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3D printing, or rapid prototyping, is a fabrication technique that is used for various engineering applications with advantages such as mass production and fine tuning of spatial-dimensional properties. Recently, this fabrication method has been adopted for tissue engineering applications due to its ability to finely tune porosity and create precise, uniform, and repeatable structures. This review aims to introduce 3D printing applications in soft tissue engineering and regenerative medicine including state-of-the-art scaffolds and key future challenges. Furthermore, 3D printing of individual cells, an evolution of traditional 3D printing technology which represents a cutting-edge technique for the creation of cell seeded scaffolds in vitro, is discussed. Key advances demonstrate the advantages of 3D printing, while also highlighting potential shortcomings to improve upon. It is clear that as 3D printing technology continues to develop, it will serve as a truly revolutionary means for fabrication of structures and materials for regenerative applications.

A novel polymer poly(caprolactone triol succinate) (PPCLSu) was synthesized from monomers polycaprolactone triol and succinic acid by direct polycondensation. The tensile strength of PPCLSu was found to be 0.33 60.03 MPa with an elongation of 47.8 61.9%. These elastomers lost about 7% of their original mass in an in vitro degradation study conducted in phosphate-buffered saline (PBS) at 37 � C up to 10 weeks. Three-dimensional (3D) porous scaffolds were created by a porogen-leaching method and these constructs were evaluated for primary rat osteoblast (PRO) proliferation and phenotype development in vitro. This elastomer pro- moted primary rat osteoblast adhesion, proliferation and increased expression of alkaline phosphatase, an early marker of osteoblastic phenotype. These preliminary results suggest that PPCLSu may be a good candidate material for scaffolding applications in tissue regeneration. V C 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3770-3777, 2013

Grafting of acrylamide onto guar gum is achieved by Ce(IV) induced free-radical polymerization to prepare interpenetrating polymer network (IPN) beads of polyacrylamide-g-guar gum with sodium alginate by crosslinking with glutaraldehyde. Two widely used pesticides, solid chlorpyrifos and liquid fenvelarate, are loaded up to 60–70% efficiency in the IPN beads. The polymer and beads are characterized by Fourier transform IR spectroscopy to confirm grafting and to understand the possible interactions between the pesticides and the polymer matrix. Scanning electron microscopy is used to study the morphology of the beads. Equilibrium swelling experiments indicate that the swelling of the beads decreases with an increase in crosslinking, as well as an increase in pesticide loading. The in vitro release studies are performed under static conditions, and the release data are fitted to an empirical relationship to evaluate the transport parameters. Diffusion coefficients are calculated for the transport of pesticides through the polymeric beads using the initial and later time approximation methods. These values show a decrease with increasing crosslinking and increasing pesticide loading. Long-term diffusion coefficients as computed by Fick's equations are found to be smaller in magnitude when compared to the initial time diffusion coefficients. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 451–457, 2003

The grafting of acrylamide onto a chitosan backbone was carried out at three acrylamide concentrations (polymer/monomer ratio = 1:1, 1:2, and 1:3). The synthesis of the grafted polymer was achieved by K2S2O8-induced free-radical polymerization. Microspheres of polyacrylamide-g-chitosan crosslinked with glutaraldehyde were prepared to encapsulate nifedipine (NFD), a calcium channel blocker and an antihypertensive drug. The microspheres of polyacrylamide-g-chitosan were produced by a water-in-oil emulsion technique with three different concentrations of glutaraldehyde as the crosslinking agent. Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC) were used to characterize the grafted copolymers, and the microspheres were prepared from them. FTIR and DSC were also used to analyze the extent of crosslinking. The microspheres were characterized by the particle size; the water transport into these microspheres, as well as the equilibrium water uptake, were studied. Scanning electron microscopy confirmed the spherical nature of the particles, which had a mean particle size of 450 μm. Individual particle dynamic swelling experiments suggested that with an increase in crosslinking, the transport became case II. The release of NFD depended on the crosslinking of the network and on the amount of drug loading. Calculating the drug diffusion coefficients with the initial time and later time approximation method further supported this. The drug release in all 27 formulations followed case II transport, and this suggested that the time dependence of the NFD release followed zero-order kinetics. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2940–2949, 2003

