Using Supercritical Fluid Technology as a Green Alternative During the Preparation of Drug Delivery Systems

Micro- and nano-carrier formulations have been developed as drug delivery systems for active pharmaceutical ingredients (APIs) that suffer from poor physico-chemical, pharmacokinetic, and pharmacodynamic properties. Encapsulating the APIs in such systems can help improve their stability by protecting them from harsh conditions such as light, oxygen, temperature, pH, enzymes, and others. Consequently, the API's dissolution rate and bioavailability are tremendously improved. Conventional techniques used in the production of these drug carrier formulations have several drawbacks, including thermal and chemical stability of the APIs, excessive use of organic solvents, high residual solvent levels, difficult particle size control and distributions, drug loading-related challenges, and time and energy consumption. This review illustrates how supercritical fluid (SCF) technologies can be superior in controlling the morphology of API particles and in the production of drug carriers due to SCF's non-toxic, inert, economical, and environmentally friendly properties. The SCF's advantages, benefits, and various preparation methods are discussed. Drug carrier formulations discussed in this review include microparticles, nanoparticles, polymeric membranes, aerogels, microporous foams, solid lipid nanoparticles, and liposomes.

Zein-based composites in biomedical applications

Considerable research efforts have been devoted to zein-based biomaterials for tissue engineering and other biomedical applications over the past decade. The attention given to zein-based polymers is primarily attributed to their biocompatibility and biodegradability. However, due to the relatively low mechanical properties of these polymers, numerous inorganic compounds (e.g., hydroxyapatite, calcium phosphate, bioactive glasses, natural clays) have been considered in combination with zein to create composite materials in an attempt to enhance zein mechanical properties. Inorganic phases also positively impact on the hydrophilic properties of zein matrices inducing a suitable environment for cell attachment, spreading, and proliferation. This review covers available literature on zein and zein-based composite materials, with focus on the combination of zein with commonly used inorganic fillers for tissue engineering and drug delivery applications. An overview of the most recent advances in fabrication techniques for zein-based composites is presented and key applications areas and future developments in the field are highlighted. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1656-1665, 2017.

Interconnected porous poly(ɛ-caprolactone) tissue engineering scaffolds fabricated by microcellular injection molding

In tissue engineering applications, a scaffold containing an interconnected porous structure is often highly desirable since these interconnected pores allow nutrients and signaling molecules to reach all of the cultured cells. In this study, microcellular injection molding, a mass production method for foamed plastic components, was combined with chemical foaming and particulate leaching methods to fabricate an interconnected porous structure using poly(ɛ-caprolactone) (PCL). Sodium bicarbonate (SB) was employed as the chemical foaming agent while carbon dioxide (CO2) was used as the physical foaming (blowing) agent. The results showed that interconnected porous structures of PCL, which depend on the composition of the materials used, could be successfully produced. Sodium bicarbonate not only generated CO2 to supplement the supercritical fluid microcellular injection molding, but also served as the nuclei for heterogeneous cell nucleation. Sodium bicarbonate and its byproduct, sodium carbonate, were also the porogens in the particulate leaching process, which further enhanced the porosity and interconnectivity. The morphologies and mechanical properties of the samples with different material compositions and porosities were discussed. The results of cell viability assays of 3T3 fibroblasts suggested that the resulting interconnected porous PCL scaffolds exhibited good biocompatibility. Cell spreading was affected by the porosity of the scaffold because of the physical restriction effect on the cell migration. Highly improved interconnectivity of the scaffold provided more space for the cells to spread.

Supercritical CO2 foaming of thermoplastic materials derived from maize: proof-of-concept use in mammalian cell culture applications

BACKGROUND

Foams are high porosity and low density materials. In nature, they are a common architecture. Some of their relevant technological applications include heat and sound insulation, lightweight materials, and tissue engineering scaffolds. Foams derived from natural polymers are particularly attractive for tissue culture due to their biodegradability and bio-compatibility. Here, the foaming potential of an extensive list of materials was assayed, including slabs elaborated from whole flour, the starch component only, or the protein fraction only of maize seeds.

METHODOLOGY/PRINCIPAL FINDINGS

We used supercritical CO2 to produce foams from thermoplasticized maize derived materials. Polyethylene-glycol, sorbitol/glycerol, or urea/formamide were used as plasticizers. We report expansion ratios, porosities, average pore sizes, pore morphologies, and pore size distributions for these materials. High porosity foams were obtained from zein thermoplasticized with polyethylene glycol, and from starch thermoplasticized with urea/formamide. Zein foams had a higher porosity than starch foams (88% and 85%, respectively) and a narrower and more evenly distributed pore size. Starch foams exhibited a wider span of pore sizes and a larger average pore size than zein (208.84 vs. 55.43 μm2, respectively). Proof-of-concept cell culture experiments confirmed that mouse fibroblasts (NIH 3T3) and two different prostate cancer cell lines (22RV1, DU145) attached to and proliferated on zein foams.

CONCLUSIONS/SIGNIFICANCE

We conducted screening and proof-of-concept experiments on the fabrication of foams from cereal-based bioplastics. We propose that a key indicator of foamability is the strain at break of the materials to be foamed (as calculated from stress vs. strain rate curves). Zein foams exhibit attractive properties (average pore size, pore size distribution, and porosity) for cell culture applications; we were able to establish and sustain mammalian cell cultures on zein foams for extended time periods.

Recent advances in food-packing, pharmaceutical and biomedical applications of zein and zein-based materials

Zein is a biodegradable and biocompatible material extracted from renewable resources; it comprises almost 80% of the whole protein content in corn. This review highlights and describes some zein and zein-based materials, focusing on biomedical applications. It was demonstrated in this review that the biodegradation and biocompatibility of zein are key parameters for its uses in the food-packing, biomedical and pharmaceutical fields. Furthermore, it was pointed out that the presence of hydrophilic-hydrophobic groups in zein chains is a very important aspect for obtaining material with different hydrophobicities by mixing with other moieties (polymeric or not), but also for obtaining derivatives with different properties. The physical and chemical characteristics and special structure (at the molecular, nano and micro scales) make zein molecules inherently superior to many other polymers from natural sources and synthetic ones. The film-forming property of zein and zein-based materials is important for several applications. The good electrospinnability of zein is important for producing zein and zein-based nanofibers for applications in tissue engineering and drug delivery. The use of zein's hydrolysate peptides for reducing blood pressure is another important issue related to the application of derivatives of zein in the biomedical field. It is pointed out that the biodegradability and biocompatibility of zein and other inherent properties associated with zein's structure allow a myriad of applications of such materials with great potential in the near future.

Fabrication of porous synthetic polymer scaffolds for tissue engineering

Tissue engineering provides a novel and promising approach to replace damaged tissue with an artificial substitute. Porous synthetic biodegradable polymers are the preferred materials for this substitution due to their microstructure, biocompatibility, biodegradability, and low cost. As a crucial element in tissue engineering, a scaffold acts as an artificial extracellular matrix (ECM) and provides support for cell migration, differentiation, and reproduction. The fabrication of viable scaffolds, however, has been a challenge in both clinical and academic settings. Methods such as solvent casting/particle leaching, thermally induced phase separation (TIPS), electrospinning, gas foaming, and rapid prototyping (additive manufacturing) have been developed or introduced for scaffold fabrication. Each method has its own advantages and disadvantages. In this review, the commonly used synthetic polymer scaffold fabrication methods will be introduced and discussed in detail, and recent progress regarding scaffold fabrication—such as combining different scaffold fabrication methods, combining various materials, and improving current scaffold fabrication methods—will be reviewed as well.