3D Printed Silicone–Hydrogel Scaffold with Enhanced Physicochemical Properties

Smart alginate inks for tissue engineering applications

Amazing achievements have been made in the field of tissue engineering during the past decades. However, we have not yet seen fully functional human heart, liver, brain, or kidney tissue emerge from the clinics. The promise of tissue engineering is thus still not fully unleashed. This is mainly related to the challenges associated with producing tissue constructs with similar complexity as native tissue. Bioprinting is an innovative technology that has been used to obliterate these obstacles. Nevertheless, natural organs are highly dynamic and can change shape over time; this is part of their functional repertoire inside the body. 3D-bioprinted tissue constructs should likewise adapt to their surrounding environment and not remain static. For this reason, the new trend in the field is 4D bioprinting - a new method that delivers printed constructs that can evolve their shape and function over time. A key lack of methodology for printing approaches is the scalability, easy-to-print, and intelligent inks. Alginate plays a vital role in driving innovative progress in 3D and 4D bioprinting due to its exceptional properties, scalability, and versatility. Alginate's ability to support 3D and 4D printing methods positions it as a key material for fueling advancements in bioprinting across various applications, from tissue engineering to regenerative medicine and beyond. Here, we review the current progress in designing scalable alginate (Alg) bioinks for 3D and 4D bioprinting in a "dry"/air state. Our focus is primarily on tissue engineering, however, these next-generation materials could be used in the emerging fields of soft robotics, bioelectronics, and cyborganics.

Natural bone inspired core-shell triple-layered gel/PCL/gel 3D printed scaffolds for bone tissue engineering

Despite technological advancements in bone tissue engineering, it is still a challenge to fabricate a scaffold with high bioactivity as well as high mechanical strength that can promote osteogenesis as well as bear load. Here we developed a 3D printed gel-polymer multi-layered hybrid scaffold. The innermost layer is porous gel-based framework made of gelatin/carboxymethyl-chitin/nano-hydroxyapatite and is cryogenically 3D printed. Further, the second and middle layer of micro-engineered polycaprolactone (PCL) is infused in the gel with controlled penetration and tuneable coating thickness. The PCL surface is further coated with a third and final thin layer of gel matrix used for the first layer. This triple-layered structure demonstrates compression strength and modulus of 13.07 ± 1.15 MPa and 21.8 ± 0.82 MPa, respectively, post 8 weeks degradation which is >3000% and >700% than gel scaffold. It also shows degradation of 6.84 ± 0.70% (83% reduction than gel scaffold) after 12 weeks and swelling of 69.09 ± 6.83% (81% reduction) as compared to gel scaffolds. Further, nearly 300%, 250%, 50%, and 440% increase in cellular attachment, proliferation, protein generation, and mineralization, respectively are achieved as compared to only PCL scaffolds. Thus, these hybrid scaffolds offer high mechanical strength, slow degradation rate, high bioactivity, and high osteoconductivity. These multifunctional scaffolds have potential for reconstructing non-load-bearing bone defects like sinus lift, jaw cysts, and moderate load-bearing like reconstructing hard palate, orbital palate, and other craniomaxillofacial bone defects.

Effect of Thixotropic Agent on the Color Stability of Platinum-Based Silicone Maxillofacial Elastomers after Artificial Aging

Maxillofacial prostheses are essential for restoring natural appearance and function in individuals with defects in the head and neck regions. Thixotropic agents, as liquid additives, are known to increase the viscosity of silicone elastomers. However, color deterioration remains a challenge in facial prostheses, leading to the need for refabrication. Despite this, there is limited research on the effect of thixotropic agents on the color stability of silicone maxillofacial elastomers. This study aims to investigate the impact of different thixotropic agent amounts on the color degradation of various maxillofacial silicone elastomers. Three elastomers (A-2000, A-2006, and A-2186) were combined with five pigments (no pigment as control, red, yellow, blue, and a mixture of red, yellow, and blue), and mixed with six thixotropic agent quantities (0, 1, 2, 3, 4, and 5 drops). A total of 450 specimens were fabricated (n = 5) and aged in an artificial aging chamber. L*, a*, b* readings were obtained before and after aging using a digital spectrophotometer. Color difference (ΔE*) means and standard deviations for 150 kj/m2, 300 kj/m2, and 450 kj/m2 were calculated. Statistical analyses, including four-way ANOVA and Fisher's PLSD test, were conducted to determine any significant differences (p < 0.05) among the groups. A comprehensive analysis revealed significant four-way interactions among the groups. In the mixed-pigmentation group, adding 4 drops of thixotropic agent resulted in ΔE* above 3 only in A-2186 silicone at 300 and 450 kj/m2 energy levels. However, the color stability of mixed-pigmented A-2000 and A-2006 remained within the acceptable thresholds of 3 ΔE* at all irradiance levels in this study. At each energy level, A-2006 exhibited the highest color stability with an increasing thixotropic agent quantity among all the silicones. Conversely, A-2186 was more affected by the increased number of thixotropic agent drops in each pigmentation group, including the control group at 450 kj/m2. The quantity of thixotropic agent plays a crucial role in determining the color stability of different silicone elastomers pigmented with various intrinsic pigments. The thixotropic agent amount has a more significant impact on color stability than the type of pigment used in the silicone elastomers. A key overarching insight from this investigation is the identification of a safety threshold for the thixotropic agent quantity of 3 drops for each silicone type, pigmentation, and energy level. These findings highlight the importance of considering the proper combination of thixotropic agents, pigments, and silicone materials to achieve optimal color stability in maxillofacial prosthetic applications.

