Shape Recovery Time Research Articles

Effect of cyclic tests on the shape memory performance of GO‐CF hybrid‐reinforced SMPC

AbstractThis study investigated the effect of cyclic tests on the shape memory performance of graphene oxide‐carbon fiber (GO‐CF) hybrid‐reinforced shape memory polymer composite (SMPC). The shape memory performance was evaluated by four indexes: shape fixation rate, shape recovery rate, recovery time, and maximum recovery force. The results showed that the service life of the GO‐CF hybrid‐reinforced SMPC prepared by the vacuum infiltration hot‐press forming experimental system process (VIHPS) was 16 cycles. After 16 repetitions, the shape fixation rate decreased by approximately 6.6% and the shape recovery rate decreased by approximately 14.0%. At this time, the shape recovery rate was lower than 90%. The recovery time increased by 62.2% and the maximum recovery force decreased by 70.1%. The maximum recovery force exhibited the highest performance attenuation. Scanning electron microscopy (SEM) was used to photograph and analyze the microstructures of the samples under various cyclic tests. It was concluded that the main factor affecting the shape memory performance of GO‐CF hybrid‐reinforced SMPC is the life of fiber bundle. During cyclic testing, the fiber was prone to kinks, tensile damage and other defects, which increased the probability of reducing the length of the fiber bundle. Therefore, the probability of fiber fracture was improved, and the shape memory performance of the GO‐CF hybrid‐reinforced SMPC was attenuated.

Competing and decisive roles of 1D/2D/3D sp2-carbons in controlling the shape switching, contact sliding, and functional properties of polymers

Stimuli-controllable shape-shifting polymers, such as shape memory polyurethane (SMPU), are promising for robotics, aerospace, sensing, automobiles, and many other applications. However, slow actuation or high shape recovery time, low recovery stress, and inadequate understanding of friction and wear characteristics of SMPU limit its widespread practical uses. Further, SMPU has been engineered with diverse foreign materials but inconsistent results and ambiguous underlying mechanisms, especially when SMPU is modified with sp2-carbon materials, are also major concerns. Here we determine and simultaneously cross-compare the role of 1D/2D/3D graphitic carbons, namely carbon nanotubes, multilayer graphene and graphite, in controlling the properties of SMPU. The designed SMPU-composites display 8–15-folds faster shape recovery in different mediums, higher recoverable stress, faster healing of the dents, 2–3-folds lower friction, better wear resistance, and improved thermal, wettability, and dielectric properties than pristine SMPU. Further, while the introduction of 2D/3D graphitic carbons massively degrade the elongation, 1D carbon nanotubes maintains the stretchability of SMPU. Eventually, we develop a novel heat alarm device employing SMPU-composite as a major component that acts as a heat sensor, an actuator, and enables the closure of the circuit. Our results uncover many unknown phenomena of engineered SMPU and pave the way for the development of smart-technologies.

Photothermally responsive smart elastomer composites based on aliphatic polycarbonate backbone for biomedical applications

Despite self-healing elastomers can autonomously heal physical damage of the interface of materials to reduce maintenance costs, utilization of self-healing ability and shape memory as sutures to promote wound healing has not been reported. Herein, novel photothermally responsive elastomer composites based on aliphatic polycarbonate and azobenzene units are reported, which exhibit highly efficient self-healing, controllable mechanical properties and excellent shape memory. Notably, the elastomer composites possess a unique, rapid light-responsive self-healing ability within only 5 min. The shape recovery time showed that the elastomer composites could completely recover to their original shape after 19.33 ± 0.29 s. The cell line studies and subsequent imaging indicate that the composites with good biocompatibility are superior biomaterials used in the biomedical arena. Spiral-like stent also displays fast response at 37 °C, finishing excellent self-expansion within only 10 s. In vivo experiments on injured wounds of rats demonstrate that the obtained composites as a self-tightening suture in keyhole surgery has the potential of excellent self-healing knotting ability and shape memory effect. The obtained self-tightening suture can quickly restore a programmed shape at physiological temperature, thereby effectively closing the injured wound surface, and without forming any scars after healing, which will provide a new direction for the future biomedical field.

