Tunable Mechanical Properties Research Articles

Additive-manufactured synthetic bone model with biomimicking tunable mechanical properties for evaluation of medical implants

Additive manufacturing has enabled the customization of biomedical systems, including transplantable medical devices, to achieve mechanical biocompatibility. For bone implants, patient-specific bone models must be used to evaluate the mechanical properties of implant compression and subsidence. This study proposes a methodology for designing and fabricating bone models to evaluate patient-specific bone implants. The method involves three-dimensional printing of infill-varied structure, with alternating high-low-high infill density regions, which undergo sequential deformation from the surficial region during compression with an implant. Based on this deformation behavior, the relationship between infill density parameters and mechanical properties was confirmed with the tunability of mechanical properties involving stiffness and failure load. The infill-varied design was applied to the inner structures of artificial vertebra models based on computed tomography scans for cadaver specimens. By tailoring the infill density conditions, the stiffness and failure load were approximated to those of the natural vertebrae. Furthermore, this infill-varied artificial vertebra could be used to evaluate additive-manufactured patient-specific implants. The patient-specific implant had greater resistance to subsidence than the commercial implant, suggesting the feasibility of a biomimicking bone model. The bone-mimetic infill-varied structure could be used to evaluate patient-specific manufactured implants and could be applied to other bone engineering structures with optimized biomechanical properties.

Conductive GelMA/alginate/polypyrrole/graphene hydrogel as a potential scaffold for cardiac tissue engineering; Physiochemical, mechanical, and biological evaluations

Cardiac failure can be a life-threatening condition that, if left untreated, can have significant consequences. Functional hydrogel has emerged as a promising platform for cardiac tissue engineering. In the proposed study, gelatin methacrylate (GelMA) and alginate, as a primary matrix to maintain cell viability and proliferation, and polypyrrole and carboxyl-graphene, to improve mechanical and electrical properties, are thoroughly evaluated. Initially, a polymer blend of GelMA/Alginate (1:1) was prepared, and then the addition of 2–5 wt% of polypyrrole was evaluated. Next, the effect of incorporating graphene-carboxyl nanosheets (1, 2, and 3 wt%) within the optimized scaffold with 2 wt% polypyrrole was thoroughly studied. The electrical conductivity of the hydrogels was significantly enhanced from 0.0615 ± 0.007 S/cm in GelMA/alginate to 0.124 ± 0.04 S/cm with the addition of 5 wt% polypyrrole. Also, 3 wt% carboxyl graphene improved the electrical conductivity to 0.27 ± 0.09 S/cm. The compressive strength of carboxyl-graphene-containing hydrogel was in the range of 175 to 520 kPa, and tensile strength was 61 and 133 kPa. The simplicity and low-cost fabrication, tunable mechanical properties, optimal electrical conductivity, blood compatibility, and non-cytotoxicity of GelMA/alginate/polypyrrole/graphene biocomposite hydrogel is a promising construct for cardiac tissue engineering.

Biocompatible polyurethanes with thermally-induced shape memory properties derived from three-arm branched poly(ε-caprolactone-co-γ-butyrolactone)-b-poly(lactide) block copolymers

A series of poly(lactide)-based polyurethanes with cross-linked network structure was synthesized through the chain extension of three-arm branched poly(ε-caprolactone-co-γ-butyrolactone)-b-poly(lactide) (3PBCL-b-PLA) block copolymers with various molecular weights and optical activities of PLA blocks. The structure of the synthesized poly(lactide)-based polyurethanes was analyzed, and it was confirmed that they contained the cross-linked network structure. Effects of the molecular weight and optical activity of PLA block on the various properties such as crystallization, mechanical property, shape memory performance, thermal stability and hydration ability were discussed. By simultaneously adjusting the molecular weight and optical activity of PLA blocks, the crystallization of flexible PCBL blocks and rigid PLA blocks in the synthesized cross-linked polyurethanes was effectively controlled, which was the basis for constructing poly(lactide)-based polyurethanes with a broad ranged mechanical property, excellent shape memory properties and controllable hydration ability. The synthesized polyurethanes exhibited adjusted tensile strength and elongation at break in the range of 11.2–32.4 MPa and 41–753 %, with the water content varying from 1.1 % to 20.1 %, respectively. The shape fixity ratio and shape recovery ratio of L-PU2.9 could reach 90.9 % and 96.0 %, respectively. The effect of stereocomplex-crystallization on the properties of poly(lactide)-based polyurethane was also analyzed. Stereocomplexed polyurethane based on 3PCL-b-PLA block copolymer enantiomers with short PLA block was constructed, which achieves fully stereocomplexation in a wide mixing ratio (20–50 %). It was found that the formation of a small number of stereocomplex-crystals can change the properties of poly(lactide)-based polyurethane in a wide range. Eventually, it was demonstrated by in vitro hemolysis test, cytotoxicity test and live/death staining that the synthetic poly(lactide)-based polyurethanes exhibited excellent biocompatibility. These results suggested that these biocompatible shape memory poly(lactide)-based polyurethanes with tunable mechanical properties and hydration ability demonstrated the promising potential as an excellent implantation for biomedical application.

