Applications of microscale technologies for regenerative dentistry

Strategies to design extrinsic stimuli-responsive dental polymers capable of autorepairing

Objectives

For many years, the requirements for dental polymers were limited to inertially filling the cavity and restoring form, function, and esthetics. Inorganic filler systems were widely enhanced to maximize the mechanical properties and optimize finishing and polishing procedures. The development of alternative photoinitiator systems also improved the carbon-carbon double bond conversion, increasing biocompatibility, wear, and stain resistance. However, despite laudable progress, the clinical life span of dental restorations is still limited, and their replacement is the most common procedure in dental offices worldwide. In the last few years, the development of materials with the potential to adapt to physiological stimuli has emerged as a key step to elevating dental polymers to a higher excellence level. In this context, using polymeric networks with self-healing properties that allow for the control of the propagation of microcracks is an appealing strategy to boost the lifetime of dental restorations. This review aims to report the current state-of-the-art of extrinsic self-healing dental polymers and provide insights to open new avenues for further developments. General classification of the self-healing polymeric systems focusing on the current extrinsic strategies used to inhibit microcracks propagation in dental polymers and recover their structural integrity and toughness are presented.

Search Strategy

An electronic search was perfomed using PubMed, Google Scholar, and Scopus databases. Only studies published in English on extrinsic self-healing polymeric systems were included.

Overall Conclusions

Self-healing materials are still in their infancy in dentistry, and the future possibilities are almost limitless. Although the mouth is a unique environment and the restorative materials have to survive chemical, physical, and mechanical challenges, which limits the use of some strategies that might compromise their physicochemical performance, there are countless untapped opportunities to overcome the challenges of the current systems and advance the field.

Fabrication of bone-derived decellularized extracellular matrix/ceramic-based biocomposites and their osteo/odontogenic differentiation ability for dentin regeneration

The goal of this study was to fabricate bioactive cell-laden biocomposites supplemented with bone-derived decellularized extracellular matrix (dECM) with calcium phosphate ceramic, and to assess the effect of the biocomponents on the osteogenic and odontogenic differentiation of human dental pulp stem cells (hDPSCs). By evaluating the rheological properties and selecting printing parameters, mechanically stable cell-laden 3D biocomposites with high initial cell-viability (>90%) and reasonable printability (≈0.9) were manufactured. The cytotoxicity of the biocomposites was evaluated via MTT assay and nuclei/F-actin fluorescent analyses, while the osteo/odontogenic differentiation of the hDPSCs was assessed using histological and immunofluorescent analyses and various gene expressions. Alkaline phosphate activity and alizarin red staining results indicate that the dECM-based biocomposites exhibit significantly higher osteogenic activities, including calcification, compared to the collagen-based biocomposites. Furthermore, immunofluorescence images and gene expressions demonstrated upregulation of dentin matrix acidic phosphoprotein-1 and dentin sialophosphoprotein in the dECM-based biocomposites, indicating acceleration of the odontogenic differentiation of hDPSCs in the printed biocomposites. The hDPSC-laden biocomposite was implanted into the subcutaneous region of mice, and the biocomposite afforded clear induction of osteo/odontogenic ectopic hard tissue formation 8 weeks post-transplantation. From these results, we suggest that the hDPSC-laden biocomposite is a promising biomaterial for dental tissue engineering.

Tissue Engineered Constructs for Periodontal Regeneration: Current Status and Future Perspectives

The periodontium, consisting of gingiva, periodontal ligament, cementum, and alveolar bone, is a hierarchically organized tissue whose primary role is to provide physical and mechanical support to the teeth. Severe cases of periodontitis, an inflammatory condition initiated by an oral bacterial biofilm, can lead to significant destruction of soft and hard tissues of the periodontium and result in compromised dental function and aesthetics. Although current treatment approaches can limit the progression of the disease by controlling the inflammatory aspect, complete periodontal regeneration cannot be predictably achieved. Various tissue engineering approaches are investigated for their ability to control the critical temporo-spatial wound healing events that are essential for achieving periodontal regeneration. This paper reviews recent progress in the field of periodontal tissue engineering with an emphasis on advanced 3D multiphasic tissue engineering constructs (TECs) and provides a critical analysis of their regenerative potential and limitations. The review also elaborates on the future of periodontal tissue engineering, including scaffold customization for individual periodontal defects, TEC's functionalization strategies for imparting enhanced bioactivity, periodontal ligament fiber guidance, and the utilization of chair-side regenerative solutions that can facilitate clinical translation.

