Foam Template Research Articles

Highly stretchable porous composite hydrogels with stable conductivity for strain sensing

Stretchable hydrogels have undergone a rapid development in recent years particularly for wearable strain sensors. However, most of hydrogel-based sensors exhibit poor mechanical properties. Here, we report a highly stretchable porous composite hydrogel that has a tensile strain of 400% and a compressive strain of 80%. The porous hydrogel is composed of polyvinyl alcohol (PVA) and copolymer of N-(3-sulfopropyl)-N-methacroyloxyethyl-N,N-dimethylammonium betaine (SBMA) and acrylamide (AM) prepared via graphene oxide (GO)-stabilized Pickering aqueous foam templates. The pore morphology of the composite hydrogels was observed by field emission scanning electron microscope (FE-SEM). The tensile and compressive curves of porous hydrogels with different GO concentrations were characterized. GO can act as Pickering stabilizer to produce aqueous foams and also construct the physical cross-linking network in porous hydrogels to improve their mechanical strength. In addition, when GO is reduced by L-ascorbic acid, it provides effective electric pathways in porous hydrogels with the conductive percolation of less than 0.40 wt%. The composite hydrogels thus exhibit excellent strain sensitivity and can be used to detect various human motions. The facile preparation process of the porous composite hydrogels with outstanding performance properties facilitates the development of flexible wearable strain sensing materials.

Orientation-Adjustable Metal-Organic Framework Nanorods for Efficient Oxygen Evolution Reaction.

A series of orientation-adjustable metal-organic framework (MOF) nanorods, CoFe(dobpdc)-I to CoFe(dobpdc)-III (dobpdc = 4,4'-dihydroxybiphenyl-3,3'-dicarboxylate), is developed on a 3D nickel foam (NF) template. By modulating the solvent composition for synthesis, the feature of MOF nanorods on the template can be varied from disorganized to a unidirectional orientation perpendicular to the NF. Well-aligned, vertically oriented CoFe(dobpdc)-III nanorods are hydrophilic and have more exposed active sites and interfacial charge transfer ability. Consequently, they exhibit a superior activity for oxygen evolution reaction (OER) with ultralow overpotentials of 176 and 240 mV at 10 and 300 mA cm-2 in 1.0 M KOH (aq), respectively. CoFe(dobpdc)-III also shows a record low overpotential of 204 mV at J10 mA cm-2 among the electrocatalysts based on CoFe MOF and an excellent overpotential at a high current density (100 mA cm-2) of 312 mV in 0.1 M KOH (aq). This is the first report of a convenient method to straighten up MOF nanorods on a template for highly efficient OER.

Lead-free piezoelectric (Ba,Ca)(Ti,Zr)O3 scaffolds for enhanced antibacterial property

Scaffolds with particularly high antimicrobial properties based on (Ba, Ca)(Ti, Zr)O3 (BCTZ), a lead-free piezoelectric material, was fabricated using the polyurethane foam template replica method. The surface of the scaffolds was successfully coated with 13–93 bioactive glass (BG) using dip coating method. The developed scaffolds were polarized using corona poling unit. The charged surfaces of BCTZ scaffolds revealed good antibacterial response. After polarization, 13–93 BG coated BCTZ scaffold recorded 42%–49% reduction in the survival rate of Escherichia coli bacteria relative to the standard sample. Overall, such an approach can be suggested to improve the biocompatibility due to bioactive glass, prevent leaching of ions from BCTZ and provide a reasonably good antibacterial characteristic to the developed scaffolds.