Optimization strategies of mesenchymal stem cell-based therapy for acute kidney injury

Considering the high prevalence and the lack of targeted pharmacological management of acute kidney injury (AKI), the search for new therapeutic approaches for it is in urgent demand. Mesenchymal stem cells (MSCs) have been increasingly recognized as a promising candidate for the treatment of AKI. However, clinical translation of MSCs-based therapies is hindered due to the poor retention and survival rates as well as the impaired paracrine ability of MSCs post-delivery. To address these issues, a series of strategies including local administration, three-dimensional culture, and preconditioning have been applied. Owing to the emergence and development of these novel biotechnologies, the effectiveness of MSCs in experimental AKI models is greatly improved. Here, we summarize the different approaches suggested to optimize the efficacy of MSCs therapy, aiming at promoting the therapeutic effects of MSCs on AKI patients.

Advanced Hydrogel systems for mandibular reconstruction

Mandibular defect becomes a prevalent maxillofacial disease resulting in mandibular dysfunctions and huge psychological burdens to the patients. Considering the routine presence of oral contaminations and aesthetic restoration of facial structures, the current clinical treatments are however limited, incapable to reconstruct the structural integrity and regeneration, spurring the need for cost-effective mandibular tissue engineering. Hydrogel systems possess great merit for mandibular reconstruction with precise involvement of cells and bioactive factors. In this review, current clinical treatments and distinct mode(s) of mandible formation and pathological resorption are summarized, followed by a review of hydrogel-related mandibular tissue engineering, and an update on the advanced fabrication of hydrogels with improved mechanical property, antibacterial ability, injectable form, and 3D bioprinted hydrogel constructs. The exploration of advanced hydrogel systems will lay down a solid foundation for a bright future with more biocompatible, effective, and personalized treatment in mandibular reconstruction.

Mechanical Evaluation of Hydrogel-Elastomer Interfaces Generated through Thiol-Ene Coupling

The formation of hybrid hydrogel-elastomer scaffolds is an attractive strategy for the formation of tissue engineering constructs and microfabricated platforms for advanced in vitro models. The emergence of thiol-ene coupling, in particular radical-based, for the engineering of cell-instructive hydrogels and the design of elastomers raises the possibility of mechanically integrating these structures without relying on the introduction of additional chemical moieties. However, the bonding of hydrogels (thiol-ene radical or more classic acrylate/methacrylate radical-based) to thiol-ene elastomers and alkene-functional elastomers has not been characterized in detail. In this study, we quantify the tensile mechanical properties of hybrid hydrogel samples formed of two elastomers bonded to a hydrogel material. We examine the impact of radical thiol-ene coupling on the crosslinking of both elastomers (silicone or polyesters) and hydrogels (based on thiol-ene crosslinking or diacrylate chemistry) and on the mechanics and failure behavior of the resulting hybrids. This study demonstrates the strong bonding of thiol-ene hydrogels to alkene-presenting elastomers with a range of chemistries, including silicones and polyesters. Overall, thiol-ene coupling appears as an attractive tool for the generation of strong, mechanically integrated, hybrid structures for a broad range of applications.

An Overview of Organ-on-a-Chip Models for Recapitulating Human Pulmonary Vascular Diseases

Traditionally, animal models have been used for recapitulating human physiology and for studying the pathological basis of many diseases affecting humankind. Indeed, over the centuries, animal models helped advance our understanding of the biology and pathology of drug therapy for humans. However, with the advent of genomics and pharmacogenomics, we now know that conventional models cannot accurately capture the pathological conditions and biological processes in humans, although humans share many physiological and anatomical features with many animals [1-3]. Species to species variation have raised concerns about the validity and suitability of animal models for studying human conditions. Over the past decade, the development and advances in microfabrication and biomaterials have spurred the growth in micro-engineered tissue and organ models (organs-on-a-chip, OoC) as alternatives to animal and cellular models [4]. This state-of-the-art technology has been used to emulate human physiology for investigating multitudes of cellular and biomolecular processes implicated in the pathological basis of disease (Fig. 13.1) [4]. Because of their tremendous potential, OoC-based models have been listed as one of the top 10 emerging technologies in the 2016 World Economic Forum [2].