Double-Network Organohydrogels Toughened by Solvent Exchange.

Double-network hydrogels based on calcium alginate are extensively exploited. Unfortunately, their low strength and unstable constitution to open environments limit their application potential. Herein, a new type of double-network organohydrogel (OHG) is proposed. By solvent exchange, a stable physical network is established based on dimethyl sulfoxide (DMSO)-alginate in the presence of a polyacrylamide network. The DMSO content endows tunable mechanical properties, with a maximum tensile strength of ≈1.7MPa. Importantly, the OHG shows much better environmental stability compared to the conventional double-network hydrogels. Due to the reversible association of hydrogen bonds, the OHG possesses some unique properties, including free-shapeability, shape-memory, and self-adhesion, that offers several promising ways to utilize alginate-based gels for wide applications.

Tunable mechanics of architectured composites from particle assemblies

Architectured materials with tunable mechanical properties and geometrical configurations hold significant promise for emulating biological organisms' behaviors, which have diverse applications in areas such as soft robotics and deployable structures. In contrast, existing architectured materials are largely restricted by the inherently fixed mechanical properties and geometries once fabricated. To address this challenge, we herein draw inspiration from the non-monolithic structures in biological organisms to design architectured composites via particle assemblies under confining stress. We utilize analytical, FEM and experimental methods to delve into the mechanical behaviors of the composite structure consisting of discrete rigid particles confined by soft prestressed ligaments. We investigate the effects of the particle geometries and the prestress in the ligaments on the mechanical tunability of our composite. Furthermore, we introduce the integration of electrothermal actuators, facilitating real-time modulation of mechanical responses. Our work aims to establish a fundamental understanding of mechanical behaviors of the architectured composites from particle assemblies confined by prestressed ligaments, which may lead to promising applications such as deployable structures and protective wearables.

Thermal fluctuations (eventually) unfold nanoscale origami

Origami is a scale invariant paradigm for morphing robotics, deployable structures (e.g. satellites, disaster relief shelters, medical stents), and metamaterials with tunable thermal, mechanical, or electromagnetic properties. There has been a resurgence of interest in using origami principles – along with 2D materials or DNA – to design a wide array of nanoscale devices. In this work, we take cognizance of the fact that small-scale devices are vulnerable to entropic thermal fluctuations and thus a foundational question underlying small-scale origami pertains to its stability, i.e. the origami structure’s propensity to “unfold” due to thermal fluctuations and the rate at which the unfolding will ensue. To properly understand the behavior of these origami-based nanodevices, we must simultaneously consider the geometric mechanics of origami along with the interplay between thermal fluctuations, entropic repulsive forces, van der Waals attraction, and other molecular-scale phenomena. In this work, to elucidate the rich behavior underpinning the evolution of an origami device at the nanoscale, we develop a minimal statistical mechanics model of folded nanoscale sheets. We use the model to investigate (1) the thermodynamic multistability of nanoscale origami structures and (2) the rate at which thermal fluctuations drive its unfolding—that is, its temporal stability. We identify, for the first time, an entropic torque that is a critical driving force for the unfolding process. Both the thermodynamic multistability and temporal stability have a nontrivial dependence on the origami’s bending stiffness, the radii of curvature of its creases, the ambient temperature, its thickness, and its interfacial energy (between folded layers). Specifically, for graphene, we show that there is a critical side length below which it can no longer be folded with stability; similarly, there exists a critical crease diameter, membrane thickness (e.g. for multilayer graphene), and temperature above which a crease cannot be stably folded. To investigate the rate of thermally driven unfolding, we extend Kramers’ escape rate theory to cases where the minima of the energy well occurs at a boundary. Rates of unfolding are found to span from effectively zero to instantaneous, and there is a clear interplay between temperature, geometry, and mechanical properties on the unfolding rate.

