Porous Sponge Research Articles

Endogenous Tissue Engineering for Chondral and Osteochondral Regeneration: Strategies and Mechanisms.

Increasing attention has been paid to the development of effective strategies for articular cartilage (AC) and osteochondral (OC) regeneration due to their limited self-reparative capacities and the shortage of timely and appropriate clinical treatments. Traditional cell-dependent tissue engineering faces various challenges such as restricted cell sources, phenotypic alterations, and immune rejection. In contrast, endogenous tissue engineering represents a promising alternative, leveraging acellular biomaterials to guide endogenous cells to the injury site and stimulate their intrinsic regenerative potential. This review provides a comprehensive overview of recent advancements in endogenous tissue engineering strategies for AC and OC regeneration, with a focus on the tissue engineering triad comprising endogenous stem/progenitor cells (ESPCs), scaffolds, and biomolecules. Multiple types of ESPCs present within the AC and OC microenvironment, including bone marrow-derived mesenchymal stem cells (BMSCs), adipose-derived mesenchymal stem cells (AD-MSCs), synovial membrane-derived mesenchymal stem cells (SM-MSCs), and AC-derived stem/progenitor cells (CSPCs), exhibit the ability to migrate toward injury sites and demonstrate pro-regenerative properties. The fabrication and characteristics of scaffolds in various formats including hydrogels, porous sponges, electrospun fibers, particles, films, multilayer scaffolds, bioceramics, and bioglass, highlighting their suitability for AC and OC repair, are systemically summarized. Furthermore, the review emphasizes the pivotal role of biomolecules in facilitating ESPCs migration, adhesion, chondrogenesis, osteogenesis, as well as regulating inflammation, aging, and hypertrophy-critical processes for endogenous AC and OC regeneration. Insights into the applications of endogenous tissue engineering strategies for in vivo AC and OC regeneration are provided along with a discussion on future perspectives to enhance regenerative outcomes.

Flexible dual-mode pressure sensor for simultaneous dynamic-static sensing with wide-range detection and ultra-fast response speed

Flexible pressure sensors play a vital role in advancing the Internet of Things (IoT). However, piezoelectric sensors are limited in static pressure detection and suffer in achieving high flexibility and high sensitivity simultaneously, while piezoresistive sensors typically have slow response speeds and insensitive high-frequency detection ability. Here, we select a novel basalt fiber fabric (BFF) that can withstand high temperatures exceeding 1000 °C as a new substrate to enable one-step fabrication of piezoelectric layer based on coherently grown Pb(Zr0.52Ti0.48)O3 (PZT) thin films via a simple dipping method. The high piezoelectric coefficient g33 of 362.5 mV·m·N−1 and the unique core-sheath structure induced stress concentration on the PZT sheath enable the flexible piezoelectric layer (bending radius < 1.45 mm) to exhibit high sensitivity (voltage sensitivity of 2.766 V/kPa within the range of 0.11–11.11 kPa) and fast response speed (11 ms). The piezoresistive layer consisting of a three-dimensional porous elastic carbon nanotube-polyurethane sponge provides a detection range comparable to piezoelectric layer (0.11–66.67 kPa). A dual-mode flexible pressure sensor (DMFS) has been developed by integrating the piezoelectric layer with the piezoresistive layer. The DMFS exhibits an extended detection range compared to previous dual-mode pressure sensors, rapid response speed, and high sensitivity, which enables simultaneous detection of both dynamic and static pressures. The DMFS can be used for keyboard and movement posture recognition, which has significant application prospects in human-machine interaction.

Adjusting Ion Diffusion Kinetics of Li Deposition Enabled by an Elastic Porous Melamine Sponge Host for Stable Lithium Metal Anodes.

Lithium (Li) dendritic growth and huge volume expansion seriously hamper Li-metal anode development. Herein, we design a lightweight 3D Li-ion-affinity host enabled by silver (Ag) nanoparticles fully decorating a porous melamine sponge (Ag@PMS) for dendrite-free and high-areal-capacity Li anodes. The compact Ag nanoparticles provide abundant preferred nucleation sites and give the host strong conductivity. Moreover, the high specific surface area and polar groups of the elastic, porous melamine sponge enhance the Li-ion diffusion kinetics, prompting homogeneity of Li deposition and stripping. As expected, the integrated 3D Ag@PMS-Li anode delivered a remarkable electrochemical performance, with a Coulombic efficiency (CE) of 97.14% after 450 cycles at 1 mA cm-2. The symmetric cell showed an ultralong lifespan of 3400 h at 1 mA cm-2 for 1 mAh cm-2. This study provides a facile and cost-effective strategy to design an advanced 3D framework for the preparation of a stable dendrite-free Li metal anode.