Polyurethane polystyrene based smart interpenetrating network with quick shape recovery through thermal actuation

Polyurethane (PU), designed with pre-polymer method involving polyol as cross-linker, has been utilized for shape memory applications. Neat PU or PU-PS (polystyrene) interpenetrating network (IPN) samples have been prepared. Functionalized multiwall carbon nanotubes (FMWCNTs) have been utilized as reinforcements. PU composites have been studied for shape recovery time and found better than neat PU. 1wt% incorporation of FMWCNTs in PU has reduced shape recovery time to 22 s for 100% shape recovery, in comparison to neat PU with 62 s of 100% shape recovery. PU-PS IPN has reduced 100% shape recovery time to 17s. Superior hydrogen bonding in neat PU has been suggested as per longer shape recovery time against thermal actuation, in comparison to PU composite and IPNs with FMWCNTs. Decreased thermal stability has been observed with FMWCNTs incorporation, indicating enhanced heat dissipation. Field emission scanning electron microscopy analyses confirmed the difference of morphologies in neat PU, PU composite, and IPNs. A distinctive filler-matrix interaction in IPNs has been observed. XRD confirmed the presence of amorphous component. Rutherford Backscattering Spectrometry and Thermal Gravimetric Analysis have been utilized for analyses. Shape recovery study has been made by a simple experimental set up prepared in lab.

Dual-functional polymer blends with rapid thermo-responsive shape memory and repeatable self-healing properties

Although dynamic chemical interaction plays a crucial role for designing sustainable materials with extended lifespans and functionality, the preparation complexity and lack of acceptable mechanical strength limit their practical application. Herein, tunable thermo-responsive shape memory and self-healing bifunctional blends are fabricated by melt blending of poly(ethylene-co-methacrylic acid sodium salt) (EMNa) and poly(ethylene-co-methyl acrylate) (EMA) copolymers. To realize the complementary advantages, the ionic interactions in the EMNa ensure the structural integrity and self-healing repeatability of the blends, meanwhile the highly elastic EMA as the dispersed phase is capable to provide the main driving force during the shape recovery process. The shape recovery behavior can be well controlled by varying the EMA content. The shape recovery time shortens with increasing EMA content, and a U-shaped EMNa with 30 wt% of EMA (EMNa/EMA-30) specimen could recover to a plate shape within 8 s. Interestingly, the EMNa/EMA-30 exhibits repeatable self-healing capability, and its tensile strength can reach 19.4 and 15.9 MPa after one and three healing cycles, respectively. The self-healed EMNa/EMA-30 still exhibits excellent reusability and endurance, and the shape fixity and shape recovery ratios are both above 95% after 15 shape memory cycles. The rapid thermo-responsive shape memory effect, superior self-healing property, and prominent programmability make the EMNa/EMA blends promising in various cutting-edge applications, such as self-deployable structures, artificial muscles, and wearable devices.

Effect of multi-walled carbon nanotube on thermal decomposition, mechanical and heat-induced shape memory properties of crosslinked poly(vinyl alcohol)

Shape memory polymers are attractive smart materials that have many promising applications in a wide variety of fields. Here the multi-walled carbon nanotube (MWCNT) was used to adjust the performance of crosslinked poly(vinyl alcohol) (PVA), which had acetal reaction with glutaraldehyde (GA). The thermal degradation, mechanical strength and shape recovery properties of PVA/MWCNT/GA membranes were investigated by thermogravimetry, differential scanning calorimetry, x-ray scattering, dynamic mechanical analysis, tensile test and thermal-stimulus shape memory recovery measurements. The MWCNT embedded in the crosslinked PVA matrix accelerated the breaking of C–O bonds. The mechanical strength and shape recovery properties of PVA/MWCNT/GA composites were tunable. As the MWCNT loading increase in crosslinked PVA matrix, the yield strength increased from 3.85 MPa to 10.21 MPa, breaking elongation decreased from 230% to 50%, the 100% shape recovery time decreased from 27 s to 16 s, respectively.

Effects of Carbon Nanotube Infiltration on a Shape Memory Polymer‐Based Device for Brain Aneurysm Therapeutics: Design and Characterization of a Joule‐Heating Triggering Mechanism

Current endovascular therapies for intracranial aneurysms face limitations that include aneurysm recurrence and incomplete occlusion. These challenges can potentially be addressed by occluding the aneurysm space with shape memory polymers (SMPs) that are tailorable to patient‐specific aneurysm geometries to improve the suboptimal treatment outcomes associated with coil embolization. However, deployment of the SMP‐based device into the aneurysm requires external stimuli to trigger shape recovery. Thus, herein, the infiltration of carbon nanotubes (CNTs) in a polyurethane SMP foam is investigated, and Joule‐heating triggering for SMP shape recovery in endovascular therapy applications is demonstrated. The results show that CNTs can be successfully infiltrated in the SMP foam, providing tunable resistivity and shape recovery time, and that CNT infiltration reduces the glass transition temperature of the SMP and alters its mechanical properties, which is evidenced with a cumulative stress reduction in cyclic compression tests. Finally, the Joule‐heating capability of the SMP material is examined using a proof‐of‐concept in vitro occlusion experiment of an idealized saccular aneurysm model. Collectively, this study indicates that CNT infiltration of SMP foams is a promising approach in the design of electrically triggered embolic devices for individualized treatment of intracranial aneurysms.