Composite Bioprinted Hydrogels Containing Porous Polymer Microparticles Provide Tailorable Mechanical Properties for 3D Cell Culture.

The mechanical and architectural properties of the three-dimensional (3D) tissue microenvironment can have large impacts on cellular behavior and phenotype, providing cells with specialized functions dependent on their location. This is especially apparent in macrophage biology where the function of tissue resident macrophages is highly specialized to their location. 3D bioprinting provides a convenient method of fabricating biomaterials that mimic specific tissue architectures. If these printable materials also possess tunable mechanical properties, they would be highly attractive for the study of macrophage behavior in different tissues. Currently, it is difficult to achieve mechanical tunability without sacrificing printability, scaffold porosity, and a loss in cell viability. Here, we have designed composite printable biomaterials composed of traditional hydrogels [nanofibrillar cellulose (cellulose) or methacrylated gelatin (gelMA)] mixed with porous polymeric high internal phase emulsion (polyHIPE) microparticles. By varying the ratio of polyHIPEs to hydrogel, we fabricate composite hydrogels that mimic the mechanical properties of the neural tissue (0.1-0.5 kPa), liver (1 kPa), lungs (5 kPa), and skin (10 kPa) while maintaining good levels of biocompatibility to a macrophage cell line.

A Promising Application of Injectable Hydrogels in Nerve Repair and Regeneration for Ischemic Stroke.

Ischemic stroke, a condition that often leads to severe nerve damage, induces complex pathological and physiological changes in nerve tissue. The mature central nervous system (CNS) lacks intrinsic regenerative capacity, resulting in a poor prognosis and long-term neurological impairments. There is no available therapy that can fully restore CNS functionality. However, the utilization of injectable hydrogels has emerged as a promising strategy for nerve repair and regeneration. Injectable hydrogels possess exceptional properties, such as biocompatibility, tunable mechanical properties, and the ability to provide a supportive environment for cell growth and tissue regeneration. Recently, various hydrogel-based tissue engineering approaches, including cell encapsulation, controlled release of therapeutic factors, and incorporation of bioactive molecules, have demonstrated great potential in the treatment of CNS injuries caused by ischemic stroke. This article aims to provide a comprehensive review of the application and development of injectable hydrogels for the treatment of ischemic stroke-induced CNS injuries, shedding light on their therapeutic prospects, challenges, recent advancements, and future directions. Additionally, it will discuss the underlying mechanisms involved in hydrogel-mediated nerve repair and regeneration, as well as the need for further preclinical and clinical studies to validate their efficacy and safety.

Blending recombinant amyloid silk proteins generates composite fibers with tunable mechanical properties

Blending diverse amyloid-silk proteins enables creation of mechanically programmable composite fibers and adjusting the blending ratio provides precise control over fiber mechanical behavior to specifically tailor them for diverse applications.

Injectable, self-healing and degradable dynamic hydrogels with tunable mechanical properties and stability by thermal-induced micellization.