3D Printed Porous Okara-Based Sponge with Hemostatic and Antimicrobial Effects for Wound Healing

3D Printed Porous Okara-Based Sponge with Hemostatic and Antimicrobial Effects for Wound Healing

Catalytic Dechlorination of Three Organochlorides by Recyclable Nano-Palladium-Engineered Natural Sponge with Formic Acid

Catalytic dechlorination of organic chlorides by palladium (Pd) with HCOOH represents one of the most effective and promising techniques for environmental remediation. In this study, we adopted alkaline-modified porous natural sponge as support of a Pd nanocatalyst (Pd@M-Sponge) and HCOOH as a hydrogen source for the hydrodechlorination of florfenicol (FF), o-chlorophenol (o-CP), and p-chlorophenol (p-CP). Favorable conversion efficiency of FF, o-CP, and p-CP was achieved at 25 °C and atmospheric pressure attributed to the small diameter and high catalytic reactivity of the prepared Pd NPs, in addition to the slight internal mass transfer limitation of the prepared Pd@M-Sponge. High reaction rate constants were obtained even in the conditions of a low molar ratio of HCOOH to p-CP (10:1) and a high concentration of p-CP (500 mg/L). The prepared catalyst also demonstrated superior recyclability without any obvious decrease in catalytic reactivity in 20 successive p-CP dechlorination cycles. This work provides an ideal recyclable and cost-effective catalyst based on renewable and biocompatible natural material for the catalytic hydrodechlorination of chlorinated organic pollutants with formic acid and a new view for the exploration and designing of highly reactive and stable catalysts for hydrodechlorination.

Green Ironmaking at Higher H2 Pressure: Reduction Kinetics and Microstructure Formation During Hydrogen-Based Direct Reduction of Hematite Pellets

Hydrogen-based direct reduction (HyDR) of iron ores has attracted immense attention and is considered a forerunner technology for sustainable ironmaking. It has a high potential to mitigate CO2 emissions in the steel industry, which accounts today for ~ 8–10% of all global CO2 emissions. Direct reduction produces highly porous sponge iron via natural-gas-based or gasified-coal-based reducing agents that contain hydrogen and organic molecules. Commercial technologies usually operate at elevated pressure, e.g., the MIDREX process at 2 bar and the HyL/Energiron process at 6–8 bar. However, the impact of H2 pressure on reduction kinetics and microstructure evolution of hematite pellets during hydrogen-based direct reduction has not been well understood. Here, we present a study about the influence of H2 pressure on the reduction kinetics of hematite pellets with pure H2 at 700 °C at various pressures, i.e., 1, 10, and 100 bar under static gas exposure, and 1.3 and 50 bar under dynamic gas exposure. The microstructure of the reduced pellets was characterized by combining X-ray diffraction and scanning electron microscopy equipped with electron backscatter diffraction. The results provide new insights into the critical role of H2 pressure in the hydrogen-based direct reduction process and establish a direction for future furnace design and process optimization.Graphical

High-Performance Multifunctional Carbon Fibrous Sponges Derived from Pitch.

3D carbon-based porous sponges are recognized for significant potential in oil absorption and electromagnetic interference (EMI). However, their widespread application is hindered by a common compromise between high performance and affordability of mass production. Herein, a novel approach is introduced that involves laser-assisted micro-zone heating melt-blown spinning (LMHMS) to address this challenge by creating pitch-based submicron carbon fibers (PSCFs) sponge with 3D interconnected structures. These structures bestow the resulting sponge exceptional characteristics including low density (≈20mgcm-3), high porosity (≈99%), remarkable compressibility (80% maximum strain), and superior conductivity (≈628 Sm-1). The resultant PSCF sponges realize an oil/organic solvent sorption capacity over 56g/g and possess remarkable regenerated ability. In addition to their effectiveness in cleaning up oil/organic solvent spills, they also demonstrated strong electromagnetic shielding capabilities, with a total shielding effectiveness (SE) exceeding 60dB across the X-band GHz range. In virtue of extreme lightweight of ≈20mgcm-3, the specific SE of the PSCF sponge reaches as high as ≈1466dB cm3g-1, surpassing the performance of numerous carbon-based porous structures. Thus, the unique blend of properties renders these sponges promising for transforming strategies in addressing oil/organic solvent contaminations and providing effective protection against EMI.

