Fiber Axis Research Articles

Engineered Protein Fibers with Reinforced Mechanical Properties Via β-Sheet High-Order Assembly.

Protein fibers are ideal alternatives to synthetic polymers due to their unique mechanical properties, biocompatibility, and sustainability. However, engineering biomimetic protein fibers with high mechanical properties remains challenging, particularly in mimicking the high molecular weight of natural proteins and regulating their complex hierarchical structures. Here, a modular design and multi-scale assembly strategy is developed to manufacture robust protein fibers using low- or medium-molecular-weight proteins. The distinct functional and structural properties of flexible, rigid, and cross-linked domains in modular proteins are skillfully harnessed. By regulating the ratio of rigid to flexible domains, the formation of high-order β-sheet crystals aligned along the fiber axis is promoted, enhancing both strength and toughness. Furthermore, the dynamic imine cross-linking network, formed by the aldehyde-amine condensation reaction of the cross-linked domains, further reinforces the protein fibers. Remarkably, fibers spun from modular proteins significantly smaller than natural spidroin exhibit outstanding mechanical properties, surpassing those of protein fibers with same or even higher molecular weights. This strategy offers a promising pathway for fabricating protein fibers suitable for diverse applications.

Random Raman Lasing in Diode-Pumped Multi-Mode Graded-Index Fiber with Femtosecond Laser-Inscribed Random Refractive Index Structures of Various Shapes

Diode-pumped multi-mode graded-index (GRIN) fiber Raman lasers provide prominent brightness enhancement both in linear and half-open cavities with random distributed feedback via natural Rayleigh backscattering. Femtosecond laser-inscribed random refractive index structures allow for the sufficient reduction in the Raman threshold by means of Rayleigh backscattering signal enhancement by +50 + 66 dB relative to the intrinsic fiber level. At the same time, they offer an opportunity to generate Stokes beams with a shape close to fundamental transverse mode (LP01), as well as to select higher-order modes such as LP11 with a near-1D longitudinal random structure shifted off the fiber axis. Further development of the inscription technology includes the fabrication of 3D ring-shaped random structures using a spatial light modulator (SLM) in a 100/140 μm GRIN multi-mode fiber. This allows for the generation of a multi-mode diode-pumped GRIN fiber random Raman laser at 976 nm with a ring-shaped output beam at a relatively low pumping threshold (~160 W), demonstrated for the first time to our knowledge.

Orientation Distribution of Crystalline β-Sheet Domains in Bombyx mori Silk Fiber Studied with Vibrational Sum Frequency Generation Spectroscopy.

Silk fibers have good biocompatibility and mechanical properties, which make them attractive in biomaterial applications as well as textile industries. It is believed that the superior mechanical property is associated with the crystalline β-sheet structure in the fiber; but a deeper understanding of the structure-property relationship is still needed for full exploitation of its physical properties. Especially, accurate information on hydrogen-bonding interactions within β-sheet domains at the nanoscale and their spatial distributions at the mesoscale are critically needed. In this study, we demonstrate the selective detection of crystalline β-sheet domains in Bombyx mori silk fiber using sum frequency generation (SFG) spectroscopy and its use to determine the angular distribution of the β-sheet crystallites with respect to the fiber axis. Numerical simulations of the SFG signal of the amide-I band were carried out using tensors based on the B2 symmetry of the D2 point group and compared with experimental data. This comparison found that the crystalline β-sheet domains are aligned along the fiber axis with a standard deviation of ∼27° and parallel to the fiber surface with a standard deviation of ∼5°. It was also found that the amide bands in the SFG spectra cannot be fully explained with the assumption that the crystalline β-sheet vibrations can be described with the D2 point group. Being able to monitor the amide group vibrations sensitive to both interchain hydrogen bonding and crystallite orientations, SFG analysis has a potential to unveil the structure-mechanical property relationship that may not be readily assessable with other characterization techniques.

Strong Silkworm Silk Fibers through CNT-Feeding and Forced Reeling.