A three-dimensional nerve guide conduit based on graphene foam/polycaprolactone

In this study, a novel nerve guide conduit was developed, based on a three-dimensional (3D) graphene conductive core grown, by chemical vapor deposition (CVD) coupled with a polycaprolactone (PCL) polymer coating. Firstly, the monolithic 3D-graphene foam (3D-GF) was synthesized on Ni foam templates via inductive heating CVD, subsequently, Ni/Graphene samples were dipped successively in PCL and cyclododecane (CDD) solutions prior to the removal of Ni from the 3D-GF/PCL scaffold in FeCl3. Our results showed that the electrical conductivity of the polymer composites reached to 25 S.m-1 after incorporation of 3D-GF. Moreover, the mechanical properties of 3D-GF/PCL composite scaffold were enhanced with respect to the same geometry of PCL scaffolds. The wettability, surface porosity, and morphology did not show any significant changes, while the PC12 cell proliferation and extension were increased for the developed 3D-GF/PCL nanocomposite. It can be concluded that 3D-GF/PCL nanocomposites could be good candidates to utilize as a versatile system for the engineering of peripheral nerve tissue.

A perspective: Is viscosity the key to open the next door for foam templating?

Macroporous polymers can be produced by curing liquid foam templates, which consist of air bubbles mechanically beaten into a liquid monomer phase. However, non-aqueous foam templates containing hydrophobic monomers as liquid phase are difficult to produce, as they cannot be stabilised by reducing their surface tension. In this perspective, we reviewed work on whipped foam templates with and without surfactant or particulate foaming agents. Once the liquid monomer phase has a suitable viscosity, froths with promising foam stability and foamability were produced. The average bubble size of the foam templates decreased with increasing viscosity of the liquid monomer phase. These foam templates were sufficiently stable to be polymerised to produce macroporous polymers with a porosity, average pore size and pore size distribution controlled by the viscosity of the monomer phase, rendering the polymer foams suitable for various potential applications. Therefore, it can be anticipated that solely the viscosity of the monomer phase is responsible to kinetically stabilise foam templates allowing for the production of macroporous polymers with well-defined properties. Tailoring the rheology of any monomer system is anticipated to be universally applicable to kinetically stabilise non-aqueous foam templates allowing for the production of useful macroporous polymers beyond the examples shown in this article, such as polyepoxides and polyacrylates. This will allow for further exploring the applicability of the foam templating technology to produce polymerizable liquid froths for printing and other shaping processes.

Monolithic Three-Dimensional Functionally Graded Hydrogels for Bioinspired Soft Robots Fabrication

Bioinspired soft robotics aims at reproducing the complex hierarchy and architecture of biological tissues within artificial systems to achieve the typical motility and adaptability of living organisms. The development of suitable fabrication approaches to produce monolithic bodies provided with embedded variable morphological and mechanical properties, typically encountered in nature, is still a technological challenge. Here we report on a novel manufacturing approach to produce three-dimensional functionally graded hydrogels (3D-FGHs) provided with a controlled porosity gradient conferring them variable stiffness. 3D-FGHs are fabricated by means of a custom-designed liquid foam templating (LFT) technique, which relies on the inclusion of air bubbles generated by a blowing agent into the monomer-based template solution during ultraviolet-induced photopolymerization. The 3D-FGHs' apparent Young's modulus ranges from 0.37 MPa (bulky hydrogel region) to 0.09 MPa (highest porosity region). A fish-shaped soft swimmer is fabricated to demonstrate the feasibility of the LFT technique to produce bioinspired robots. Mobility tests show a significant improvement in terms of swimming speed when the robot is provided with a graded body. The proposed manufacturing approach constitutes an enabling solution for the development of macroscopic functionally graded hydrogel-based structures usable in biomimetic underwater soft robotics applications.

Open-Cellular Alumina Foams with Hierarchical Strut Porosity by Ice Templating: A Thickening Agent Study.

Alumina replica foams were manufactured by the Schwartzwalder sponge replication technique and were provided with an additional strut porosity by a freeze-drying/ice-templating step prior to thermal processing. A variety of thickeners in combination with different alumina solid loads in the dispersion used for polyurethane foam template coating were studied. An additional strut porosity as generated by freeze-drying was found to be in the order of ~20%, and the spacings between the strut pores generated by ice-templating were in the range between 20 µm and 32 µm. In spite of the lamellar strut pore structure and a total porosity exceeding 90%, the compressive strength was found to be up to 1.3 MPa. Combining the replica process with freeze-drying proves to be a suitable method to enhance foams with respect to their surface area accessible for active coatings while preserving the advantageous flow properties of the cellular structure. A two-to-threefold object surface-to-object volume ratio of 55 to 77 mm−1 was achieved for samples with 30 vol% solid load compared to 26 mm−1 for non-freeze-dried samples. The freeze-drying technique allows the control of the proportion and properties of the introduced pores in an uncomplicated and predictable way by adjusting the process parameters. Nevertheless, the present article demonstrates that a suitable thickener in the dispersion used for the Schwartzwalder process is inevitable to obtain ceramic foams with sufficient mechanical strength due to the necessarily increased water content of the ceramic dispersion used for foam manufacturing.