Silicon-Based Scaffold for Wound Healing Skin Regeneration Applications: A Concise Review

Silicon has made its breakthrough in various industries, including clinical and biomedical applications. Silicon-based biomaterials that were fabricated into various types of scaffolds may attract interest due to their highly favorable properties covering their excellent biocompatibility, high surface area, mechanical strength, and selectivity depending on their application including film, hydrogel, nanoparticles, and so on. Silicon-based materials have also shown exciting results involving cell culture, cell growth, as well as tissue engineering. In this article, a simple review compromising the evaluation of silicon's unique properties has been discussed and followed by the application of the silicone-based product in future perspectives in biomedical fields. The review goals are to widen and inspire broader interest in silicone-based materials in wound healing research.

Multilayer Soft Photolithography Fabrication of Microfluidic Devices Using a Custom-Built Wafer-Scale PDMS Slab Aligner and Cost-Efficient Equipment

We present a robust, low-cost fabrication method for implementation in multilayer soft photolithography to create a PDMS microfluidic chip with features possessing multiple height levels. This fabrication method requires neither a cleanroom facility nor an expensive UV exposure machine. The central part of the method stays on the alignment of numerous PDMS slabs on a wafer-scale instead of applying an alignment for a photomask positioned right above a prior exposure layer using a sophisticated mask aligner. We used a manual XYZR stage attached to a vacuum tweezer to manipulate the top PDMS slab. The bottom PDMS slab sat on a rotational stage to conveniently align with the top part. The movement of the two slabs was observed by a monocular scope with a coaxial light source. As an illustration of the potential of this system for fast and low-cost multilayer microfluidic device production, we demonstrate the microfabrication of a 3D microfluidic chaotic mixer. A discussion on another alternative method for the fabrication of multiple height levels is also presented, namely the micromilling approach.

The Manufacture of Unbreakable Bionics via Multifunctional and Self-Healing Silk-Graphene Hydrogels

Biomaterials capable of transmitting signals over longer distances than those in rigid electronics can open new opportunities for humanity by mimicking the way tissues propagate information. For seamless mirroring of the human body, they also have to display conformability to its curvilinear architecture, as well as, reproducing native-like mechanical and electrical properties combined with the ability to self-heal on demand like native organs and tissues. Along these lines, a multifunctional composite is developed by mixing silk fibroin and reduced graphene oxide. The material is coined "CareGum" and capitalizes on a phenolic glue to facilitate sacrificial and hierarchical hydrogen bonds. The hierarchal bonding scheme gives rise to high mechanical toughness, record-breaking elongation capacity of ≈25 000%, excellent conformability to arbitrary and complex surfaces, 3D printability, a tenfold increase in electrical conductivity, and a fourfold increase in Young's modulus compared to its pristine counterpart. By taking advantage of these unique properties, a durable and self-healing bionic glove is developed for hand gesture sensing and sign translation. Indeed, CareGum is a new advanced material with promising applications in fields like cyborganics, bionics, soft robotics, human-machine interfaces, 3D-printed electronics, and flexible bioelectronics.

Nanoclay Reinforced Biomaterials for Mending Musculoskeletal Tissue Disorders

Nanoclay-reinforced biomaterials have sparked a new avenue in advanced healthcare materials that can potentially revolutionize treatment of musculoskeletal defects. Native tissues display many important chemical, mechanical, biological, and physical properties that engineered biomaterials need to mimic for optimal tissue integration and regeneration. However, it is time-consuming and difficult to endow such combinatorial properties on materials via feasible and nontoxic procedures. Fortunately, a number of nanomaterials such as graphene, carbon nanotubes, MXenes, and nanoclays already display a plethora of material properties that can be transferred to biomaterials through a simple incorporation procedure. In this direction, the members of the nanoclay family are easy to functionalize chemically, they can significantly reinforce the mechanical performance of biomaterials, and can provide bioactive properties by ionic dissolution products to upregulate cartilage and bone tissue formation. For this reason, nanoclays can become a key component for future orthopedic biomaterials. In this review, we specifically focus on the rapidly decreasing gap between clinic and laboratory by highlighting their application in a number of promising in vivo studies.