Electrospun nanofiber matrices have attracted a great deal of attention as matrices for skin repair and regeneration. The current manuscript reports the fabrication and characterization of a bioactive polycaprolactone (PCL) fiber matrix for its ability to deliver multiple factors. Bioactive PCL matrices were created by incorporating a model angiogenic factor and a model antibiotic drug. Chitosan coating on the fiber matrices significantly improved the ability to hold moisture and contributed to antibiotic activity. These fiber matrices have a modulus of 5.8 61.3 MPa and matrices subjected to degradation over 4 weeks did not lose their tensile properties due to slow degradation rate. Chitosan coating avoided the initial burst release commonly associated with fiber matrices and only 60% of the encapsulated drug was released over a period of 15 days. Control PCL-chitosan matrices were able to reduce Staphylococcus aureus (S. aureus) growth both in static and dynamic condition as compared to formulations with 50 mg gentamicin. In general, all the fiber matrices were able to support fibroblast growth and maintained normal cell morphology. Such bioactive ban- dages may serve as versatile and less expensive alternatives for the treatment of complex wounds. V C 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41879.

Microspheres of polyacrylamide-grafted-chitosan crosslinked with glutaraldehyde were prepared and used to encapsulate indomethacin, a nonsteroidal anti-inflammatory drug. The microspheres were produced by the water/oil emulsion technique and encapsulation of indomethacin was carried out before crosslinking of the matrix. The extent of crosslinking was analyzed by Fourier transform infrared spectroscopy and differential scanning calorimetry. Microspheres were characterized for drug-entrapment efficiency, particle size, and water transport into the polymeric matrix as well as for drug-release kinetics. Scanning electron microscopy confirmed the spherical nature and surface morphology of the particles with a mean particle size of 525 μm. Dynamic swelling experiments suggested that, with an increase in crosslinking, the transport mechanism changed from Fickian to non-Fickian. The release of indomethacin depends upon the crosslinking of the network and also on the amount of drug loading. This was further supported by the calculation of drug-diffusion coefficients using the initial time approximation. The drug release in all the formulations followed a non-Fickian trend and the diffusion was relaxation-controlled. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1525–1536, 2003

Acrylic-based copolymers of methyl methacrylate (MMA) and methacrylic acid (MAA) have been prepared by solution and bulk polymerization techniques using benzoyl peroxide (BPO) as an initiator. Three polymers were prepared with a varying ratio of MMA/MAA. In an effort to increase the hydrophilicity of the matrix, one MMA/MAA polymer was prepared by adding an additional amount of 2-hydroxy ethyl methacrylate (HEMA). All the polymers were crosslinked in situ by ethylene glycol dimethacrylate (EGDMA). These polymers were characterized by Fourier transform infrared spectroscopy and differential scanning calorimetry. Viscous flow characteristics were determined from solution viscosity and rheological measurements. Dynamic and equilibrium swelling experiments were carried out under varying pH conditions (i.e., 0.1N NaOH, 0.1N HCl, and double-distilled water). Partially crosslinked hydrogels show varying hydrophilicity because of the presence of carboxylic acid groups making them pH-responsive. Swelling increased with an increasing number of —COOH groups on the polymer backbone and the hydrophilicity varied with changing pH. Cypermethrin, a widely used pesticide, and cupric sulfate, a model micronutrient, were loaded into these pH-sensitive hydrogels to investigate their controlled release characteristics. The in vitro release rates of both compounds have been carried out under static dissolution conditions at 30°C. Release data have been fitted to an empirical relation to estimate transport parameters. The release results have been discussed in terms of the varying hydrophilicity of the hydrogel network polymers. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 394–403, 2003