Multifunctional porous SiC nanowire scaffolds

Porous SiC nanowire (SiCNW) ceramics have great potentials for engineering applications. We herein report the fabrication of 3D SiCNW scaffolds with tuneable microstructures, densities, and therefore properties, by regulating the solid loading content in the reticulated melamine foam (MF) template and finalizing with a modified carbothermal reaction. We first demonstrate the resulting samples exhibiting high strength (modulus up to ∼167.3 kPa), good recoverability (11% residual strain and 72% maximum stress after 100 compressive cycles at a ε = 20%), low thermal conductivity of 32−54 mW m−1 K−1 at room temperature, and good fire retardance performance. We further show that these unique and tuneable SiCNW scaffolds could act as efficient organic solvent/oil absorbent, as excellent support for MOF-derived TiO2-C catalyst to achieve enhanced photocatalytic performance, and as competent electromechanical strain sensors. These multifunctionalities could find niche applications in energy and environment devices.

Tailoring and visualising pore openings in gelatin-based hydrogel foams

HypothesisWhile tailoring the pore diameters in hydrogel foams has been demonstrated in numerous studies, fine control over the diameters of the pore openings is still a challenge. We hypothesise that this can be achieved by controlling the size of the thin films which separate the bubbles in the liquid foam template. If this is the case, systematic changes of the template’s gas fraction ϕ (the higher ϕ, the larger are the thin films) will lead to corresponding changes of the pore opening diameter. ExperimentsSince the size of the thin films depends on both bubble size 〈Db〉 and gas fraction ϕ, we need to decouple both parameters to control the film size. Thus, we generated foams with constant bubble sizes via microfluidics and adjusted the gas fractions via two different techniques. The foams were solidified using UV light. Subsequently, they were analysed with confocal fluorescence microscopy. FindingsWe were able to change the pore opening diameter 〈dp〉 at a constant pore diameter 〈Dp〉 by adjusting the gas fraction of the foam template. The obtained 〈dp〉/〈Dp〉 ratios are between those obtained theoretically for disordered foams and FCC ordered foams, respectively.

Fabrication of 3D monolithic graphene foam/polycaprolactone porous nanocomposites for bioapplications

Aiming at the production of light, porous, conductive, biosafe composites, in this paper we are presenting a novel fabrication method for monolithic, three-dimensional (3D) graphene foam (GF)/porous polymer composites. The synthesis adopts a novel process architecture by using Ni foam templates in an inductive heating chemical vapor deposition growth process, and by removing Ni chemically while retaining graphene integrity by the reversible application of cyclododecane (CD); finally, nondestructive coating procedures with polycaprolactone (PCL) solutions have been developed. The composites can be optimized to enhance electrical conduction, flexibility and mechanical properties, while mixing PCL and CD allows to coat the GF with a novel mesoporous polymer coating. By tuning the GF properties, the typical electrical resistance of the 3D forms can be reduced to a few 10 s of Ohms, values that are maintained after the PCL coatings. The current study achieved a GF fraction ranging between 1 and 7.3 wt%, with even the lower graphene content composites showing acceptable electrical and mechanical properties. The properties of these conductive 3D-GF/PCL composites are in line with the requirements for applications in the field of nerve tissue engineering.Graphical abstract

Fabrication and characterization of Ag- and Ga-doped mesoporous glass-coated scaffolds based on natural marine sponges with improved mechanical properties.