Salt-resistant hierarchically porous wood sponge coated with graphene flake/polyaniline nanocomposite for interfacial solar steam production and wastewater treatment

This research presents an innovative approach for developing a flexible wood sponge (WS) with a hierarchically porous structure. This unique structure is achieved through a sequential process involving balsa wood delignification, freeze-drying, and subsequent coating with graphene flake (GF)/polyaniline (PANI) nanocomposite. The resulting GF/PANI nanocomposite significantly reduces electron transfer resistivity, thereby enhancing heat generation for water evaporation. This results in self-cleaning photoabsorber, benefiting from GF’s salt-rejecting properties, PANI’s ionic network, and the WS’s porous structure. The photoabsorber demonstrates improved mechanical strength, reduced thermal conductivity, and a single water route design atop a drilled insulator foam, effectively minimizing heat loss during solar desalination. Under 1 sun (1 sun = 1 kW m−2) illumination, the photoabsorber achieves an impressive evaporation flux of 1.49 kg m−2h−1 and a high solar to thermal efficiency of 95.51 %. Importantly, continuous 10-cycle testing under 1 sun illumination reveals no salt deposition on the surface. Furthermore, the photoabsorber demonstrates promising applications in wastewater treatment, effectively purifying dye-contaminated seawater and desalinating both acidic and alkaline seawater. The investigation into the GF/PANI nanocomposite’s effect on steam generation, conducted through electrochemical impedance spectroscopy in a 0.5 M Na2SO4 electrolyte, reveals enhanced interfacial charge transfer, surpassing both PANI and GF due to reduced electrochemical resistance. Evaluation of desalinated seawater and purified wastewater demonstrates a significant decrease in major cation concentrations, meeting WHO and EPA drinking water standards. These findings underscore the potential of the GF/PANI nanocomposite in superior steam generation applications.

Novel Janus polyurethane sponges with asymmetric wettability enabling all-weather stable and efficient remediation of crude oil spills in harsh environments

Developing high-performance adsorbent materials featuring with all-weather service and high sorption efficiency to recover viscous crude oil spills in harsh environments is urgently required, yet it still remains an onerous challenge. Herein, a prominent Janus polydimethylsiloxane (PDMS) & Ti3C2TX MXene @ polyurethane sponge (JPMPU) was exploited via a convenient anisotropic successive depositing technology. Thanks to controllable asymmetric decoration of Ti3C2Tx MXene/PDMS and hydrophilic components on two antithetic sides, the resulting sponges exhibited a fascinating asymmetric superwettability, rendering JPMPUs superb interfacial floating stability and overturning-resistance property. Moreover, Janus sponges displayed outstanding photothermal and Joule-heating effects. Particularly, ultrahigh temperatures of 99.5 °C and 102.1 °C were acquired for JPMPU under the sunlight intensity of 1.5 kW m−2 and the applied voltage of 10 V, respectively. On the basis of these favorable features, JPMPUs exhibited a remarkable in situ sorption capability for high-viscous crude oil spills, which can absorb >16.1 and 19.0 times its own weight of crude oil driven by the solar heat and Joule heat, respectively. Consequently, together with robust recyclability, this 3D porous Janus thermal management sponge with superior self-heating integration functionality and robust self-floating stability shows great potential for fast all-weather crude oil recovery in harsh environments.