A dentin-derived hydrogel bioink for 3D bioprinting of cell laden scaffolds for regenerative dentistry

Recent studies in tissue engineering have adopted extracellular matrix (ECM) derived scaffolds as natural and cytocompatible microenvironments for tissue regeneration. The dentin matrix, specifically, has been shown to be associated with a host of soluble and insoluble signaling molecules that can promote odontogenesis. Here, we have developed a novel bioink, blending printable alginate (3% w/v) hydrogels with the soluble and insoluble fractions of the dentin matrix. We have optimized the printing parameters and the concentrations of the individual components of the bioink for print accuracy, cell viability and odontogenic potential. We find that, while viscosity, and hence printability of the bioinks, was greater in the formulations containing higher concentrations of alginate, a higher proportion of insoluble dentin matrix proteins significantly improved cell viability; where a 1:1 ratio of alginate and dentin (1:1 Alg-Dent) was most suitable. We further demonstrate high retention of the soluble dentin molecules within the 1:1 Alg-Dent hydrogel blends, evidencing renewed interactions between these molecules and the dentin matrix post crosslinking. Moreover, at concentrations of 100 μg ml^-1, these soluble dentin molecules significantly enhanced odontogenic differentiation of stem cells from the apical papilla encapsulated in bioprinted hydrogels. In summary, the proposed novel bioinks have demonstrable cytocompatibility and natural odontogenic capacity, which can be a used to reproducibly fabricate scaffolds with complex three-dimensional microarchitectures for regenerative dentistry in the future.

Dental Tissue Engineering Research and Translational Approaches towards Clinical Application

Stem cell-based dental tissue regeneration is a new and exciting field that has the potential to transform the way that we practice dentistry. It is, however, imperative its clinical application is supported by solid basic and translational research. In this way, the full extent of the potential risks involved in the use of these technologies will be understood, and the means to prevent them will be discovered. Therefore, the aim of this chapter is to analyze the state-of-the-science with regard to dental pulp stem cell research in dental tissue engineering, the new developments in biomimetic scaffold materials customized for dental tissue applications, and to give a prospectus with respect to translational approaches of these research findings towards clinical application.

Recent advancements in regenerative dentistry: A review

Although human mouth benefits from remarkable mechanical properties, it is very susceptible to traumatic damages, exposure to microbial attacks, and congenital maladies. Since the human dentition plays a crucial role in mastication, phonation and esthetics, finding promising and more efficient strategies to reestablish its functionality in the event of disruption has been important. Dating back to antiquity, conventional dentistry has been offering evacuation, restoration, and replacement of the diseased dental tissue. However, due to the limited ability and short lifespan of traditional restorative solutions, scientists have taken advantage of current advancements in medicine to create better solutions for the oral health field and have coined it "regenerative dentistry." This new field takes advantage of the recent innovations in stem cell research, cellular and molecular biology, tissue engineering, and materials science etc. In this review, the recently known resources and approaches used for regeneration of dental and oral tissues were evaluated using the databases of Scopus and Web of Science. Scientists have used a wide range of biomaterials and scaffolds (artificial and natural), genes (with viral and non-viral vectors), stem cells (isolated from deciduous teeth, dental pulp, periodontal ligament, adipose tissue, salivary glands, and dental follicle) and growth factors (used for stimulating cell differentiation) in order to apply tissue engineering approaches to dentistry. Although they have been successful in preclinical and clinical partial regeneration of dental tissues, whole-tooth engineering still seems to be far-fetched, unless certain shortcomings are addressed.

Integration of 3D Printed and Micropatterned Polycaprolactone Scaffolds for Guidance of Oriented Collagenous Tissue Formation In Vivo

Scaffold design incorporating multiscale cues for clinically relevant, aligned tissue regeneration has potential to improve structural and functional integrity of multitissue interfaces. The objective of this preclinical study is to develop poly(ε-caprolactone) (PCL) scaffolds with mesoscale and microscale architectural cues specific to human ligament progenitor cells and assess their ability to form aligned bone-ligament-cementum complexes in vivo. PCL scaffolds are designed to integrate a 3D printed bone region with a micropatterned PCL thin film consisting of grooved pillars. The patterned film region is seeded with human ligament cells, fibroblasts transduced with bone morphogenetic protein-7 genes seeded within the bone region, and a tooth dentin segment positioned on the ligament region prior to subcutaneous implantation into a murine model. Results indicate increased tissue alignment in vivo using micropatterned PCL films, compared to random-porous PCL. At week 6, 30 μm groove depth significantly enhances oriented collagen fiber thickness, overall cell alignment, and nuclear elongation relative to 10 μm groove depth. This study demonstrates for the first time that scaffolds with combined hierarchical mesoscale and microscale features can align cells in vivo for oral tissue repair with potential for improving the regenerative response of other bone-ligament complexes.