Natural marine sponges were used as sacrificial template for the fabrication of bioactive glass-based scaffolds. After sintering at 1050°C, the resulting samples were additionally coated with a silicate solution containing biologically active ions (Ag and Ga), well-known for their antibacterial properties. The produced scaffolds were characterized by superior mechanical properties (maximum compressive strength of 4MPa) and total porosity of ~80% in comparison to standard scaffolds made by using PU foam templates. Direct cell culture tests performed on the uncoated and coated samples showed positive results in terms of adhesion, proliferation, and differentiation of MC3T3-E1 cells. Moreover, vascular endothelial growth factor (VEGF) secretion from cells in contact with scaffold dissolution products was measured after 7 and 10 days of incubation, showing promising angiogenic results for bone tissue engineering applications. The antibacterial potential of the produced samples was assessed by performing agar diffusion tests against both Gram-positive and Gram-negative bacteria.

Removal of antibiotics from aqueous solution by using porous adsorbent templated from eco-friendly Pickering aqueous foams

The aqueous foam template without any solvent and only using the particles stabilizer has attracted much attention for preparation of the porous adsorbents. Herein, a novel porous adsorbent was fabricated via thermal-initiated polymerization of Pickering aqueous foams, which was stabilized by the natural sepiolite (Sep) and pine pollen, and utilized for the removal of antibiotic from aqueous solution. The stabilizing mechanism of Pickering aqueous foam of that the Sep was modified with the leaching substance from pine pollen and arranged orderly around the bubble to form a dense “shell” structure was revealed. The adsorbents possessed the hierarchical porous structure and excellent adsorption performance for antibiotic of chlorotetracycline hydrochloride (CTC) and tetracycline hydrochloride (TC). The equilibrium adsorption capacities of CTC and TC were achieved with 465.59 and 330.59 mg/g within 60 min at 25°C, respectively. The adsorption process obeyed Langmuir model and pseudo-second-order adsorption kinetic model. This work provided eco-friendly approach for fabricate porous adsorbents for wastewater treatment.

Creation of Bony Microenvironment with Extracellular Matrix Doped-Bioactive Ceramics to Enhance Osteoblast Behavior and Delivery of Aspartic Acid-Modified BMP-2 Peptides.

IntroductionDecellularized matrix from porcine small intestinal submucosa (SIS) endows scaffolds with an ECM-like surface, which enhances stem cell self-renewal, proliferation, and differentiation. Mesoporous bioactive glass (MBG) is extensively recognized as an excellent bio-ceramic for fabricating bone grafts.Materials and MethodsIn the current study, SIS was doped on an MBG scaffold (MBG/SIS) using polyurethane foam templating and polydopamine chemistry method. To mimic the bony environment of a natural bone matrix, an ECM-inspired delivery system was constructed by coupling the BMP2-related peptide P28 to a heparinized MBG/SIS scaffold (MBG/SIS-H-P28). The release of P28 from MBG/SIS-H-P28 and its effects on the proliferation, viability, and osteogenic differentiation of bone marrow stromal stem cells were investigated in vitro and in vivo.ResultsOur research indicated that the novel tissue-derived ECM scaffold MBG/SIS has a hierarchical and interconnected porous architecture, and superior biomechanical properties. MBG/SIS-H-P28 released P28 in a controlled manner, with the long-term release time of 40 d. The results of in vitro experiments showed improvements in cell proliferation, cell viability, alkaline phosphatase activity, and mRNA expression levels of osteogenesis-related genes (Runx-2, OCN, OPN, and ALP) compared to those of MBG/SIS or MBG/SIS-P28 and MBG/SIS-H-P28. The in vivo results demonstrated that MBG/SIS-H-P28 scaffolds evidently increased bone formation in rat calvarial critical-sized defect compared to that in controls.ConclusionMBG/SIS-H-P28 scaffolds show potential as ideal platforms for delivery of P28 and for providing a bony environment for bone regeneration.