Design and characterization of 3D printed, neomycin-eluting poly-L-lactide mats for wound-healing applications

This study evaluates the suitability of 3D printed biodegradable mats to load and deliver the topical antibiotic, neomycin, for up to 3 weeks in vitro. A 3D printer equipped with a hot melt extruder was used to print bandage-like wound coverings with porous sizes appropriate for cellular attachment and viability. The semicrystalline polyester, poly-l-lactic acid (PLLA) was used as the base polymer, coated (post-printing) with polyethylene glycols (PEGs) of MWs 400 Da, 6 kDa, or 20 kDa to enable manipulation of physicochemical and biological properties to suit intended applications. The mats were further loaded with a topical antibiotic (neomycin sulfate), and cumulative drug-release monitored for 3 weeks in vitro. Microscopic imaging as well as Scanning Electron Microscopy (SEM) studies showed pore dimensions of 100 × 400 µm. These pore dimensions were achieved without compromising mechanical strength; because of the "tough" individual fibers constituting the mat (Young's Moduli of 50 ± 20 MPa and Elastic Elongation of 10 ± 5%). The in vitro dissolution study showed first-order release kinetics for neomycin during the first 20 h, followed by diffusion-controlled (Fickian) release for the remaining duration of the study. The release of neomycin suggested that the ability to load neomycin on to PLLA mats increases threefold, as the MW of the applied PEG coating is lowered from 20 kDa to 400 Da. Overall, this study demonstrates a successful approach to using a 3D printer to prepare porous degradable mats for antibiotic delivery with potential applications to dermal regeneration and tissue engineering. Illustration of the process used to create and characterize 3D printed PLLA mats.

Molecular-Gated Drug Delivery Systems Using Light-Triggered Hydrophobic-to-Hydrophilic Switches

A photoresponsive molecular-gated drug delivery system (DDS) based on silicone-hydrogel (poly(HEMA-co-PEGMEA)) interpenetrating polymer networks (IPNs) functionalized with carboxylated spiropyran (SPCOOH) was designed and demonstrated as an on-demand DDS. The triggered-release mechanism relies on controlling the wetting behavior of the surface by light, exploiting different hydrophobicities between the "closed" and "open" isomers of spiropyran as a photoswitchable molecular gate on the surface of IPN (SP-photogated IPN). Light-triggered release of doxycycline (DOX) as a model drug indicated that the spiropyran (SP) molecules provide a hydrophobic layer around the drug carrier and have a good gate-closing efficiency for IPNs with 20-30% hydrogel content. Upon UV light irradiation, SP converts into an open hydrophilic merocyanine state, which triggers the release of DOX. These results were compared with a previously developed SP-bulk modified IPN using the same hydrogel as a control, proving the efficiency of the gated IPN system. The covalent attachment of SPCOOH to the alcohol groups of the hydrogel and the structural change caused by UV light was indicated with FTIR analysis. XPS results also confirm the presence of SP by indicating the atomic percentage of nitrogen with respect to the hydrogel content.

On-Demand Reversible UV-Triggered Interpenetrating Polymer Network-Based Drug Delivery System Using the Spiropyran-Merocyanine Hydrophobicity Switch

A reversible switchable on-demand UV-triggered drug delivery system (DDS) based on interpenetrating polymer networks (IPNs) with silicone as the host polymer and spiropyran (SP)-functionalized guest polymer is designed and demonstrated. The photo-responsive IPNs provide a new triggered drug delivery concept as they exploit the change in intermolecular interactions (work of adhesion) among the drug, matrix, and solvent when the incorporated hydrophobic SP moieties transform into the hydrophilic merocyanine form upon light irradiation without degradation and disruption of the DDS. The change in how the copolymer composition (hydrophilicity and content) and the lipophilicity of the drug (log P) affect the release profile was investigated. A thermodynamic model, based on Hansen solubility parameters, was developed to design and optimize the polymer composition of the IPNs to obtain the most efficient light-triggered drug release and suppression of the premature release. The developed IPNs showed excellent result for dopamine, l-dopa, and prednisone with around 90-95% light-triggered release. The model was applied to study the release behavior of drugs with different log P and to estimate if the light-induced hydrophobic-to-hydrophilic switch can overcome the work of adhesion between polymers and drugs and hence the desorption and release of the drugs. To the best of our knowledge, this is the first time that work of adhesion is used for this aim. Comparing the result obtained from the model and experiment shows that the model is useful for evaluating and estimating the release behavior of specific drugs merocyanine, IPN, DDS, and spiropyran.

On the progress of 3D-printed hydrogels for tissue engineering

ABSTRACT

Additive manufacturing or more commonly known as 3D printing, is currently driving innovations and applications in diverse fields such as prototyping, manufacturing, aerospace, education, and medicine. Recent technological and materials research breakthroughs have enabled 3D bioprinting, where biomaterials and cells are used to create scaffolds and functional living tissues (e.g. skin, cartilage, etc.). This prospective focuses on the classification and applications of hydrogels, and design considerations in their production (i.e. physical and biological parameters). The materials for 3D printing of hydrogels, such as biopolymers, synthetic polymers, and nanocomposites, are mainly discussed. More importantly, future perspectives on 3D printing hydrogels including new materials, 4D printing, emerging printing technologies, etc. and their importance in biomedical and bioengineering applications are discussed.

