrades. To engineer fully-degradable polyelectrolyte complexes and delivery vehicles for cationic therapeutics, we sought to invert PBAE net charge to generate net anionic PBAEs. Since PBAEs can carry up to a net charge of +1 per tertiary amine, we synthesized a series of alkyne-functionalized PBAEs that allowed installation of 2 anionic thiol-containing molecules per tertiary amine via a radical thiol-yne reaction. Finding dialysis in aqueous solution to lead to PBAE degradation, we developed a preparative size exclusion chromatography method to remove unreacted thiol from the net anionic PBAEs without triggering hydrolysis. The net anionic PBAEs display non-monotonic solution behavior as a function of pH, being more soluble at pH 4 and 10 than in intermediate pH ranges. Like cationic PBAEs, these net anionic PBAEs degrade in aqueous environments with hydrophobic content-dependent hydrolysis, as determined by 1H NMR spectroscopy. Further, these net anionic PBAEs form complexes with the cationic peptide (GR)10, which disintegrate over time as the polymer hydrolyzes. Together, these studies outline a synthesis and purification route to make previously inaccessible net anionic PBAEs with tunable solution and degradation behavior, allowing for user-determined complexation and release rates and providing opportunities for degradable polyelectrolyte complexes and cationic therapeutic delivery.

Poly(β-Amino Esters): Synthesis, Formulations, and Their Biomedical Applications

Poly(β-amino ester) (abbreviated as PBAE or PAE) refers to a polymer synthesized from an acrylate and an amine by Michael addition and has properties inherent to tertiary amines and esters, such as pH responsiveness and biodegradability. The versatility of building blocks provides a library of polymers with miscellaneous physicochemical and mechanical properties. When used alone or together with other materials, PBAEs can be fabricated into different formulations in order to fulfill various requirements in drug delivery (for instance, gene, anticancer drugs, and antimicrobials delivery) and natural complex mimicry (nanochaperones). This progress report discusses the recent developments in design, synthesis, formulations, and applications of PBAEs in biomedical fields and provides a perspective view for the future of the PBAEs.

A Biodegradable Stent with Surface Functionalization of Combined-Therapy Drugs for Colorectal Cancer

In-stent restenosis caused by tumor ingrowth is a major problem for patients undergoing stent placement because conventional stents often lack sustainable antitumor capabilities. The aim of this work is to develop a silk fibroin (SF)-based nanofibrous membrane that is loaded with combined-therapy drugs by using electrospinning technologies, which is further coated on a polydioxanone (PDO) stent and used for the treatment of colorectal cancer (CRC). In order to improve treatment effectiveness, a combination of therapeutic drugs, i.e., curcumin (CUR) and 5-fluorouracil (5-FU), is dissolved into SF solution and then eletrospun onto the surface of the PDO stent. The morphology, secondary structure, and in vitro drug release profiles of the membranes are characterized. The antitumor efficacy is assessed in vitro and in vivo using a human CRC cell line and normal cells, and tumor-bearing nude mice. In vitro and in vivo studies on the nanofibrous memembrane-coating demonstrate improved antitumor effects for the CUR/5-FU dual drug system which can be attributed to cell cycle arrest in the S phase in association with induced apoptosis in tumor cells by blocking signal transducer and activator of transcription3 (Stat3) and nuclear factor kappa beta (NF-kB) signaling pathways, suggesting potential in the treatment of CRC in the future.

Bisphosphonate-functionalized poly(β-amino ester) network polymers

Three novel bisphosphonate-functionalized secondary diamines are synthesized and incorporated into poly(β-amino ester)s (PBAEs) to investigate the effects of bisphosphonates on biodegradation and toxicity of PBAE polymer networks. These three novel amines, BPA1, BPA2, and BPA3, were prepared from the reactions of 1,4-butanediamine, 1,6-hexanediamine, or 4,9-dioxa-1,12-dodecanediamine with tetraethyl vinylidene bisphosphonate, respectively. The PBAE macromers were obtained from the aza-Michael addition reaction of these amines to 1,6-hexane diol diacrylate (HDDA) and poly(ethylene glycol) diacrylate (PEGDA, M_n  = 575) and photopolymerized to produce biodegradable gels. These gels with different chemistries exhibited similar degradation behavior with mass loss of 53-73% within 24 h, indicating that degradation is mostly governed by the bisphosphonate group. Based on the in vitro cytotoxicity evaluation against NIH 3T3 mouse embryonic fibroblast cells, the degradation products do not exhibit significant toxicity in most cases. It was also shown that PBAE macromers can be used as cross-linkers for the synthesis of 2-hydroxyethyl methacrylate hydrogels, conferring small and customizable degradation rates upon them. The materials reported have potential to be used as nontoxic degradable biomaterials. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1412-1421, 2017.

Application of a bilayer tubular scaffold based on electrospun poly(l-lactide-co-caprolactone)/collagen fibers and yarns for tracheal tissue engineering

An electrospun bilayer tubular scaffold based on collagen/P(LLA–CL) was prepared and preprocessing with autologous tracheal cells and vascularization was done for the purpose of tracheal tissue engineering.