Synthesis and thermal transport properties of high-surface area hexagonal boron nitride foam structures

Continuous and porous foam structures of thermally conducting and electrically insulating hexagonal boron nitride (h-BN) have been synthesized for heat dissipation and other applications. However, the volume fraction and effective thermal conductivity of existing h-BN foam structures are still limited by the large pore size and small specific surface area of the sacrificial reticular nickel foam templates used for chemical vapor deposition (CVD) of h-BN. Here we report experiments for increasing the volume fraction and effective thermal conductivity of CVD h-BN foams with the use of sintered nickel powder templates with reduced pore sizes. The volume fraction of the obtained high-surface area h-BN foam sample HS1 reaches 0.34 ± 0.06 %, which is an increase by a factor of 2.8 compared to a baseline h-BN foam grown on a reticulated nickel foam with the same growth time. Increasing the growth time by a factor of 3 results in a further increase of the volume fraction by an additional factor of 2 for sample HS2. With poly(methyl methacrylate) (PMMA) filled into the pore space of the h-BN foam, the room-temperature effective thermal conductivity of the composite increases from 0.31 ± 0.02 W m−1 K−1 for the baseline structure to 0.70 ± 0.05 W m−1 K−1 for HS2. Based on numerical analysis of the measured effective thermal conductivity, the solid thermal conductivity of the h-BN struts of the baseline structure is 580 ± 150 W m−1 K−1, which is comparable to a prior first principles calculation. The increases in the volume fraction and effective thermal conductivity are accompanied by a reduction of the h-BN solid thermal conductivity to 410 ± 80 W m−1 K−1 for HS1 and 250 ± 30 W m−1 K−1 for HS2 due to increased defect concentrations and surface curvature. The numerical simulation reveals that the thermal interface resistance between h-BN and PMMA plays only a small role on the effective thermal conductivity due to the dominant thermal transport contribution from the continuous high-thermal conductivity struts.

Preceramic polymer-derived ceramic foams with lamellar strut porosity

Open porous replica foams with additional lamellar strut pores were prepared by preceramic polymer processing combined with a freeze-drying process. Polymer foam templates were coated with an aqueous polymethylsilesquioxane system followed by freezing at - 20 °C to generate hierarchical porosity within the foam struts. The cellular structure was preserved during freeze drying, cross-linking, template removal and pyrolysis, and additional strut porosity was formed. The size of the lamellar strut pores was found to depend on the solid load of the preceramic slurry and its pretreatment procedure. The cellular materials obtained in this work possess additional lamellar strut pore channels with a thickness of approximately 40 µm and a total foam porosity of > 90 %.

Polyfoam: Foam-Templated Microcellular Polymers.

High internal phase emulsion (HIPE) templating has been developed as a robust method for the synthesis of porous polymers with designed porosity and void size. However, it has low starting material efficiency, requiring the removal of over 74% of the starting material to form interconnected porous structures. Foam templating methods attempt to resolve this issue by replacing the internal liquid phase with a gas phase. The current challenge for foam templating is to reach small void sizes (<100 μm), especially when using free-radical polymerization. We use a rapid gas dispersion method to form foam-templated hydrogels, which is an energy-effective method in comparison to the typical HIPE templating approach. We report hydrogels with an average void diameter of 63 μm at 70% porosity. The morphology and properties of foam-templated hydrogels are evaluated to show comparability to the HIPE-templated hydrogels of the same material.

Metal-Organic Complexes@Melamine Foam Template Strategy to Prepare Three-Dimensional Porous Carbon with Hollow Spheres Structures for Efficient Organic Vapor and Small Molecule Gas Adsorption.

Three-dimensional (3D) porous carbon materials have received substantial attention owing to their unique structural features. However, the synthesis of 3D porous carbon, especially 3D porous carbon with hollow spheres structures at the connection points, still pose challenges. Herein, we first develop a metal-organic complexes@melamine foam (MOC@MF) template strategy, by using hot-pressing and carbonization method to synthesize 3D porous carbon with hollow spheres structures (denoted as NOPCs). The formation mechanism of NOPCs can be attributed to the difference in Laplace pressure and surface energy gradient between the carbonized MOC and carbonized MF. These rare 3D porous carbons exhibit high BET surface area (2453.8 m2 g-1), N contents (10.5%), and O contents (16.3%). Moreover, NOPCs show significant amounts of toluene and methanol at room temperature, reaching as high as 1360 and 1140 mg g-1. The adsorption amounts of SO2 and CO2 for NOPCs are up to 93.1 and 445 mg g-1. Theoretical calculation indicates surfaces of porous carbon with N and O coexistence could strongly enhance adsorption with high adsorption energy of -65.83 kJ mol g-1.