High-Temperature-Aging Induced Sequential Recovery of Shape Memory Nitrile Butadiene Rubber Composites.

The sacrificial bonds in natural materials have inspired the preparation of shape memory polymer (SMP), which can be prepared through the construction of dual cross-linking networks in a polymer matrix. With the rise of 4D printing technology, fine control over the shape recovery of SMPs, especially control over the recovery time, is urgently needed. In this study, the high-temperature aging method is adopted to tune the shape recovery time of dual cross-linked SMPs. Shape memory acrylonitrile butadiene rubber composite (i.e., NBR-C) is prepared by introducing Zn2+-C≡N coordination bonding and sulfur covalent cross-linking networks into the rubber matrix and then thermal aging at 180 °C for various time frames. Aging increases the covalent cross-linking density, ruptures rubber chains, and generates imine structures. Moreover, the composition of the coordination bonding network becomes diversified because of the formation of coordination bonds between imines and Zn2+ ions. The mechanical "tough-brittle" transition of aged NBR-C is observed, and its glassy temperature increases with aging time, which in turn changes the shape recovery time at the same recovery temperature. On the basis of these findings, the special shape memory rubber components with sequential recovery are fabricated by partially aging the NBR-C strings. This methodology provides novel solutions for the preparation of sequential SMP products without programming heating design or using redundant chemical materials. We believe that this work will be able to help promote comprehensive research of SMPs and widen applications of SMPs in the industry.

Functionalized carbon nanotubes based thermo-responsive shape memory blends with enhanced mechanical properties for potential robotics applications

A two-step approach was used to synthesize a thermo-responsive polyurethane and its blends with amino-functionalized-polystyrene (PS) and multiwall carbon nanotubes (MWCNTs) aiming at enhanced mechanical, thermal and shape memory properties. The synthesis of novel shape-memory polyurethane was confirmed by FTIR spectroscopy. SEM analysis of samples revealed excellent interfacial interaction due to chemical and physical interlinking between both polymers (PU synthesized and polystyrene functionalized) and functionalized multiwall carbon nanotubes (FMWCNTs) filler. The significant improvement in mechanical and thermal properties was observed for synthesized blends (PU/modified PS) as the filler content increased. The mechanical properties of PU/modified-PS blend having 3% loading amount of FMWCNTs were enhanced from 28.6 to 59.3 MPa as compared to those of neat PU. Due to proper fabrication and strong interfacial interaction, enhancement in thermal properties was also evident from the results with increasing filler loading amount. A sharp decrease in thermal, mechanical and recovery properties was also evident due to agglomerates net-points formation when loading amount of carbon filler increased from a certain level. Almost 100% shape recoveries were achieved for all samples, but the recovery durations of the samples were different. Modified-PS and FMWCNTs with PU formed three-dimensional interlocked networks which provided excellent mechanical strength, thermal stabilities and efficient shape recovery to the synthesized blends. Shape recovery response time of blends and nanocomposite was also found to decrease almost half of that of the pristine PU (less than 37 s for blends). Enhanced thermal stabilities, tensile properties, smaller shape recovery time, almost 100% shape recovery capabilities and sustainability, all factors favor the potential use of these blended composite materials in robotics, aeronautics, medical devices and high-performance materials in auto-industry.

Stimuli responsive optical polymers through omnidirectional and reconfigurable porosity

Manufacturing of porous materials with reconfigurable porosity is beneficial to systems with tailored material properties. This study describes the design and characterization of dynamic omnidirectional porosity in thermoplastic porous networks and their associated applications as novel indicators of temperature and chemical vapors. Herein we report on porous thermo-responsive films that undergo considerable pore size reduction (~40%) and pore density reduction (~86%) when heated to an activation temperature (Ta = 100 °C). These films are synthesized using a solid-state, high-pressure gas saturation and phase separation process on physically crosslinked block copolymers such as poly (styrene-butadiene-styrene) (SBS), and poly (styrene-ethylene/butylene-styrene) (SEBS). The pores act as light-scattering centers, leading to optical opacity of the film. Upon activation, there is a noticeable volume change (~18%) and a concomitant optical opaque-to-transparent transition (OTT) by virtue of the omnidirectional shrinkage of pores. The transparency of these films changes from 45% to 95%, with modulation occurring indiscriminately across a broad spectrum (e.g., UV, visible, and IR). We detail the underlying mechanism for the thermo-responsive dynamic porosity and its interlinked relationship with OTT, and control of OTT temperature onset and its associated shape recovery time. Furthermore, we demonstrate the utility of dynamic porosity for actuation and chemical vapor sensing purposes.