3D Printing of a Polydimethylsiloxane/Polytetrafluoroethylene Composite Elastomer and its Application in a Triboelectric Nanogenerator

Silicone rubber elastomers are broadly used in various fields, where the three-dimensional (3D) printing of silicone rubber elastomers is important for the free construction of complex structures. Herein, a series of polydimethylsiloxane/polytetrafluoroethylene composite inks for direct-ink-writing 3D printing are developed. The inks are prepared by directly mixing a silicone rubber liquid precursor with polytetrafluoroethylene micropowder. The polytetrafluoroethylene micropowder serves as a thixotropic agent to regulate the rheological properties of the polydimethylsiloxane precursor to fulfill the requirement of 3D printing and endow the composite material with high electron affinity. The printed polydimethylsiloxane/polytetrafluoroethylene composite elastomer exhibits excellent elasticity and cyclic stability. A high-performance triboelectric nanogenerator is constructed with the 3D-printed polydimethylsiloxane/polytetrafluoroethylene composite as the triboelectric layer and elastic structure. This work establishes a new method of 3D printing polydimethylsiloxane-based elastomers and thus provides a new technique for constructing complex structures in flexible devices.

Stable and Antibacterial Magnesium-Graphene Nanocomposite-Based Implants for Bone Repair

Magnesium (Mg)-based alloys are promising biodegradable materials for bone repair applications. However, due to their rapid degradation and high corrosion rate, Mg-based alloys are typically associated with in vivo infections and implant failure. This study evaluated the synergistic stability and anti-inflammatory properties that could potentially be achieved by the modification of the Mg alloy with graphene nanoparticles (Gr). Incorporation of low dosages of Gr (0.18 and 0.50 wt %) in a Mg alloy with aluminum (Al, 1 wt %) and copper (Cu, 0.25 wt %) was successfully achieved by a spark plasma sintering (SPS) method. Notably, the degradation rate of the Mg-based alloys was reduced approximately 4-fold and the bactericidal activity was enhanced up to 5-fold with incorporation of only 0.18 wt % Gr to the Mg-1Al-Cu matrix. Moreover, the modified Mg-based nanocomposites with 0.18 wt % Gr demonstrated compressive properties within the range of native cancellous bone (modulus of approximately 6 GPa), whereas in vitro studies with human mesenchymal stromal cells (hMSCs) showed high cytocompatibility and superior osteogenic properties compared to non-Gr-modified Mg-1Al-Cu implants. Overall, this study provides foundations for the fabrication of stable, yet fully resorbable, Mg-based bone implants that could reduce implant-associated infections.

Extrusion 3D Printing of Polymeric Materials with Advanced Properties

3D printing is a rapidly growing technology that has an enormous potential to impact a wide range of industries such as engineering, art, education, medicine, and aerospace. The flexibility in design provided by this technique offers many opportunities for manufacturing sophisticated 3D devices. The most widely utilized method is an extrusion-based solid-freeform fabrication approach, which is an extremely attractive additive manufacturing technology in both academic and industrial research communities. This method is versatile, with the ability to print a range of dimensions, multimaterial, and multifunctional 3D structures. It is also a very affordable technique in prototyping. However, the lack of variety in printable polymers with advanced material properties becomes the main bottleneck in further development of this technology. Herein, a comprehensive review is provided, focusing on material design strategies to achieve or enhance the 3D printability of a range of polymers including thermoplastics, thermosets, hydrogels, and other polymers by extrusion techniques. Moreover, diverse advanced properties exhibited by such printed polymers, such as mechanical strength, conductance, self-healing, as well as other integrated properties are highlighted. Lastly, the stimuli responsiveness of the 3D printed polymeric materials including shape morphing, degradability, and color changing is also discussed.