Degradation of acetalated dextran can be broadly tuned based on cyclic acetal coverage and molecular weight

Microparticles (MPs) derived from acid-sensitive biopolymers enable rapid degradation and cargo release under acidic conditions, such as at tumor microenvironments, within lysosomal/phagosomal compartments inside phagocytic cells, or at sites of inflammation. One such acid-sensitive biopolymer, acetalated dextran (Ace-DEX), has tunable degradation rates and pH-neutral degradation byproducts consisting of dextran, acetone, and ethanol. By studying the degradation profiles of Ace-DEX MPs with varying cyclic acetal coverage (CAC) and dextran molecular weight (MW), we concluded that MPs composed of low CAC or high MW polymer degraded the fastest at both pH 7.4 and 5.0. To further understand the properties of this unique polymer, we encapsulated a model drug resiquimod, which is a toll-like receptor (TLR) 7/8 agonist, into Ace-DEX MPs of different polymer CAC and dextran MW. It was observed that resiquimod was released faster from MPs of lower CAC or higher MW. By evaluating the activation of RAW macrophages cultured with different types of resiquimod-loaded Ace-DEX MPs, we found that MPs of lower CAC or higher MW promoted greater nitrite production and resulted in more robust cell activation. Our results indicate we can precisely control the degradation profile, release kinetics, and bioactivity of encapsulated cargos by altering CAC and MW, furthering Ace-DEX MPs' novelty as a drug carrier.

Meniscus tissue engineering using a novel combination of electrospun scaffolds and human meniscus cells embedded within an extracellular matrix hydrogel

Meniscus injury and degeneration have been linked to the development of secondary osteoarthritis (OA). Therapies that successfully repair or replace the meniscus are, therefore, likely to prevent or delay OA progression. We investigated the novel approach of building layers of aligned polylactic acid (PLA) electrospun (ES) scaffolds with human meniscus cells embedded in extracellular matrix (ECM) hydrogel to lead to formation of neotissues that resemble meniscus-like tissue. PLA ES scaffolds with randomly oriented or aligned fibers were seeded with human meniscus cells derived from vascular or avascular regions. Cell viability, cell morphology, and gene expression profiles were monitored via confocal microscopy, scanning electron microscopy (SEM), and real-time polymerase chain reaction (PCR), respectively. Seeded scaffolds were used to produce multilayered constructs and were examined via histology and immunohistochemistry. Morphology and mechanical properties of PLA scaffolds (with and without cells) were influenced by fiber direction of the scaffolds. Both PLA scaffolds supported meniscus tissue formation with increased COL1A1, SOX9, and COMP, yet no difference in gene expression was found between random and aligned PLA scaffolds. Overall, ES materials, which possess mechanical strength of meniscus and can support neotissue formation, show potential for use in cell-based meniscus regeneration strategies.

From repair to regeneration: biomaterials to reprogram the meniscus wound microenvironment

When the field of tissue engineering first arose, scaffolds were conceived of as inert three-dimensional structures whose primary function was to support cellularity and tissue growth. Since then, advances in scaffold and biomaterial design have evolved to not only guide tissue formation, but also to interact dynamically with and manipulate the wound environment. At present, these efforts are being directed towards strategies that directly address limitations in endogenous wound repair, with the goal of reprogramming the local wound environment (and the cells within that locality) from a state that culminates in an inferior tissue repair into a state in which functional regeneration is achieved. This review will address this approach with a focus on recent advances in scaffold design towards the resolution of tears of the knee meniscus as a case example. The inherent limitations to endogenous repair will be discussed, as will specific examples of how biomaterials are being designed to overcome these limitations. Examples will include design of fibrous scaffolds that promote colonization by modulating local extracellular matrix density and delivering recruitment factors. Furthermore, we will discuss scaffolds that are themselves modulated by the wound environment to alter porosity and modulate therapeutic release through precise coordination of scaffold degradation. Finally, we will close with emerging concepts in local control of cell mechanics to improve interstitial cell migration and so advance repair. Overall, these examples will illustrate how emergent features within a biomaterial can be tuned to manipulate and harness the local tissue microenvironment in order to promote robust regeneration.

The fusion of tissue spheroids attached to pre-stretched electrospun polyurethane scaffolds

