3D Printing of Hybrid-Hydrogel Materials for Tissue Engineering: a Critical Review.

Purpose: Key natural polymers, known as hydrogels, are an important group of materials in design of tissue-engineered constructs that can provide suitable habitat for cell attachment and proliferation. However, in comparison to tissues within the body, these hydrogels display poor mechanical properties. Such properties cause challenges in 3D printing of hydrogel scaffolds as well as their surgical handling after fabrication. For this reason, the purpose of this study is to critically review the 3D printing processes of hydrogels and their characteristics for tissue engineering application. Methods: A search of Google Scholar and PubMed has been performed from 2003 to February 2022 using a combination of keywords. A review of the types of 3D printing is presented. Additionally, different types of hydrogels and nano-biocomposite materials for 3D printing application are critically reviewed. The rheological properties and crosslinking mechanisms for the hydrogels are assessed. Results: Extrusion-based 3D printing is the most common practice for constructing hydrogel-based scaffolds, and it allows for the use of varying types of polymers to enhance the properties and printability of the hydrogel-based scaffolds. Rheology has been found to be exceedingly important in the 3D printing process; however, shear-thinning and thixotropic characteristics should also be present in the hydrogel. Despite these features of extrusion-based 3D printing, there are limitations to its printing resolution and scale. Conclusion: Combining natural and synthetic polymers and a variety of nanomaterials, such as metal, metal oxide, non-metal, and polymeric, can enhance the properties of hydrogel and provide additional functionality to their 3D-printed constructs.

Development of a Modular Reinforced Bone Tissue Engineering Scaffold with Enhanced Mechanical Properties.

A modular design composed of 3D-printed polycaprolactone (PCL) as the load-bearing module, and dual porosity gelatin foam as the bio-reactive module, was developed and characterized in this study. Surface treatment of the PCL module through aminolysis-aldehyde process was found to yield a stronger interface bonding compared to NaOH hydrolysis, and therefore was used in the fabrication procedure. The modular scaffold was shown to significantly improve the mechanical properties of the gelatin foam. Both compressive modulus and ultimate strength was found to increase over 10 times when the modular design was employed. The bio-reactive module i.e., gelatin foam, presented a dual porosity network of 100-300 µm primary and <10 µm secondary pores. SEM images revealed excellent attachment of DPSCs to the bio-reactive module.

In vivo efficacy of 3D-printed elastin-gelatin-hyaluronic acid scaffolds for regeneration of nasal septal cartilage defects.

Nasal septal cartilage perforations occur due to the different pathologies. Limited healing ability of cartilage results in remaining defects and further complications. This study sought to assess the efficacy of elastin-gelatin-hyaluronic acid (EGH) scaffolds for regeneration of nasal septal cartilage defects in rabbits. Defects (4 × 7 mm) were created in the nasal septal cartilage of 24 New Zealand rabbits. They were randomly divided into four groups: Group 1 was the control group with no further intervention, Group 2 received EGH scaffolds implanted in the defects, Group 3 received EGH scaffolds seeded with autologous auricular chondrocytes implanted in the defects, and Group 4 received EGH scaffolds seeded with homologous auricular chondrocytes implanted in the defects. After a 4-month healing period, computed tomography (CT) and magnetic resonance imaging (MRI) scans were obtained from the nasal septal cartilage, followed by histological evaluations of new tissue formation. Maximum regeneration occurred in Group 2, according to CT, and Group 3, according to both T1 and T2 images with 7.68 ± 1.36, 5.44 ± 2.41, and 8.72 ± 3.02 mm2 defect area respectively after healing. The difference in the defect size was statistically significant after healing between the experimental groups. Group 3 showed significantly greater regeneration according to CT scans and T1 and T2 images. The neocartilage formed over the underlying old cartilage with no distinct margin in histological evaluation. The EGH scaffolds have the capability of regeneration of nasal cartilage defects and are able to integrate with the existing cartilage; yet, they present the best results when pre-seeded with autologous chondrocytes.

Osteo-mucosal engineered construct: In situ adhesion of hard-soft tissues.

OBJECTIVES: The aim of this work was to combine engineered hard and soft tissue, adopting a new method for interfacial adhesion of osteo-mucosal construct. We hypothesized that the chemical procedure involved in this method not only adheres the components, but also improves the cell growth inside them. METHODS: 3D-printed functionally-graded porous hard-tissue scaffolds were characterized, functionalized by aminolysis and tyrosinase, and accommodated by human osteoblast cells. Introducing amino groups through aminolysis and inducing dopaquinones by tyrosinase can take part in the Michael additions to cause the adhesion. Subsequently, fully-differentiated engineered oral mucosa was formed directly on the surface of hard tissue. Constructs were assessed in term of morphology, structure, chemical composition, histology, and cytocompatibility. Interfacial adhesion was compared to a control group prepared by using a biological glue for the attachment of the soft and hard tissues. RESULTS: The data confirmed higher proliferation of osteoblast cells via aminolysis and improved osteoblast cells distribution and differentiation by incorporation of tyrosinase in collagen. There was evidence of multilayered, stratified epithelium on the osteo-mucosal model with viable fibroblasts and osteoblasts within the lamina propria and bone tissue layers. Our method of adhesion resulted in cohesive debonding within the engineered soft tissue; while in the control group, adhesive debonding and complete separation of the oral mucosa from the hard tissue was observed. Although the shear strength of the osteo-mucosal model (157.6 kDa ± 25.1) was slightly higher than that of the control group (149.4 kDa ± 23.1), there was no statistically significant difference between them (p > 0.05). However, the advantage of our in situ adhesion approach is the absence of a barrier like glue which can disrupt direct cellular communications between tissues. SIGNIFICANCE: This study provides a novel method of directly combining tissue-engineered human bone with oral mucosa, which has the potential to improve cell-ingrowth and tissue integration. This engineered tissue construct, after further optimization, can be used clinically as a graft material in various oral surgeries and can also be employed as an in vitro model to investigate many aspects of oral diseases and examine dental materials and oral health care products as a replacement of in vivo models.

Pressure-Assisted Coating of Ceramics on 3D-Printed Polymeric Scaffolds.

Pressure-assisted coating (PAC) is introduced to coat 3D-printed polymeric scaffolds with ß-tricalcium phosphate (ß-TCP) for tissue engineering applications. The method consists of four steps: infiltration of ceramic particles into the porous structure of the polymeric scaffold, dehydration of the slurry, compaction of ceramic particles around the scaffold, and heat treatment. The optimal coating is obtained at an infiltration speed of 400 mm/min followed by complete dehydration, compaction under ca. 8 MPa pressure, and subsequent heat treatment at 65 °C. The outcome is a uniformly coated scaffold with no deformation or structural defects, as confirmed by micro-CT analysis and laser and scanning electron microscopy. Scaffolds coated using the PAC method present superior interface bonding strength compared to those coated with a biomimetic approach. The contact angle decreased from 75.2 ± 1.4° for the uncoated scaffold to 39.6 ± 9.6° for the PAC specimen. PAC also increased the surface roughness from 0.66 ± 0.08 to 6.89 ± 0.26 µm and doubled the number of attached cells on the 3rd day of culture. The described method is applicable to different structures, object sizes, pore sizes, and shapes. For instance, in-depth coating of a 10 mm × 10 mm (D × H) cone with a 58 ± 4 µm-thick layer of ß-TCP can be achieved using PAC. The method can be used to coat other polymers, such as poly(lactic-co-glycolic acid) (PLGA). Successful coating of ß-TCP on 3D-printed PLGA scaffolds is also presented as a proof of concept.