Tough magnesium phosphate-based 3D-printed implants induce bone regeneration in an equine defect model

One of the important challenges in bone tissue engineering is the development of biodegradable bone substitutes with appropriate mechanical and biological properties for the treatment of larger defects and those with complex shapes. Recently, magnesium phosphate (MgP) doped with biologically active ions like strontium (Sr²⁺) have shown to significantly enhance bone formation when compared with the standard calcium phosphate-based ceramics. However, such materials can hardly be shaped into large and complex geometries and more importantly lack the adequate mechanical properties for the treatment of load-bearing bone defects. In this study, we have fabricated bone implants through extrusion assisted three-dimensional (3D) printing of MgP ceramics modified with Sr²⁺ ions (MgPSr) and a medical-grade polycaprolactone (PCL) polymer phase. MgPSr with 30 wt% PCL (MgPSr-PCL30) allowed the printability of relevant size structures (>780 mm³) at room temperature with an interconnected macroporosity of approximately 40%. The printing resulted in implants with a compressive strength of 4.3 MPa, which were able to support up to 50 cycles of loading without plastic deformation. Notably, MgPSr-PCL30 scaffolds were able to promote in vitro bone formation in medium without the supplementation with osteo-inducing components. In addition, long-term in vivo performance of the 3D printed scaffolds was investigated in an equine tuber coxae model over 6 months. The micro-CT and histological analysis showed that implantation of MgPSr-PCL30 induced bone regeneration, while no bone formation was observed in the empty defects. Overall, the novel polymer-modified MgP ceramic material and extrusion-based 3D printing process presented here greatly improved the shape ability and load-bearing properties of MgP-based ceramics with simultaneous induction of new bone formation.

Trends in Utilization of Virtual Surgical Planning in Pediatric Craniofacial Surgery

INTRODUCTION

While the use of virtual surgical planning (VSP) has been well described in the adult craniofacial literature, there has been little written about pediatric uses or trends. The purpose of this study is to evaluate the evolving utilization of VSP for pediatric craniofacial procedures.

METHODS

The authors' prospective institutional review board-approved craniofacial registry was queried for index craniofacial procedures from January 2011 through December 2018. Data was collected regarding utilization of traditional surgical planning versus VSP, as well as the extent of VSP's influence on the operative procedure. These data were analyzed for trends over time and compared using appropriate statistics.

RESULTS

During the study period, a total of 1131 index craniofacial cases were performed, of which 160 cases (14.1%) utilized VSP. Utilization of VSP collectively increased over time, from 2.0% in 2011 to 18.6% in 2018 (P < 0.001). Utilization rates of VSP varied across procedures from 0% of craniosynostosis cases and fronto-orbital advancement cases to 67% of osteocutaneous free tissue transfers (P < 0.001). The most profound contributor to increase in VSP utilization was orthognathic surgery, utilized in 0% of orthognathic procedures in 2011 to 68.3% of orthognathic procedures in 2018 (P < 0.001).

CONCLUSIONS

Utilization of virtual surgical planning for pediatric craniofacial procedures is increasing, especially for complex orthognathic procedures and osteocutaneous free tissue transfers. Utilization patterns of individual components of the VSP system demonstrate unique footprints across the spectrum of craniofacial procedures, which reinforces the specific and variable benefits of this workflow for treating pediatric craniofacial disorders.

Engineered 3D Polymer and Hydrogel Microenvironments for Cell Culture Applications

The realization of biomimetic microenvironments for cell biology applications such as organ-on-chip, in vitro drug screening, and tissue engineering is one of the most fascinating research areas in the field of bioengineering. The continuous evolution of additive manufacturing techniques provides the tools to engineer these architectures at different scales. Moreover, it is now possible to tailor their biomechanical and topological properties while taking inspiration from the characteristics of the extracellular matrix, the three-dimensional scaffold in which cells proliferate, migrate, and differentiate. In such context, there is therefore a continuous quest for synthetic and nature-derived composite materials that must hold biocompatible, biodegradable, bioactive features and also be compatible with the envisioned fabrication strategy. The structure of the current review is intended to provide to both micro-engineers and cell biologists a comparative overview of the characteristics, advantages, and drawbacks of the major 3D printing techniques, the most promising biomaterials candidates, and the trade-offs that must be considered in order to replicate the properties of natural microenvironments.

Permeability and mechanical properties of gradient porous PDMS scaffolds fabricated by 3D-printed sacrificial templates designed with minimal surfaces

In the present study, polydimethylsiloxane (PDMS) porous scaffolds are designed based on minimal surface architectures and fabricated through a low-cost and accessible sacrificial mold printing approach using a fused deposition modeling (FDM) 3D printer. The effects of pore characteristics on compressive properties and fluid permeability are studied. The results suggest that radially gradient pore distribution (as a potential way to enhance mechanically-efficient scaffolds with enhanced cell/scaffold integration) has higher elastic modulus and fluid permeability compared to their uniform porosity counterparts. Also, the scaffolds are fairly strain-reversible under repeated loading of up to 40% strain. Among different triply periodic minimal surface pore architectures, P-surface was observed to be stiffer, less permeable and have lower densification strain compared to the D-surface and G-surface-based pore shapes. The biocompatibility of the created scaffolds is assessed by filling the PDMS scaffolds using mouse embryonic fibroblasts with cell-laden gelatin methacryloyl which was cross-linked in situ by UV light. Cell viability is found to be over 90% after 4 days in 3D culture. This method allows for effectively fabricating biocompatible porous organ-shaped scaffolds with detailed pore features which can potentially tailor tissue regenerative applications. STATEMENT OF SIGNIFICANCE: Printing polymers with chemical curing mechanism required for materials such as PDMS is challenging and impossible to create high-resolution uniformly cured structures due to hard control on the base polymer and curing process. An interconnected porous mold with ordered internal architecture with complex geometries were 3D printed using low-cost and accessible FDM technology. The mold acted as a 3D sacrificial material to form internally architected flexible PDMS scaffolds for tissue engineering applications. The scaffolds are mechanically stable under high strain cyclic loads and provide enough pore and space for viably integrating cells within the gradient architecture in a controllable manner.