Current overview on challenges in regenerative endodontics

INTRODUCTION

Regenerative endodontics provides hope of converting the non-vital tooth into vital once again. It focuses on substituting traumatized and pathological pulp with functional pulp tissue. Current regenerative procedures successfully produce root development but still fail to re-establish real pulp tissue and give unpredictable results. There are several drawbacks that need to be addressed to improve the quality and efficiency of the treatment.

AIM

The aim of this review article is to discuss major priorities that ought to be dealt before applications of regenerative endodontics flourish the clinical practice.

MATERIALS AND METHODS

A web-based research on MEDLINE was done using filter terms Review, published in the last 10 years and Dental journals. Keywords used for research were "regenerative endodontics," "dental stem cells," "growth factor regeneration," "scaffolds," and "challenges in regeneration." This review article screened about 150 articles and then the relevant information was compiled.

RESULTS

Inspite of the impressive growth in regenerative endodontic field, there are certain loopholes in the existing treatment protocols that might sometimes result in undesired and unpredictable outcomes.

CONCLUSION

Considerable research and development efforts are required to improve and update existing regenerative endodontic strategies to make it an effective, safe, and biological mode to save teeth.

Micro-Assembly of a Vascular-Like Micro-Channel with Railed Micro-Robot Team-Coordinated Manipulation

The 3D assembly of cellular structures is important for the fabrication of biological substitutes in tissue engineering. In particular, a micro-channel with a 200 μm diameter is of interest because of its promising ability to construct the vascular network for oxygen and nutrition delivery in thick biological substitutes in the future. In this paper, a novel rail-guided micro-robot-team system is proposed for the micro-assembly of a cellular structure. The cellular two-dimensional (2D) component was fabricated by ultraviolet (UV) illumination of a cross-linkable hydrogel. The modular rail-guided micro-robotic system was set up with multi-micromanipulators as the modules and controlled with hybrid motors to achieve an operation resolution of 30 nm. To realize the bottom-up fabrication of the cellular micro-channel, different micro-assembly strategies with multi-manipulators were developed. The micro-assembly success rate and the efficiency of the different strategies were evaluated based on the assembly of micro-donuts. Through the novel, designed, concentric movement of the multi-manipulators along the rail, arbitrary change of the approaching angle and the coordination posture was achieved to improve the micro-assembly's flexibility. The operation range for every micromanipulator in different coordinated manipulation modes was analysed to avoid the breakdown of the assembled 3D structure. The image processing for the target location and end-effector identification was conducted to improve assembly efficiency in the micro-robot-team system. Finally, the assembly of the cellular vascular-like micro-channel was achieved with coordinated manipulation in the rail-guided micro-robot-team system.

Tooth Tissue Engineering: Potential and Pitfalls

All over the world a large number of people suffer from tooth diseases like dental caries, tooth abscess, and plaques. Tooth loss or damage, which occurs frequently in our society are generally repaired by applying several conventional methods, such as root-canal treatment, direct pulp capping and dental implants. These methods are quite painful, create damage to the surrounding tooth tissues and also may at times have adverse side-effects. The limitations of the conventional methods can be overcome by applying the concept of tooth tissue engineering. Tooth tissue engineering is the application of biosciences and engineering to regenerate a biofunctional tooth, which can be used to replace the missing tooth or repair the damaged tooth. Tissue engineering involves three key elements - cell, scaffold and growth factors, which interact with each other to regenerate a specific tissue. The success of tissue engineering depends on the proper selection of these three key elements and understanding the interactions among them. To bring us close to the realization of a tissue-engineered tooth, immense progress is going on in understanding how tooth is first developed, and there is a good advancement in tooth regeneration. In this review, “tooth tissue engineering” will be discussed, along with the recent advancements and challenges in bring a biofunctional tooth from laboratory out into clinical use.