Preparation of modified multi‐walled carbon nanotubes as a reinforcement for epoxy shape‐memory polymer composites

Effectively improving the mechanical properties and thermal resistance of epoxy shape‐memory polymers (ESMPs) without affecting their shape‐memory performance is necessary to expand these polymers in practical applications. In this article, modified multi‐walled carbon nanotubes (MWCNTs) were prepared and used as efficient reinforcement for enhancing the comprehensive properties of ESMPs. Increases of nearly 289% to 444% for impact strength and 112% to 184% for tensile force were obtained by adding only 0.1 to 1 wt% epoxy‐modified MWCNTs. The addition of unmodified and carboxyl‐modified MWCNTs was also investigated but showed less impact on the mechanical properties of the ESMPs than epoxy‐modified MWCNTs. Thermogravimetry analysis (TGA) and dynamic mechanical analyses (DMA) showed that less than 1 wt% modified MWCNTs can enhance the heat resistance of ESMPs greatly. Although the shape recovery time for composite materials increased upon adding the MWCNTs, the entire recovery time was still less than 1 minute, and the shape recovery rate was relatively high, nearly 100%.

Three-Dimensional-Printed Shape Memory Biomass Composites for Thermal-Responsive Devices.

In this study, unique three-dimensional (3D)-printed shape memory biomass composites were prepared by the melt blending and extrusion of polyurethane, polycaprolactone (PCL), and wood flour (WF) with adjustable contents. The addition content of PCL was used to adjust the shape memory transition temperature and improve the shape fixing rate of composites. The crystallization, thermal, mechanical, and shape memory properties of different composites were investigated. The results of X-ray diffraction and differential scanning calorimetry tests showed that the crystallization peak and melting temperature of different composites were not obviously changed. As the PCL content increased, the tensile strength of the composites decreased first and then increased, and the elongation at break gradually decreased. Thermal response shape memory test results showed that, when the PCL content was 30 wt.%, the composites had high shape recovery rate and fixed rate (both ∼100%). In addition, carbon black (CB) was added as a photothermal conversion material to the composite with a preferred ratio to achieve the photothermal response shape memory performance. With the addition of CB, the thermal conductivity of composites was improved. Under the same conditions, the thicker the 3D-printed specimens, the longer the specimen shape recovery time; the greater the light intensity, the shorter the specimen shape recovery time. Compared with the composite without CB, the flower model printed with the composites containing CB had a better photothermal response shape memory performance.

Additive manufacturing of shape memory polymers: effects of print orientation and infill percentage on shape memory recovery properties

Purpose The purpose of this paper is to study the shape memory properties of SMP samples produced through a MEAM process. Fused deposition modeling or, as it will be referred to in this paper, material extrusion additive manufacturing (MEAM) is a technique in which polymeric materials are extruded though a nozzle creating parts via accumulation and joining of different layers. These layers are fused together to build three-dimensional objects. Shape memory polymers (SMP) are stimulus responsive materials, which have the ability to recover their pre-programmed form after being exposed to a large strain. To induce its shape memory recovery movement, an external stimulus such as heat needs to be applied. Design/methodology/approach This project investigates and characterizes the influence of print orientation and infill percentage on shape recovery properties. The analyzed shape recovery properties are shape recovery force, shape recovery speed and time elapsed before activation. To determine whether the analyzed factors produce a significant variation on shape recovery properties, t-tests were performed with a 95% confidence factor between each analyzed level. Findings Results proved that print angle and infill percentage do have a significant impact on recovery properties of the manufactured specimens. Originality/value The manufacturing of SMP objects through a MEAM process has a vast potential for different applications; however, the shape recovery properties of these objects need to be analyzed before any practical use can be developed. These have not been studied as a function of print parameters, which is the focus of this study.