Micropatterned Cell Orientation of Cyanobacterial Liquid-Crystalline Hydrogels

Control of cell extension direction is crucial for the regeneration of tissues, which are generally composed of oriented molecules. The scaffolds of highly oriented liquid crystalline polymer chains were fabricated by casting cyanobacterial mega-saccharides, sacran, on parallel-aligned micrometer bars of polystyrene (PS). Polarized microscopy revealed that the orientation was in transverse direction to the longitudinal axes of the PS bars. Swelling behavior of the micropatterned hydrogels was dependent on the distance between the PS bars. The mechanical properties of these scaffolds were dependent on the structural orientation; additionally, the Young's moduli in the transverse direction were higher than those in the parallel direction to the major axes of the PS bars. Further, fibroblast L929 cells were cultivated on the oriented scaffolds to be aligned along the orientation axis. L929 cells cultured on these scaffolds exhibited uniaxial elongation.

Development and Application of an Additively Manufactured Calcium Chloride Nebulizer for Alginate 3D-Bioprinting Purposes

Three-dimensional (3D)-bioprinting enables scientists to mimic in vivo micro-environments and to perform in vitro cell experiments under more physiological conditions than is possible with conventional two-dimensional (2D) cell culture. Cell-laden biomaterials (bioinks) are precisely processed to bioengineer tissue three-dimensionally. One primarily used matrix material is sodium alginate. This natural biopolymer provides both fine mechanical properties when gelated and high biocompatibility. Commonly, alginate is 3D bioprinted using extrusion based devices. The gelation reaction is hereby induced by a CaCl₂ solution in the building chamber after material extrusion. This established technique has two main disadvantages: (1) CaCl₂ can have toxic effects on the cell-laden hydrogels by oxygen diffusion limitation and (2) good printing resolution in the CaCl₂ solution is hard to achieve, since the solution needs to be removed afterwards and substituted by cell culture media. Here, we show an innovative approach of alginate bioprinting based on a CaCl₂ nebulizer. The device provides CaCl₂ mist to the building platform inducing the gelation. The necessary amount of CaCl₂ could be decreased as compared to previous gelation strategies and limitation of oxygen transfer during bioprinting can be reduced. The device was manufactured using the MJP-3D printing technique. Subsequently, its digital blueprint (CAD file) can be modified and additive manufactured easily and mounted in various extrusion bioprinters. With our approach, a concept for a more gentle 3D Bioprinting method could be shown. We demonstrated that the concept of an ultrasound-based nebulizer for CaCl₂ mist generation can be used for 3D bioprinting and that the mist-induced polymerization of alginate hydrogels of different concentrations is feasible. Furthermore, different cell-laden alginate concentrations could be used: Cell spheroids (mesenchymal stem cells) and single cells (mouse fibroblasts) were successfully 3D printed yielding viable cells and stable hydrogels after 24 h cultivation. We suggest our work to show a different and novel approach on alginate bioprinting, which could be useful in generating cell-laden hydrogel constructs for e.g., drug screening or (soft) tissue engineering applications.

Surface-Selective Control of Cell Orientation on Cyanobacterial Liquid Crystalline Gels

Liquid crystalline hydrogels (LCGs) with layer structures and oriented pores were created using sacran which is a cyanobacterial heteropolysaccharide possessing functional sulfate, carboxylate, and amide groups in common with glycosaminoglycan. The LCG biocompatibility with L929 mouse fibroblasts was confirmed under the appropriate conditions. Enhanced growth and proliferation of L929 cells without exhibiting any toxicity were confirmed. The water contact angle and protein adsorption ability on the LCG were well-controlled by the cross-linking degree. Additionally, fibroblasts were finely oriented on the LCG side face where layer edges made a striped morphology on its surface, whereas the flat top faces of the LCG did not induce any specific cell orientation.