Dynamic hydrogels possessing injectable, degradable and self-healing abilities have attracted considerable attention in the biomedical field in recent years, but it is difficult to tune the mechanical properties and stability of conventional dynamic hydrogels. In this work, we synthesized ABA-triblock copolymers via RAFT polymerization, where the A block consisted of thermo-sensitive poly(N-isopropylacrylamide-co-diacetone acrylamide) and the B block was hydrophilic poly(acrylamide). Subsequently, dynamic hydrogels were obtained based on the acylhydrazone bonds between the triblock copolymers and adipic acid dihydrazide (ADH). The obtained hydrogels exhibited injectable and self-healable abilities. In response to the thermal-induced micellization of their temperature-responsive blocks, the mechanical strength of the hydrogels not only increased, but also they exhibited high stability even at pH 2.0. Moreover, the hydrogel in the stable state could be degraded by the fracture of its trithiocarbonate groups. In addition, the hydrogels exhibited good cytocompatibility and controlled release behavior for doxorubicin (DOX). Considering these attractive tunable properties, these dynamic hydrogels show various potential applications in the biomedical field, such as drug carriers and cell or tissue engineering scaffolds.

Antibacterial polylysine-containing hydrogels for hemostatic and wound healing applications: preparation methods, current advances and future perspectives.

The treatment of various types of wounds such as dermal wounds, multidrug resistant bacteria-infected wounds, and chronic diabetic wounds is one of the critical challenges facing healthcare systems. Delayed wound healing can impose a remarkable burden on patients and health care professionals. In this case, given their unique three-dimensional porous structure, biocompatibility, high hydrophilicity, capability to provide a moist environment while absorbing wound exudate, permeability to both gas and oxygen, and tunable mechanical properties, hydrogels with antibacterial function are one of the most promising candidates for wound healing applications. Polylysine is a cationic polymer with the advantages of inherent antibacterial properties, biodegradability, and biocompatibility. Therefore, its utilization to engineer antibacterial hydrogels for accelerating wound healing is of great interest. In this review, we initially discuss polylysine properties, and then focus on the most recent advances in polylysine-containing hydrogels (since 2016) prepared using various chemical and physical crosslinking methods for hemostasis and wound healing applications. Finally, the challenges and future directions in the engineering of these antibacterial hydrogels for wound healing are discussed.

In-situ preparation of high oxygen content titanium via wire arc additive manufacturing with tunable mechanical properties

High oxygen content titanium (HOC-Ti) was manufactured in-situ via wire arc additive manufacturing (WAAM), and its compositional segregation, microstructure and mechanical properties were studied. The results show that the microstructure of the HOC-Ti component is mainly manifested as acicular α colonies. As oxygen content increases, the acicular α of the sample is broadened and the colonies become larger. Due to the strengthening of oxygen and the grain size effect, the strength change trend is from high to low, and the elongation is reduced significantly. The fracture mechanism is changed from ductile rupture to cleavage fracture. When the oxygen content is 0.37 wt.%, its elongation is about 7%, andthe ultimate tensile strength is more than 730 MPa, which is around double that of commercially pure titanium (CP-Ti).

3D printing confectionaries with tunable mechanical properties

3D printing confectionaries with tunable mechanical properties

Development of bioactive short fiber-reinforced printable hydrogels with tunable mechanical and osteogenic properties for bone repair.

Personalized bone-regenerative materials have attracted substantial interest in recent years. Modern clinical settings demand the use of engineered materials incorporating patient-derived cells, cytokines, antibodies, and biomarkers to enhance the process of regeneration. In this work, we formulated short microfiber-reinforced hydrogels with platelet-rich fibrin (PRF) to engineer implantable multi-material core-shell bone grafts. By employing 3D bioprinting technology, we fabricated a core-shell bone graft from a hybrid composite hydroxyapatite-coated poly(lactic acid) (PLA) fiber-reinforced methacryolyl gelatin (GelMA)/alginate hydrogel. The overall concept involves 3D bioprinting of long bone mimic microstructures that resemble a core-shell cancellous-cortical structure, with a stiffer shell and a softer core with our engineered biomaterial. We observed a significantly enhanced stiffness in the hydrogel scaffold incorporated with hydroxyapatite (HA)-coated PLA microfibers compared to the pristine hydrogel construct. Furthermore, HA non-coated PLA microfibers were mixed with PRF and GelMA/alginate hydrogel to introduce a slow release of growth factors which can further enhance cell maturation and differentiation. These patient-specific bone grafts deliver cytokines and growth factors with distinct spatiotemporal release profiles to enhance tissue regeneration. The biocompatible and bio-responsive bone mimetic core-shell multi-material structures enhance osteogenesis and can be customized to have materials at a specific location, geometry, and material combination.