Oleogel Fabrication Based on Sodium Caseinate, Hydroxypropyl Methylcellulose, and Beeswax: Effect of Concentration, Oleogelation Method, and Their Optimization

AbstractIn the present study, the oleogel preparation with hydroxypropyl methylcellulose (HPMC) (0–2 g/100 g), sodium caseinate (CN) (0–4 g/100 g), beeswax (0–5 g/100 g), and oleogelation method (foam and emulsion template) was optimized using response surface methodology (RSM) to attain the desirable oil retaining ability, rheological, and textural characteristics. For all the chosen responses, the quadratic model was the best‐fitting model with a determination coefficient of R2 &gt; 0.91. Results exhibited that the HPMC and CN concentrations were the most influential tested factors on the oil binding capacity, textural, and rheological characteristics of the oleogels due to the formation of more complex and strong network. There was a significant improvement in oil binding capacity and structure recovery of samples by beeswax addition. To produce oleogel similar to industrial shortening, the optimization method was done based on maximum oil binding capacity and thixotropic recovery and other responses were chosen equal to those of shortening values (ɳa = 330 Pa.s, G′ = 276,543 Pa, A value = 164,308 Pa s rad−1, and firmness = 44.99 g). As regards the optimized level of structuring agents and responses (ɳa = 317 and 329 Pa.s, G′ = 249,782 and 260,997 Pa, A value = 180,022 and 180,373 Pa s rad−1, and firmness = 44.37 and 36.98 g corresponding to Optimization 1 and Optimization 2, respectively), fabrication of oleogels with at least 90 g/100 g trans‐free and low saturated oil and attributes close to industrial shortening is possible.

Synthesis of Graphene Foam Filter by Chemical Vapor Deposition Using Palm Oil as a Carbon Source for PM2.5 Capture

Particulate matter which has aerodynamic diameter less than 2.5 µm (PM2.5) has seriously impacted on human health such as cardio-respiratory diseases. Graphene foam is a 3-dimensional graphene, in which carbon atoms are arranged into a honeycomb lattice. In this study, PM2.5 capture using graphene foam filter is demonstrated. The graphene foam is synthesized by chemical vapor deposition using nickel foam and palm oil as a template and carbon source, respectively. After that the sample is soaked with FeCl3 to dissolve the nickel foam template. The presence of graphene on nickel foam is confirmed by Raman spectroscopy. The morphologies of graphene and PM2.5 particles are measured by field emission scanning electron microscope. The element composition of PM2.5 is investigated by energy-dispersive X-ray spectroscopy (EDX). The results show that graphene foam can capture various types of PM2.5 particles (such as aluminium, silicon, phosphorus and so on) without additional energy.

Graphene Oxide Modified Porous Graphene for Aqueous Alcohol Detection

Nowadays, more and more people are paying attention to healthcare monitoring. The graphene-based sensors with the flexibility, low cost, and high performance display great potential applications in the field of gas detection due to the exceptional physiochemical properties resulting from the atomic structure. Herein, we have prepared the graphene oxide modified porous graphene sensor for the aqueous alcohol detection. Via the combination of nickel foam template, chemical vapor deposition, and chemical dealloying, the porous graphene network can be obtained for surface modification. Under different aqueous alcohol concentrations, gas sensing ability has been investigated. The sensor shows a sensitivity of 0.087% per unit weight and fast response time of 195 s. Moreover, the absorption and desorption of the water and alcohol molecules on the graphene surface are discussed. Because alcohol detection in the moisture atmosphere shows a similar scenario for the practical application, it would be a promising candidate for drink monitoring.