Ultrasensitive Wearable Strain Sensors of 3D Printing Tough and Conductive Hydrogels

3D printing of Rg3-loaded hydrogel scaffolds: anti-inflammatory and scar-formation related collagen inhibitory effects for scar-free wound healing

During the process of wound healing, the stimulation of inflammatory factors often leads to abnormal proliferation of blood vessels and collagen, ultimately resulting in scar formation. To address this challenge, we fabricate a novel dermal extracellular matrix (DECM) hydrogel scaffold loaded with ginsenoside Rg3 (Rg3) using 3D printing technology. Mesoporous silica nanoparticles (MSNs) are introduced into the system to encase the Rg3 to control its release rate and enhance its bioavailability. We systematically evaluate the biological, physicochemical, and wound healing properties of this scaffold. In vitro studies demonstrate that the hydrogel exhibits excellent biocompatibility and solid-like rheological properties, ensuring its successful printing. In vivo studies reveal that the composite hydrogel scaffolds effectively accelerate wound healing and achieve scar-free wound healing within three weeks. Histological and immunohistochemical (IHC) analyses show that the composite hydrogel scaffolds reduce the inflammatory response and inhibit excessive collagen accumulation. These combined effects underscore the potential of our approach in effectively inhibiting scar formation.

Self-Healable and 4D Printable Hydrogel for Stretchable Electronics

Materials with high stretchability and conductivity are used to fabricate stretchable electronics. Self-healing capability and four-dimensional (4D) printability are becoming increasingly important for these materials to facilitate their recovery from damage and endow them with stimuli-response properties. However, it remains challenging to design a single material that combines these four strengths. Here, a dually crosslinked hydrogel is developed by combining a covalently crosslinked acrylic acid (AAC) network and Fe3+ ions through dynamic and reversible ionically crosslinked coordination. The remarkable electrical sensitivity (a gauge factor of 3.93 under a strain of 1500%), superior stretchability (a fracture strain up to 1700%), self-healing ability (a healing efficiency of 88% and 97% for the mechanical and electrical properties, respectively), and 4D printability of the hydrogel are demonstrated by constructing a strain sensor, a two-dimensional touch panel, and shape-morphing structures with water-responsive behavior. The hydrogel demonstrates vast potential for applications in stretchable electronics.

Temperature-Controlled 3D Cryoprinting Inks Made of Mixtures of Alginate and Agar

Temperature-controlled 3D cryoprinting (TCC) is an emerging tissue engineering technology aimed at overcoming limitations of conventional 3D printing for large organs: (a) size constraints due to low print rigidity and (b) the preservation of living cells during printing and subsequent tissue storage. TCC addresses these challenges by freezing each printed voxel with controlled cooling rates during deposition. This generates a rigid structure upon printing and ensures cell cryopreservation as an integral part of the process. Previous studies used alginate-based ink, which has limitations: (a) low diffusivity of the CaCl2 crosslinker during TCC's crosslinking process and (b) typical loss of print fidelity with alginate ink. This study explores the use of an ink made of agar and alginate to overcome TCC protocol limitations. When an agar/alginate voxel is deposited, agar first gels at above-freezing temperatures, capturing the desired structure without compromising fidelity, while alginate remains uncrosslinked. During subsequent freezing, both frozen agar and alginate maintain the structure. However, agar gel loses its gel form and water-retaining ability. In TCC, alginate crosslinking occurs by immersing the frozen structure in a warm crosslinking bath. This enables CaCl2 diffusion into the crosslinked alginate congruent with the melting process. Melted agar domains, with reduced water-binding ability, enhance crosslinker diffusivity, reducing TCC procedure duration. Additionally, agar overcomes the typical fidelity loss associated with alginate ink printing.

Development of an Ultrastretchable Double-Network Hydrogel for Flexible Strain Sensors

The weak mechanical properties of hydrogels due to the inefficient dissipation of energy in the intrinsic structures limit their practical applications. Here, a double-network (DN) hydrogel has been developed by integrating an ionically cross-linked agar network, a covalently cross-linked acrylic acid (AAC) network, and the dynamic and reversible ionically cross-linked coordination between the AAC chains and Fe³⁺ ions. The proposed model reveals the mechanisms of the improved mechanical performances in the DN agar/AAC-Fe³⁺ hydrogel. The hydrogen-bond cross-linked double helices of agar and ionic-coordination interactions of AAC-Fe³⁺ can be temporarily sacrificed during large deformation to readily dissipate the energy, whereas the reversible AAC-Fe³⁺ interactions can be regenerated after stress relief, which greatly increases the material toughness. The developed DN hydrogel demonstrates a remarkable stretchability with a break strain up to 3174.3%, high strain sensitivity with the gauge factor being 0.83 under a strain of 1000%, and good 3D printability, making the material a desirable candidate for fabricating flexible strain sensors, electronic skin, and soft robots.

A Review of Conductive Hydrogel Used in Flexible Strain Sensor

Hydrogels, as classic soft materials, are important materials for tissue engineering and biosensing with unique properties, such as good biocompatibility, high stretchability, strong adhesion, excellent self-healing, and self-recovery. Conductive hydrogels possess the additional property of conductivity, which endows them with advanced applications in actuating devices, biomedicine, and sensing. In this review, we provide an overview of the recent development of conductive hydrogels in the field of strain sensors, with particular focus on the types of conductive fillers, including ionic conductors, conducting nanomaterials, and conductive polymers. The synthetic methods of such conductive hydrogel materials and their physical and chemical properties are highlighted. At last, challenges and future perspectives of conductive hydrogels applied in flexible strain sensors are discussed.

Shape-Adaptive Metastructures with Variable Bandgap Regions by 4D Printing

This article shows how four-dimensional (4D) printing technology can engineer adaptive metastructures that exploit resonating self-bending elements to filter vibrational and acoustic noises and change filtering ranges. Fused deposition modeling (FDM) is implemented to fabricate temperature-responsive shape-memory polymer (SMP) elements with self-bending features. Experiments are conducted to reveal how the speed of the 4D printer head can affect functionally graded prestrain regime, shape recovery and self-bending characteristics of the active elements. A 3D constitutive model, along with an in-house finite element (FE) method, is developed to replicate the shape recovery and self-bending of SMP beams 4D-printed at different speeds. Furthermore, a simple approach of prestrain modeling is introduced into the commercial FE software package to simulate material tailoring and self-bending mechanism. The accuracy of the straightforward FE approach is validated against experimental observations and computational results from the in-house FE MATLAB-based code. Two periodic architected temperature-sensitive metastructures with adaptive dynamical characteristics are proposed to use bandgap engineering to forbid specific frequencies from propagating through the material. The developed computational tool is finally implemented to numerically examine how bandgap size and frequency range can be controlled and broadened. It is found out that the size and frequency range of the bandgaps are linked to changes in the geometry of self-bending elements printed at different speeds. This research is likely to advance the state-of-the-art 4D printing and unlock potentials in the design of functional metastructures for a broad range of applications in acoustic and structural engineering, including sound wave filters and waveguides.