Engineering mechanical compliance in polymers and composites for the design of smart flexible sensors.

Polymers are one of the most popular materials for next-generation flexible sensing device fabrication due to their tunable mechanical and electrical properties. A series of prior research studies in the field of smart flexible and wearable sensing illustrates the potential of various polymer and composite materials to be applied in sensor development. In this direction, mechanical compliance plays a vital role as it ensures the stability and reliability of the fabricated sensor. Therefore, engineering mechanical compliance for the development of smart flexible solutions has emerged as a significant area of research. Furthermore, the usage of flexible sensing devices is rapidly increasing in the field of healthcare devices and robotic automation. This feature article summarizes the relevant contributions of the authors in the field of engineered polymers and composites for flexible sensor development with a focus on healthcare and physical sensing applications. We discuss the polymer and composite materials, their characteristics, fabrication technologies, finite element method analysis, and examples of flexible physical sensors, i.e. pressure, strain, and temperature sensors, for various wearable healthcare applications and robotic automation. Finally, we discuss examples of multi-sensory systems having flexible sensors.

Smart materials for flexible electronics and devices: hydrogel.

In recent years, flexible conductive materials have attracted considerable attention for their potential use in flexible energy storage devices, touch panels, sensors, memristors, and other applications. The outstanding flexibility, electricity, and tunable mechanical properties of hydrogels make them ideal conductive materials for flexible electronic devices. Various synthetic strategies have been developed to produce conductive and environmentally friendly hydrogels for high-performance flexible electronics. In this review, we discuss the state-of-the-art applications of hydrogels in flexible electronics, such as energy storage, touch panels, memristor devices, and sensors like temperature, gas, humidity, chemical, strain, and textile sensors, and the latest synthesis methods of hydrogels. Describe the process of fabricating sensors as well. Finally, we discussed the challenges and future research avenues for flexible and portable electronic devices based on hydrogels.

A whole-course-repair system based on ROS/glucose stimuli-responsive EGCG release and tunable mechanical property for efficient treatment of chronic periodontitis in diabetic rats.

Elevated glucose levels, multiple pro-inflammatory cytokines and the generation of excessive reactive oxygen species (ROS) are pivotal characteristics within the microenvironments of chronic periodontitis with diabetes mellitus (CPDM). Control of inflammation and modulation of immune system are required in the initial phase of CPDM treatment, while late severe periodontitis requires a suitable scaffold to promote osteogenesis, rebuild periodontal tissue and reduce alveolar bone resorption. Herein, a whole-course-repair system is introduced by an injectable hydrogel using phenylboronic acid functionalized oxidized sodium alginate (OSA-PBA) and carboxymethyl chitosan (CMC). Epigallocatechin-3-gallate (EGCG) was loaded to simultaneously adjust the mechanical property of the OSA-PBA/CMC + EGCG hydrogel (OPCE). This hydrogel has distinctive adaptability, injectability, and ROS/glucose-triggered release of EGCG, making it an ideal drug delivery carrier. As expected, OPCE hydrogel shows favourable antioxidant and anti-inflammatory properties, along with a regulatory influence on the phenotypic transition of macrophages, providing a favourable immune microenvironment. Apart from that, it provides a favourable mechanical support for osteoblast/osteoclast differentiation regulation at the late proliferation stage of periodontal regeneration. The practical therapeutic effects of OPCE hydrogels were also confirmed when applied for treating periodontitis in diabetic rats. In summary, OPCE hydrogel may be a promising whole-course-repair system for the treatment of CPDM.

A "Mesh Scaffold" that regulates the mechanical properties and restricts the phase transition-induced volume change of the PNIPAM-based hydrogel for wearable sensors.