Topical application of a hyaluronic acid-based hydrogel integrated with secretome of human mesenchymal stromal cells for diabetic ulcer repair

This preclinical proof-of-concept study aimed to evaluate the effectiveness of secretome therapy in diabetic mice with pressure ulcers. We utilized a custom-made hyaluronic acid (HA)-based porous sponge, which was rehydrated either with normal culture medium or secretome derived from human mesenchymal stromal cells (MSCs) to achieve a hydrogel consistency. Following application onto skin ulcers, both the hydrogel-only and the hydrogel + secretome combination accelerated wound closure compared to the vehicle group. Notably, the presence of secretome significantly enhanced the healing effect of the hydrogel, as evidenced by a thicker epidermis and increased revascularization of the healed area compared to the vehicle group. Notably, molecular analysis of healed skin revealed significant downregulation of genes involved in delayed wound healing and abnormal inflammatory response in ulcers treated with the hydrogel + secretome combination, compared to those treated with the hydrogel only. Additionally, we found no significant differences in therapeutic outcomes when comparing the use of secretome from fetal dermal MSCs to that from umbilical cord MSCs. This observation is supported by the proteomic profile of the two secretomes, which suggests a shared molecular signature responsible of the observed therapeutic effects.

Mild preparation of hyaluronic acid/silk fibroin sponges by modified crosslinking method

The composites composed of hyaluronic acid (HA) and silk fibroin (SF) exhibit great potential in diverse biomedical applications. However, the utilization of commercial crosslinkers such as 1,4-butanediol diglycidyl ether (BDDE) for crosslinking HA typically necessitates harsh conditions involving strong alkaline, which greatly limits its potential applications. In this study, a mild modified approach was developed to fabricate HA/SF blend sponges crosslinked by BDDE without alkaline conditions. The blend solutions were cryo-concentrated to induce crosslinking reactions. The mechanism of freezing crosslinking was elucidated by investigating the effects of ice crystal growth and HA molecular weight on the degree of crosslinking. The results revealed that HA achieved efficient crosslinking when its molecular weight exceeds 1000 kDa and freezing temperatures ranged from −40 °C to −20 °C. After introducing SF, multiple crosslinks were formed between SF and HA chains, producing water-stable porous sponges. The SEM results demonstrated that the introduction of SF effectively enhanced the interconnectivity between macropores through creating subordinate holes onto the pores wall. Raising the SF content significantly enhanced compression strength, resistance to enzymatic degradation and cell viability of blend sponges. This study provides a novel strategy for designing bioactive HA/SF blend sponges as substitutes for tissue repair and wound dressing.

Underoil superhydrophilic flame-retardant 3D porous composite for efficient on-demand emulsion separation: Interface engineering design on sphagnum moss

Oil pollution and the energy crisis make oil-water separation an urgent for human need. The widespread use of materials with a single emulsion separation capability is limited. Multifunctional on-demand separation materials can adapt to a wide range of application scenarios, thus having a wider range of applications. The underoil superhydrophilic surface is of great significance for realizing the on-demand separation of oil/water emulsions through the removal of water in the oil and oil in the water. A 3D porous emulsion separation material based on the superhydrophilic principle of sphagnum moss was designed. The material was prepared in a simple step by taking advantage of the adhesion of polydopamine and the introduction of the as-prepared superhydrophilic BaSO4 nanoparticles to achieve superhydrophilicity with a water contact angle (WCA) of 0° and an oil contact angle (OCA) of 157.3°, resulting in excellent separation performance for both water-in-oil and oil-in-water emulsions. Underoil superhydrophilic porous composite (OSPC) can complete two kinds of emulsion separations by filtration or adsorption. It adsorbs water from water-in-oil emulsion to achieve separation, with a good adsorption capacity of 74.38 g/g and efficiency up to 99%. It can also filter oil-in-water emulsions with an efficiency of 99.92%. The separation efficiencies are all almost unchanged after ten separation cycles. Furthermore, the material has excellent flame retardancy, which reduces the possibility of secondary disasters. The three-dimensional porous sponge has excellent on-demand separation performance for multiple emulsions. It provides a new preparation strategy for underoil superhydrophilic materials and a new idea for the design direction of special wetting materials for the on-demand separation of oil/water emulsions.

Direct synthesis of lightweight fiber sponges with interconnected nanopores structured fibers for absorbing low-frequency noise

Noise pollution from transportation industries poses many challenges to people’s health and the global economy. Most noise-absorbing materials suffer from the difficulty of achieving good noise absorption while maintaining light weight. In this study, we propose a feasible strategy to assemble porous fiber sponges (PPFS) based on electrospinning method. Modulating the phase separation rate of charged jet allows for the direct assembly of fiber sponges consisting of interconnected nanopores (30–70 nm) structured fibers. The resultant PPFS possess lightweight feature (volume density of 9.06 mg cm−3) and good mechanical properties (full recovery from 60% deformation). The sponges also show excellent low-frequency sound-absorbing performance (the sound absorption coefficient at 500 Hz is 0.78). Meanwhile, the noise reduction coefficient (NRC) of the fiber sponges reaches 0.57 when thickness is 30 mm, which could absorb white noise and high decibel noise efficiently. This work offers a fresh solution to develop lightweight and efficient sound-absorbing materials.