Rapid remote actuation in shape memory hyperbranched polyurethane composites using cross-linked photothermal reduced graphene oxide networks

We report near-infrared (NIR) laser-driven rapid shape recovery in hyperbranched polyurethane/epoxy (HBPU/EP) composites based on cross-linked graphene photothermal networks. Graphene-based photothermal reinforcements were synthesized by functionalizing graphene oxide (GO) with the epoxy bisphenol A diglycidil ether (DGEBA) to form DGEBA-f-GO, and by subsequent reduction of DGEBA-f-GO to DGEBA-f-reduced graphene oxide (DGEBA-f-rGO). Upon irradiation with an 808 nm NIR laser, the composites with 1 wt% cross-linked DGEBA-f-rGO exhibited a remarkable shape recovery time of 6.5 s (one-fourth that of the composites with 1 wt% GO). The rapid actuation in these composites results from excellent photothermal conversion by the cross-linked DGEBA-f-rGO owing to enhanced π conjugation in rGO and homogeneous dispersion of graphene networks in the polymer matrix due to covalent interactions. An increase in the epoxy groups on the GO surface also enhanced the mechanical properties of the composites by forming denser cross-linked networks. Graphene-based photothermal networks may be suitable for fabricating robust and remote-controllable rapid shape recovery composites.

Mechanical Signature of Red Blood Cells Flowing Out of a Microfluidic Constriction Is Impacted by Membrane Elasticity, Cell Surface-to-Volume Ratio and Diseases.

Despite the fact that Red Blood Cells (RBCs) have been intensively studied in the past 50 years to characterize mechanical phenotypes associated with both healthy and pathological states, only ektacytometry (i.e., laser diffractometry) is currently used by hematologists to screen for RBC membrane disorders. Therefore, the development of new diagnostic tools able to perform analysis at the scale of a single cell, over a statistically relevant population, would provide important complementary information. But these new diagnostic tools would have to be able to discriminate between different disorders causing a change in RBCs mechanical properties. We evaluated the mechanical response of artificially rigidified RBCs flowing through a microfluidic constriction. The geometry consists in a 50 μm wide channel with a succession of 14 tooth-like patterns, each composed of a 5 μm wide and 10 μm long constriction, associated with a 25 μm wide and 10 μm long enlargement. RBCs deformability was altered using two chemical treatments, known to affect RBCs membrane surface area and membrane deformability, lysolecithine (LPC) and diamide, respectively. Differences between samples were highlighted by the representation of the inverse of the shape recovery time (1/τr), versus the extension at the exit of the constriction, Dout. The results demonstrate that our approach is able to provide a direct signature of RBCs membrane composition and architecture, as it allows discriminating the effect of changes in RBCs membrane surface area from changes in RBCs membrane deformability. Finally, in order to evaluate the potential of our microsystem to detect pathological cells, we have performed preliminary experiments on patients with Hereditary Spherocytosis (HS) or Sickle Cell Anemia (SCA).

Shape memory hybrid based on polyvinyl alcohol and 0D silver nanoparticles

Shape memory hybrids (SMH) have drawn significant attention because they allow an easy alternative for the design of shape memory materials with tailored properties or features. In this work, a shape memory hybrid was made, based on polyvinyl alcohol (PVA) as the elastic domain and 0D silver nanoparticles (Agnps) as the transition domain, a dissolution method of mixing both components, and evaporation of water afterward allowed the formation of films of the hybrid material. Two different size distributions of silver nanoparticles were used (13.7 ± 2.6 and 67.9 ± 14.1 nm), in order to study the effect of the size on the shape memory effect (SME) of the final SMH, under temperature stimuli. The materials obtained were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy, infrared spectroscopy (IR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques. The crystallinity of PVA was slightly altered with the addition of Agnps. Finally, the shape memory effect was tested on both hybrid materials, resulting in a better response to temperature for the SMH prepared with Agnps of 68 nm, also the shape recovery time can be tuned varying both the increase of temperature and the size distribution of Agnps used.