Poly(N-isopropylacrylamide) (PNIPAM) is capable of improving the reversibility and responsiveness of flexible electronics. However, its phase transition-induced volume variation and poor adhesiveness remain limitations for expending its applications. Herein, a pressure-sensitive adhesive (PSA), which is a type of mesh scaffold, is constructed inside the network of PNIPAM, providing the hydrogel with a constant volume in response to different temperatures, in situ tunable mechanical properties, and superior adhesiveness. The reversible density of the mesh scaffold adjusts the aggregation state of the hydrogel chains, whereupon it is capable of changing its mechanical modulus from 6.7 kPa to 45.3 kPa. This mechanical mechanism contributes to hydrogel-based flexible devices for multiple applications, especially in pressure-related sensors. The mesh scaffold restricts the phase-transition-induced volume variation, which allows the hydrogel sensor to stably monitor the external pressure at various temperatures. The high adhesion enables the effective interfacial interaction with the skin, avoiding the loss of sensing signals during the detection of human body movements. When it is assembled into an electronic device, it can transmit information and recognize sign language via Morse code. Thus, herein, we report a hydrogel sensor that is promising for pressure detection in temperature-unstable environments, especially for managing the health of patients who require emergency medical care through sign language recognition.

Alginate-CaCO3 hybrid colloidal hydrogel with tunable physicochemical properties for cell growth

Biomaterials composed of food polysaccharides are of great interest for future biomedical applications due to their great biocompatibility, tunable mechanical properties, and complex architectural designs that play a crucial role in the modulation of cell adhesion and proliferation. In this work, a facile approach was designed to obtain novel 3D alginate-CaCO3 hybrid hydrogel particles in situ. Controlling the gel concentration from 3 to 20 mg·mL−1 allows us to control the alginate-CaCO3 hydrogel particles' size and density (size variation from 1.86 to 2.34 mm and density from 1.22 to 1.29 mg/mm3). This variable also has a considerable influence on the mineralization process resulting in CaCO3 particles with varied sizes and amounts within the hydrogel beads. The measurements of Young's modulus showed that the inclusion of CaCO3 particles into the alginate hydrogel improved its mechanical properties, and Young's modulus of these hybrid hydrogel particles had a linear relationship with alginate content and hydrogel particle size. Cell experiments indicated that alginate-CaCO3 hybrid hydrogel particles can support osteoblastic cell proliferation and growth. In particular, the amount of hydroxyapatite deposition on the cell membrane significantly increased after the treatment of cells with hybrid hydrogel particles, up to 20-fold. This work offers a strategy for constructing inorganic particle-doped polysaccharide hybrid hydrogel scaffolds that provide the potential to support cell growth.

Engineering a mussel-inspired hemostatic sealant with a strong tissue anchor as a first-aid tissue adhesive

During injury, achieving quick hemostasis, while maintaining adhesive performances on wet tissue and providing appropriate dynamical mechanical properties remain crucial clinical requests for the current tissue adhesives. Here, we design a double-crosslinking tissue adhesive with tunable degradation rate, adhesion strength, mechanical properties, and cell responses, using thiolated gelatin (Gel-SH) or gelatin and tannic acid (TA). We also investigate the role of different concentrations of gelatin (70 and 150 mg/ml), TA (7, 20, and 70 mg/ml), and crosslinkers (sodium periodate (11.3 and 16.8 mg/ml) and Fe3+ ions (1.7 and 4.8 mg/ml)) to provide a balance of covalent and noncovalent interactions leading to establish muscular tissue adhesive strength and superior hemostasis. Our findings reveal that the Gel-SH(gelatin)/TA adhesives have a range of swelling ability (300–700 %), compressive modulus (0.2–1.2 kPa), strength (0.1–0.8 kPa), and toughness (10–15 kJ/m3). In addition, Gel-SH/TA sample shows rapid self-healing ability and offers robust adhesiveness to wet cow skin and cardiac tissue (50–70 kPa) compared to others mainly owing to the improved cohesive strength originating from the range of chemical interaction within the adhesive network and abundant phenolic hydroxyl groups, surface water removal, and dynamic phenol-quinone balance at the tissue interfaces. Meanwhile, Gel-SH/TA adhesive also exhibits high hemocompatibility and hemostatic properties and appropriate in vitro biocompatibility. Notably, Gel-SH/TA adhesive could support attachment, viability and proliferation, of H9c2 cells. Overall, the multifunctional Gel-SH/TA adhesive hydrogel, owing to the range of molecular interactions opens a new opportunity for quick sealing hemostasis and wound healing and could potentially be harnessed for the development of clinically relevant tissue adhesives.