Polyacrylamide-grafted sodium alginate (PAAm-g-Na-Alg) copolymeric membranes have been prepared, characterized, and used in the pervaporation separation of 10–80 mass % water-containing tetrahydrofuran mixtures. Totally three membranes were prepared: (1) neat Na-Alg with 10 mass % of polyethylene glycol (PEG) and 5 mass % of polyvinyl alcohol (PVA), (2) 46 % grafted PAAm-g-Na-Alg membrane containing 10 mass % of PEG and 5 mass % of PVA, and (3) 93 % grafted PAAm-g-Na-Alg membrane containing 10 mass % of PEG and 5 mass % of PVA. Using the transport data, important parameters like permeation flux, selectivity, pervaporation separation index, swelling index, and diffusion coefficient have been calculated at 30°C. Diffusion coefficients were also calculated from sorption gravimetric data of water–tetrahydrofuran mixtures using Fick's equation. Arrhenius activation parameters for the transport processes were calculated for 10 mass % of water in the feed mixture using flux and diffusion data obtained at 30, 35, and 40°C. The separation selectivity of the membranes ranged between 216 and 591. The highest permeation flux of 0.677 kg/m2 h was observed for 93% grafted membrane at 80 mass % of water in the feed mixture. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 272–281, 2002

Interpenetrating network polymeric beads of poly(vinyl alcohol)-grafted-acrylamide with sodium alginate have been prepared by crosslinking with glutaraldehyde. Cypermethrin, a widely used pesticide, was loaded with 80% efficiency in these hydrogel beads. The beads were characterized by Fourier transform infrared spectroscopy to confirm the grafting. Scanning electron microscopy was used to know the morphology of the beads. Equilibrium swelling experiments indicated that swelling of the beads decreased with an increase in crosslinking. The in vitro release studies were performed under static conditions and the release data have been fitted to an empirical relation to estimate the transport parameters. The diffusion coefficients have been calculated for the transport of pesticide through the polymeric beads using the initial time approximation method. These values showed decrease with increasing crosslinking as well as increasing pesticide loading. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 552–560, 2002; DOI 10.1002/app.10306

This article presents our preliminary experimental data on the release kinetics and encapsulation efficiency of urea formaldehyde (UF) crosslinked matrices of starch (St), guar gum (GG), and starch + guar gum (St + GG) for the controlled release of solid (chlorpyrifos) and liquid (neem seed oil) pesticides. The data reveal variable release rates in relation to the polymer type and especially the pesticide type. It is possible to slow the release rates of pesticides using cheaply available materials such as starch and guar gum. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2863–2866, 2001

Injectable hydrogels have attracted a great deal of attention as cell carriers and bioactive agents in regenerative medicine due to their ability to fill complex three-dimensional (3D) tissue gaps and relative ease of in vivo administration. Polysaccharide-based hydrogels can provide microenvironments that favor tissue regeneration and biocompatibility due to their chemical similarities with native extracellular matrix components. This manuscript reports the in vitro application of an injectable chitosan-based polysaccharide hydrogel for cell and protein delivery. Crosslinked hydrogels were produced by the reaction between the amino functionality of chitosan and the aldehyde of dextran aldehyde resulting in an imine bond (Schiff's base) formation in aqueous solutions. This approach eliminated the use of additional crosslinking agents which may pose undesired side effects regarding cytotoxicity and biocompatibility. Additionally, we demonstrate versatility of the gel in terms of its fabrication, and ability to alter mechanical properties by changing the crosslinking extent due to aldehyde content. Bovine serum albumin (BSA), used as a model protein, followed a steady release pattern from the gel. BSA release was dependent on the extent of hydrogel crosslinking. Increase in crosslinking extent resulted in improved mechanical properties and sustained release of BSA. Human fetal osteoblasts encapsulated into the hydrogel showed at least 70% viability and continued to proliferate under in vitro culture. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39934.