Hybrid PGS-PCL microfibrous scaffolds with improved mechanical and biological properties

Poly(glycerol sebacate) (PGS) is a biodegradable elastomer that has generated great interest as a scaffold material due to its desirable mechanical properties. However, the use of PGS in tissue engineering is limited by difficulties in casting micro- and nanofibrous structures, due to high temperatures and vacuum required for its curing and limited solubility of the cured polymer. In this paper, we developed microfibrous scaffolds made from blends of PGS and poly(ε-caprolactone) (PCL) using a standard electrospinning set-up. At a given PGS:PCL ratio, higher voltage resulted in significantly smaller fibre diameters (reduced from ∼4 µm to 2.8 µm; p < 0.05). Further increase in voltage resulted in the fusion of fibres. Similarly, higher PGS concentrations in the polymer blend resulted in significantly increased fibre diameter (p < 0.01). We further compared the mechanical properties of electrospun PGS:PCL scaffolds with those made from PCL. Scaffolds with higher PGS concentrations showed higher elastic modulus (EM), ultimate tensile strength (UTS) and ultimate elongation (UE) (p < 0.01) without the need for thermal curing or photocrosslinking. Biological evaluation of these scaffolds showed significantly improved HUVEC attachment and proliferation compared to PCL-only scaffolds (p < 0.05). Thus, we have demonstrated that simple blends of PGS prepolymer with PCL can be used to fabricate microfibrous scaffolds with mechanical properties in the range of a human aortic valve leaflet.

Sequential assembly of cell-laden hydrogel constructs to engineer vascular-like microchannels

Microscale technologies, such as microfluidic systems, provide powerful tools for building biomimetic vascular-like structures for tissue engineering or in vitro tissue models. Recently, modular approaches have emerged as attractive approaches in tissue engineering to achieve precisely controlled architectures by using microengineered components. Here, we sequentially assembled microengineered hydrogels (microgels) into hydrogel constructs with an embedded network of microchannels. Arrays of microgels with predefined internal microchannels were fabricated by photolithography and assembled into 3D tubular construct with multi-level interconnected lumens. In the current setting, the sequential assembly of microgels occurred in a biphasic reactor and was initiated by swiping a needle to generate physical forces and fluidic shear. We optimized the conditions for assembly and successfully perfused fluids through the interconnected constructs. The sequential assembly process does not significantly influence cell viability within the microgels indicating its promise as a biofabrication method. Finally, in an attempt to build a biomimetic 3D vasculature, we incorporated endothelial cells and smooth muscle cells into an assembled construct with a concentric microgel design. The sequential assembly is simple, rapid, cost-effective, and could be used for fabricating tissue constructs with biomimetic vasculature and other complex architectures.

Fabrication and characterization of tough elastomeric fibrous scaffolds for tissue engineering applications

Development of biodegradable tough elastomeric scaffolds are important for engineering tissues such as myocardium and heart valves that experience dynamic environments in vivo. Biomaterial scaffolds should ideally provide appropriate physical, chemical and mechanical cues to the seeded cells to closely mimic the native ECM. Collagen fibers form an important component of native myocardium as well as heart valve leaflets and provide necessary tensile properties to these tissues. Amongst various polymers, collagen mimicking biodegradable elastomer, Poly-(glycerol-sebacate) (PGS) has shown great promise in microfabricated scaffolds for cardiac tissue engineering. However, its use is limited by its solubility and the ability to cast nano-/microfibrous structures. For its superior mechanical properties, thermal or UV crosslinking of the pre-polymer is required under high temperatures and vacuum limiting fabrication of fibers. In this work, we fabricated electrospun PGS fibers were fabricated by simply blending it with biodegradable polycaprolactone (PCL) polymer without any post-processing. It was hypothesized that microfibrous PGS-PCL scaffolds would provide appropriate physical (fibrous structure) and chemical (balanced hydrophilicity and hydrophobicity) to the cells in addition to the mechanical properties.

Hypoxia enhances colony formation and proliferation but inhibits differentiation of human dental pulp cells

The hypoxia condition was expected to be suitable for the establishment and maintenance of human dental pulp cells (hDPCs), because they reside in a low-oxygen environment in vivo. Therefore, we presently examined the effects of hypoxia on the proliferation and differentiation of hDPCs in vitro. hDPCs grown under 3% O(2) showed a significantly higher proliferation rate than those under 21% O(2). Then, we prepared hypoxic cultures of hDPCs from older patients' teeth having inflammation and succeeded in recovering and expanding a small number of hDPCs. These cells were confirmed to have capability for osteo/odontogenic differentiation. Hypoxia suppressed the osteo/odontogenic differentiation of hDPCs in vitro and increased the number of cells expressing STRO-1, an early mesenchymal stem cell marker. This simple method will increase the possibility to obtain living hDPCs from damaged and/or aged tissues, from which it is ordinarily difficult to isolate living stem cells with differentiation capability.