A SERS-based point-of-care testing approach for efficient determination of diquat and paraquat residues using a flexible silver flower-coated melamine sponge

Diquat (DQ) and paraquat (PQ) residues in food are potential hazards to consumers' health. Point-of-care testing (POCT) of them remains challenging. Based on surface-enhanced Raman spectroscopy (SERS) technology, we developed a POCT strategy for DQ and PQ on apple surface and in apple juice. A point-of-use composite was fabricated using a piece of porous melamine sponge (MS) modified with silver nanoflowers (AgNFs), combining the specificity of the SERS fingerprint and the excellent adsorption capacity of MS. Using this dual-functional AgNFs@MS, the on-site determination of the DQ and PQ residues was completed within 3 min without pretreatment. Clear trends were observed between SERS intensity and logarithmic concentrations, with r values from 0.962 to 0.984. The limit of detection of DQ and PQ were 0.14–0.70 ppb in apple juice and on apple surface. This study provides a new point-of-use alternative for rapidly detecting DQ and PQ residues in nonlaboratory settings.

Three-dimensional NiO/Ni self-floating porous composite materials for efficient solar interfacial evaporation

The scarcity of freshwater resources and energy crisis are major challenges to the progress of human society. Recently, solar-driven interfacial water evaporation utilizes photothermal conversion materials to convert solar energy into thermal energy, which possesses great application potential in seawater desalination and sewage treatment. Here, we fabricated porous nickel sponge (NSP) composite photothermal material (NiO-NSP) containing abundant NiO nanoneedles array. This in situ-grown NiO with enhanced bonding between photothermal material and substrate can be used as a solar absorber directly without consuming additional photothermal material compared to conventional coating and cladding on the carrier surface. Due to the multi-scattering induced light trapping effect of the semiconductor NiO nanoneedles array on the surface of nickel fibers, resulting in light absorption up to 96.5 %. NiO-NSP evaporators have an abundance of interconnected porous channels for water supply, which results in strong capillary action to pump water to the evaporation interface efficiently. Moreover, the highly dispersed nanoneedles array expand the evaporation surface area. Consequently, the NiO-NSP evaporator delivers an excellent evaporation rate of 1.78 kg m−2 h−1 and a photothermal conversion efficiency of 84.7 % at one sun radiation. In addition, the micron-sized porous mesh structure of NiO-NSP dissolve salt crystals and diffuse them downward into the bulk brine, exhibiting excellent salt resistance and remarkable seawater desalination stability. Moreover, NiO-NSP solar evaporators has been demonstrated to possess excellent ability to purify dye and heavy metal ion wastewater. Therefore, the NiO-NSP high-performance evaporator produced by straightforward process proved to be suitable for solar seawater desalination and wastewater purification.

Flexible porous nanofibrous sponge with antioxidant and antibacterial characteristics for promoting diabetic wound healing

Excessive reactive oxygen species (ROS) and bacterial burden delay diabetic wound healing. Although antioxidants and antibacterials have the potential to mitigate excessive ROS and bacterial burden, their efficacy is compromised by the presence of abundant exudate. In this study, a porous chitosan/polyvinyl alcohol nanofibrous sponge immobilized with protocatechualdehyde (CPNS-PA) is developed as a flexible dressing. The CPNS-PA exhibits favorable liquid management ability, high liquid absorption capacity (3867.73 %), superior air permeability (281.1 mm/s), and great compressive recovery at a strain of 50 %. Immobilizing protocatechualdehyde endows CPNS-PA with improved antioxidant and antibacterial properties. Besides, to enhance the stability of CPNS-PA in an aqueous environment, heat treatment is employed instead of toxic chemical agents, thereby preserving its excellent cytocompatibility. The experiments conducted on a diabetic mouse wound model demonstrate that CPNS-PA effectively promotes diabetic wound healing, indicating its promising potential for clinical applications in diabetic wound healing.