3D printing integrated with controlled delivery for musculoskeletal tissue engineering

3D printing is an emerging tool to fabricate scaffolds for tissue engineering and regenerative medicine, benefited by customized design, tunable internal microstructure and a wide range of applicable materials. As a recent technical advancement, 3D-printed scaffolds have been incorporated with a controlled delivery of growth factors and/or other bioactive cues to facilitate tissue regeneration, in addition to providing a temporal structural substrate for cell and tissue ingrowth. This review covers a number of the existing approaches to incorporate a controlled delivery system in 3D-printed scaffolds from hydrogel adsorption and surface coating to chemical integration and embedding microspheres. In addition, we discuss the advantages and disadvantages of each delivery method integrated in 3D-printed scaffolds, outstanding challenges and future directions.

Paving the Way Toward Complex Blood-Brain Barrier Models Using Pluripotent Stem Cells

A tissue with great need to be modeled in vitro is the blood-brain barrier (BBB). The BBB is a tight barrier that covers all blood vessels in the brain and separates the brain microenvironment from the blood system. It consists of three cell types [neurovascular unit (NVU)] that contribute to the unique tightness and selective permeability of the BBB and has been shown to be disrupted in many diseases and brain disorders, such as vascular dementia, stroke, multiple sclerosis, and Alzheimer's disease. Given the progress that pluripotent stem cells (PSCs) have made in the past two decades, it is now possible to produce many cell types from the BBB and even partially recapitulate this complex tissue in vitro. In this review, we summarize the most recent developments in PSC differentiation and modeling of the BBB. We also suggest how patient-specific human-induced PSCs could be used to model BBB dysfunction in the future. Lastly, we provide perspectives on how to improve production of the BBB in vitro, for example by improving pericyte differentiation protocols and by better modeling the NVU in the dish.

Co-release of dicloxacillin and thioridazine from catheter material containing an interpenetrating polymer network for inhibiting device-associated Staphylococcus aureus infection

Approximately half of all nosocomial bloodstream infections are caused by bacterial colonization of vascular catheters. Attempts have been made to improve devices using anti-adhesive or antimicrobial coatings; however, it is often difficult to bind coatings stably to catheter materials, and the low amounts of drug in thin-film coatings limit effective long-term release. Interpenetrating polymer networks (IPNs) are polymer hybrid materials with unique drug release properties. While IPNs have been extensively investigated for use in tablet- or capsule-based drug delivery systems, the potential for use of IPNs in drug release medical devices remains largely unexplored. Here, we investigated the use of silicone-hydrogel IPNs as a catheter material to provide slow anti-bacterial drug-release functionality. IPN catheters were produced by the sequential method, using supercritical CO₂ as a solvent to polymerize and crosslink poly(2-hydroxyethyl methacrylate) (PHEMA) in silicone elastomer. The design was tested against Staphylococcus aureus colonization after loading with dicloxacillin (DCX) alone or in combination with thioridazine (TDZ), the latter of which is known to synergistically potentiate the antibacterial effect of DCX against both methicillin-sensitive and methicillin-resistant S. aureus. The hydrophilic PHEMA component allowed for drug loading in the catheters by passive diffusion and provided controlled release properties. The drug-loaded IPN material inhibited bacterial growth on agar plates for up to two weeks and in blood cultures for up to five days, and it withstood 24h of seeding with resilient biofilm aggregates. The combined loading of DCX+TDZ enhanced the antibacterial efficiency in static in vitro experiments, although release analyses revealed that this effect was due to an enhanced loading capacity of DCX when co-loaded with TDZ. Lastly, the IPN catheters were tested in a novel porcine model of central venous catheter-related infection, in which drug-loaded IPN catheters were found to significantly decrease the frequency of infection.

Engineering complex tissue-like microgel arrays for evaluating stem cell differentiation

Development of tissue engineering scaffolds with native-like biology and microarchitectures is a prerequisite for stem cell mediated generation of off-the-shelf-tissues. So far, the field of tissue engineering has not full-filled its grand potential of engineering such combinatorial scaffolds for engineering functional tissues. This is primarily due to the many challenges associated with finding the right microarchitectures and ECM compositions for optimal tissue regeneration. Here, we have developed a new microgel array to address this grand challenge through robotic printing of complex stem cell-laden microgel arrays. The developed microgel array platform consisted of various microgel environments that where composed of native-like cellular microarchitectures resembling vascularized and bone marrow tissue architectures. The feasibility of our array system was demonstrated through localized cell spreading and osteogenic differentiation of human mesenchymal stem cells (hMSCs) into complex tissue-like structures. In summary, we have developed a tissue-like microgel array for evaluating stem cell differentiation within complex and heterogeneous cell microenvironments. We anticipate that the developed platform will be used for high-throughput identification of combinatorial and native-like scaffolds for tissue engineering of functional organs.