Effective cell invasion into thick electrospun biomimetic scaffolds is an unsolved problem. One possible strategy to biofabricate tissue constructs of desirable thickness and material properties without the need for cell invasion is to use thin (<2 µm) porous electrospun meshes and self-assembling (capable of tissue fusion) tissue spheroids as building blocks. Pre-stretched electrospun meshes remained taut in cell culture and were able to support tissue spheroids with minimal deformation. We hypothesize that elastic electrospun scaffolds could be used as temporal support templates for rapid self-assembly of cell spheroids into higher order tissue structures, such as engineered vascular tissue. The aim of this study was to investigate how the attachment of tissue spheroids to pre-stretched polyurethane scaffolds may interfere with the tissue fusion process. Tissue spheroids attached, spread, and fused after being placed on pre-stretched polyurethane electrospun matrices and formed tissue constructs. Efforts to eliminate hole defects with fibrogenic tissue growth factor-β resulted in the increased synthesis of collagen and periostin and a dramatic reduction in hole size and number. In control experiments, tissue spheroids fuse on a non-adhesive hydrogel and form continuous tissue constructs without holes. Our data demonstrate that tissue spheroids attached to thin stretched elastic electrospun scaffolds have an interrupted tissue fusion process. The resulting tissue-engineered construct phenotype is a direct outcome of the delicate balance of the competing physical forces operating during the tissue fusion process at the interface of the pre-stretched elastic scaffold and the attached tissue spheroids. We have shown that with appropriate treatments, this process can be modulated, and thus, a thin pre-stretched elastic polyurethane electrospun scaffold could serve as a supporting template for rapid biofabrication of thick tissue-engineered constructs without the need for cell invasion.

Simple fabrication method for a porous poly(vinyl alcohol) matrix by multisolvent mixtures for an air-exposed model of the lung epithelial system

We introduce a simple and easy method for fabricating a thin and porous matrix that can be used as an extracellular matrix (ECM). A porous poly(vinyl alcohol) (PVA) matrix was created through recrystallization by multiple solvents under distilled water (DW), isopropyl alcohol (IPA), and a combination of DW and IPA. The crysatllization was driven by precipitating and dissolving a solute in a solution of a solvent and a nonsolvent, which induced the formation of microspheres in the IPA. The crystal structure depended on the ratio of the solvent/nonsolvent and the concentration of the PVA aqueous solution; these properties were used to tune the thickness, size, and porosity of the matrices. The resulting PVA matrix was chemically stabilized through a reaction with glutaraldehyde in the IPA solution. We demonstrated that a very thin and porous PVA matrix provided an effective functional model of the lung epithelial system. Lung epithelial cells (A549) displayed a high affinity for this matrix, which was permeable to the culture medium. These properties facilitated culturing under the air environment.

Decoupling polymer properties to elucidate mechanisms governing cell behavior

Determining how a biomaterial interacts with cells ("structure-function relationship") reflects its eventual clinical applicability. Therefore, a fundamental understanding of how individual material properties modulate cell-biomaterial interactions is pivotal to improving the efficacy and safety of clinically translatable biomaterial systems. However, due to the coupled nature of material properties, their individual effects on cellular responses are difficult to understand. Structure-function relationships can be more clearly understood by the effective decoupling of each individual parameter. In this article, we discuss three basic decoupling strategies: (1) surface modification, (2) cross-linking, and (3) combinatorial approaches (i.e., copolymerization and polymer blending). Relevant examples of coupled material properties are briefly reviewed in each section to highlight the need for improved decoupling methods. This follows with examples of more effective decoupling techniques, mainly from the perspective of three primary classes of synthetic materials: polyesters, polyethylene glycol, and polyacrylamide. Recent strides in decoupling methodologies, especially surface-patterning and combinatorial techniques, offer much promise in further understanding the structure-function relationships that largely govern the success of future advancements in biomaterials, tissue engineering, and drug delivery.

The influence of fibrous elastomer structure and porosity on matrix organization

Fibrous scaffolds are finding wide use in the field of tissue engineering, as they can be designed to mimic many native tissue properties and structures (e.g., cardiac tissue, meniscus). The influence of fiber alignment and scaffold architecture on cellular interactions and matrix organization was the focus of this study. Three scaffolds were fabricated from the photocrosslinkable elastomer poly(glycerol sebacate) (PGS), with changes in fiber alignment (non-aligned (NA) versus aligned (AL)) and the introduction of a PEO sacrificial polymer population to the AL scaffold (composite (CO)). PEO removal led to an increase in scaffold porosity and maintenance of scaffold anisotropy, as evident through visualization, mechanical testing, and mass loss studies. Hydrated scaffolds possessed moduli that ranged between ∼3-240 kPa, failing within the range of properties (<300 kPa) appropriate for soft tissue engineering. CO scaffolds were completely degraded as early as 16 days, whereas NA and AL scaffolds had ∼90% mass loss after 21 days when monitored in vitro. Neonatal cardiomyocytes, used as a representative cell type, that were seeded onto the scaffolds maintained their viability and aligned along the surface of the AL and CO fibers. When implanted subcutaneously in rats, a model that is commonly used to investigate in vivo tissue responses to biomaterials, CO scaffolds were completely integrated at 2 weeks, whereas ∼13% and ∼16% of the NA and AL scaffolds, respectively remained acellular. However, all scaffolds were completely populated with cells at 4 weeks post-implantation. Polarized light microscopy was used to evaluate the collagen elaboration and orientation within the scaffold. An increase in the amount of collagen was observed for CO scaffolds and enhanced alignment of the nascent collagen was observed for AL and CO scaffolds compared to NA scaffolds. Thus, these results indicate that the scaffold architecture and porosity are important considerations in controlling tissue formation.