Photocatalytic synthesis of high-concentration hydrogen peroxide by efficient photothermal synergy

We here report an energy-saving and cost-effective strategy to produce high-concentration H2O2 in commercial sponge with aqueous storage functions. Thereinto, the rationally designed hybrid consisting of thiourea-group-grafted covalent triazine frameworks (CTFs) and carbon dots (CDs) uniformly wraps the skeletons of the porous sponge to serve as photocatalytic system. This system allows solar radiation to be localized at the surface of photocatalyst and meantime suppresses the tremendous energy consumption of evaporation significantly by a simple seal way, thus giving rise to the required reaction temperature. With it, not only solar-converted heat as by-product in photocatalysis is fully utilized in H2O2 production, but also high-concentration H2O2 solution (105.2 mM for per gram photocatalyst in 1 h) is gained due to catalysis confinement effect. For application, our presented design exhibits more superior activities in degrading different organic pollutants (such as Rhodamine B, phenols and tetracycline) than conventional particulate photocatalysis thanks to in situ formation of concentrated H2O2 in sponge.

Silk fibroin sponge impregnated with fish bone collagen: A promising wound healing scaffold and skin tissue regeneration.

In the present study, porous silk fibroin sponges (SFS) were prepared using silk fibroin (SF), fish bone collagen (FBC), and olive oil (OO). The study investigates the potential use of using this sponge as skin tissue regeneration. The sponge was characterized for its physicochemical, mechanical, antimicrobial, and drug release properties. An in vitro study was carried out using human keratinocyte cell line (HaCaT). Biodegradation study using enzymatic method was carried out. The results showed that the mechanical properties such as tensile strength (23.40 ± 0.05 MPa), elongation at break (14.25 ± 0.02%), and water absorption (30.23 ± 0.01%) of the SFS were excellent, indicating promising performance. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays proved the biocompatible nature of the SFS. The SFS exhibited outstanding antibacterial properties against E. coli (4.72 ± 0.05 mm) and S. aureus (4.98 ± 0.07 mm). The developed SFS promote a promising solution for skin tissue regeneration and wound dressing.

3D porous sponge-like sensors prepared from various conductive nanohybrids-filled melamine sponge toward human motion detections

Melamine sponge (MS) is a commercial material with low density, high porosity, and superior elasticity, which has been utilized to design and produce three‐dimensional (3D) porous sponge-like sensors. Notably, this material is environment‐friendly and inexpensive. In the present study, an MS filled with various hybrid source bridges, as 3D porous sponges, was designed and prepared by using a simple and low-cost immerse-filling method and then submerged into various hybrid source mixtures with the support of an MS framework with a hole-opened structure. The structurally morphological, chemical, thermal, and mechanical properties of the as-obtained 3D porous sponges were further determined for potential changes in the structure of these sponges after filling the hybrid source mixtures in/on the MS framework. The presence of functional hybrid sources in the 3D porous sponges indicated that such sponges have reached a good sensitivity, stretch ability, and repeatability under compressing, stretching, bending, and twisting deformations. In addition, these sponges were effectively applied for detecting and monitoring concomitantly small- and large-scale movements of the human body, that is, the various motions of the knee, elbow, finger, wrist, and foot insole. Therefore, the 3D porous sponges prepared in this study can be considered as a promising candidate for practical applications as flexible or wearable devices.

Low-/Negative-Pressure Hydrocephalus: To Understand the Formation Mechanism from the Perspective of Clinicians.

Low-/negative-pressure hydrocephalus (LPH/NePH) is uncommon in clinical practice, and doctors are unfamiliar with it. LPH/NePH is frequently caused by other central nervous system diseases, and patients are frequently misdiagnosed with other types of hydrocephalus, resulting in delayed treatment. LPH/NePH therapy evolved to therapeutic measures based on "external ventricular drainage below atmospheric pressure" as the number of patients with LPH/NePH described in the literature has increased. However, the mechanism of LPH/NePH formation is unknown. Thus, understanding the process of LPH/NePH development is the most important step in improving diagnosis and treatment capability. Based on case reports of LPH/NePH, we reviewed theories of transcortical pressure difference, excessive cerebral venous drainage, brain viscoelastic changes, and porous elastic sponges.