Near-infrared light photothermally induced shape memory and self-healing effects of epoxy resin coating with polyaniline nanofibers

Polyaniline (PANI) nanofibers were synthesized by interfacial polymerization at 20 °C. The ultraviolet–visible–near-infrared light (NIR) absorption spectra indicated two absorptions at 450 nm and 848 nm, which exhibited the NIR-absorbing characteristic of PANI. The photothermal conversion temperature of PANI nanofibers increased with the NIR power density of 808 nm. The nanofibers were added to the epoxy resin to prepare NIR-induced shape memory and self-healing coating. The shape memory effect of the composites was measured via NIR irradiation of 2 W cm−2 808 nm. The surface morphologies of the scratched and healed coatings were investigated by optical stereomicroscopy. The healing performance of the intact, damaged, and healed coatings was also evaluated by linear sweep voltammetry and polarization curves. Results indicated that the shape recovery time of the composite decreased as PANI content increased. The composite with 5 wt% PANI exhibited the best shape memory effect and multiple self-healing performances. Moreover, PANI nanofibers improved the protection performance of the epoxy resin coatings. The electrochemical results indicated that the coating with 5 wt% PANI had good self-healing performance when the scratched area was irradiated by NIR. This finding was attributed to the photothermal conversion effect and passivation function of the PANI nanofibers.

PEEK modified PLA shape memory blends: towards enhanced mechanical and deformation properties

Polylactic acid (PLA) is one of the most promising shape memory polymers with outstanding biocompatibility, while poly(ether ether ketone) (PEEK) is a special engineering plastic with excellent mechanical performance. In this work, PEEK was selected to modify PLA, and a series of PLA blended with different PEEK contents (PLA/PEEK blends) were obtained. The effects of PEEK on thermodynamic, mechanical and shape memory properties of PLA/PEEK blends were investigated. The results showed that the thermal stability of the PLA/PEEK blend was improved with the PEEK content increase. The tensile strength reached the highest value of 20.6 MPa when the PEEK content was 10%. While the best shape memory performance occurred with the PEEK content of 15%, the shape recovery time was less than 2 s, and the shape fixation/recovery ratio was more than 99%. Furthermore, the programmable mimetic flower opening process was achieved by using PLA/PEEK blends with different PEEK content ratios. The above results indicated that the blend of an appropriate proportion of PEEK had positive effects on mechanical and deformation performances of PLA.

Numerical Simulation of Liquid Droplet Coalescence and Breakup

Abstract This paper studied the evolution of binary droplet collision in liquid and also a mathematical calculation method of coalescence time. Binary droplet collisions occur in many engineering applications; however, the accurate models to predict the collision of droplets in the liquid are still lacking. In this work, the binary collision processes of droplets were simulated through computational fluid dynamic (CFD) method, where the interfaces between the two phases were tracked by the volume of fluid (VOF) approach. The results reveal that Weber number determines the results of the head-on collisions, and the cases with the same Weber number present similar evolution processes. If coalescence happens, the collision time decreases with increase in relative velocity, whereas the shape recovery time is independent with the relative velocity, but depends on droplet diameter. It is derived from this research that the collision time is proportional to the droplet diameter, and the shape recovery time is proportional to the 3/2 power of droplet diameter. The droplet moving directions play an important role in the collision results, and the case of two droplets moving toward each other with equal velocity is the easiest way to coalesce. When two droplets with different sizes collide, besides relative velocity, the coalescence and breakup are determined by the absolute velocities, the size, and size ratio of the two droplets. The increase in viscosity of continuous phase results an increase in collision time, but decrease in coalescence time.

Dual shape recovery of red blood cells flowing out of a microfluidic constriction.

Micropipette aspiration, optical tweezers, rheometry, or ecktacytometry have been used to study the shape recovery of healthy human Red Blood Cells (RBCs) and measure associated relaxation times of the order of 100-300 ms. These measurements are in good agreement with the Kelvin-Voigt model, which describes the cell as a visco-elastic material, predicting that its relaxation time only depends on cell intrinsic properties. However, such mechanical solicitation techniques are far from being relevant regarding RBC solicitation in vivo. In this paper, we report for the first time the existence of two different behaviors of the RBC shape recovery while flowing out of a microfluidic constricted channel. The calculation of the viscous stress corresponding to the frontier between the two recovery modes confirms that the RBC resistance to shear is the elastic property dominating the transition between the two recovery behaviors. We also quantified associated recovery times and report values as low as 4 ms-which is almost two decades smaller than the typical RBC relaxation time-at high viscosity and flow velocity of the carrier fluid. Although we cannot talk about relaxation time because the cell is never at rest, we believe that the measured shape recovery time arises from the coupling of the cell intrinsic deformability and the hydrodynamic stress. Depending on the flow conditions, the cell mechanics becomes dominant and drives the shape recovery process, allowing the measurement of recovery times of the same order of magnitude than relaxation times previously published. Finally, we demonstrated that the measurement of the shape recovery time can be used to distinguish Plasmodium falciparum (causing malaria) infected RBCs from healthy RBCs.