High-performance silk fibers, with their eco-friendly degradability and renewability, have long captivated researchers as an alternative to synthetic fibers. Spider dragline silk, renowned for its exceptional strength (>1 GPa), has an extremely low yield, hindering its widespread use. While domesticated silkworms (Bombyx mori) can produce silk fibers industrially, their moderate strength (≈0.5 GPa) pales in comparison to the formidable spider dragline silk. In this study, naturally produced strong silkworm silk fibers are reported with a tensile strength of ≈1.2 GPa achieved through combining feeding carbon nanotubes (CNTs) to silkworms and in situ forced reeling for alignment. Molecular dynamics simulations confirm the interaction between the CNTs and silk fibroin, while the forced reeling process aligns these reinforcing fillers and the silk fibroin β-sheet nanocrystals along the fiber axis. Structural analysis reveals a significant enhancement in the content and alignment of β-sheet nanocrystals within the silk fibers, accounting for their superior mechanical properties, including tensile strength of ≈1.2 GPa and Young's modulus of 24.4 GPa, surpassing various types of silkworm silk and spider silk. This advancement addresses the historical trade-off between the strength and scalability of silk, potentially paving the way for eco-friendly, biodegradable, and renewable alternatives to synthetic fibers in a variety of applications.

Concurrent experimental testing and computational micromechanical modeling of a UD NCF GFRP composite ply

The current research work presents a detailed and thoroughly validated computational micromechanical modeling methodology to study the damage initiation and propagation in a uni-directional (UD) glass fiber-reinforced non-crimp fabric (NCF) composite ply. Under the applied transverse tension and compression loads, the effect of various microscale material and geometrical parameters on the ply level stress–strain behavior is studied. To this end, along with the distinctly modeled individual microscale constituents of the UD NCF composite ply (axial fibers, backing fibers, and matrix), the generated RVE (Representative Volume Element) model consists of manufacturing-induced defects and variabilities such as voids as well as the backing fiber’s out-of-plane waviness. The fiber/matrix interface failure behavior in the generated RVE model is simulated using cohesive zone formulations that follow bi-linear traction-separation law. Whereas the hydrostatic pressure-dependent non-linear stress–strain response followed by the fracture of the epoxy matrix is captured using the linear Drucker-Prager plasticity model coupled with the stress triaxiality-based failure criterion. The backing fibers stress–strain and failure behavior is modeled using the linear elastic and isotropic material model in conjunction with the maximum stress criterion.The proposed numerical methodology is thoroughly validated both qualitatively and quantitatively by conducting detailed experiments on a neat epoxy as well as at the composites laminate level. Upon comparing the experimental and computational results, the observed knee or slope change in the bi-linear stress–strain response of the UD NCF composite ply under transverse tension is attributed to the loss of the load-carrying ability of the axial fiber bundle due to fiber/matrix interface debonding followed by the ply splitting. Under the application of transverse compression load, the composite ply failure behavior is governed by the formation of a dominant matrix plastic shear band in the axial fiber bundles, which is in turn triggered by the fiber/matrix interface failure. Under the applied loads in the matrix-dominated direction, the presented research work provides a detailed insight into the microscale damage initiation and propagation in a UD NCF composite ply and its consequent influence on the macroscale stress–strain response.

Study of the effect of interfacial damage and friction on stress transfer in short fiber-reinforced composites

The purpose of this study is to investigate the influence of different stages of different interfaces evolution under external forces on stress transfer within composite materials, which is crucial for analyzing reinforcement mechanisms in composite materials. Analytical solutions are derived to explore the impact of these distinct phases, both at the interfaces along the fiber length direction and at the fiber ends, on the complex stress distribution profiles within composite materials. Furthermore, the frictional effect at the interface serves to impede the debonding process in the composite. Under the same load, the debonding length of the interface decreases as the frictional effect increases. The increase in fiber aspect ratio (AR) effectively reduces the length of the damage and debonding interface and increases the axial fiber stress. Additionally, the theoretical results agree well with numerical simulation and experimental results. In essence, this model provides analytical solutions that are instrumental for analyzing stress transfer in fiber-reinforced composites during different stages of interface evolution.

Magnetoresponsive Anisotropic Fiber‐Integrating Hydrogels for Neural Tissue Regeneration

The physiological functions of specific areas of the nervous system, such as the spinal cord, are directional dependent, relying on the ordered arrangement of cells and extracellular matrix. Here, an automated electrospinning method is used to make fibers with different diameters by changing the concentrations of polycaprolactone/gelatine solutions. The impact of fiber diameter, orientation, and the inclusion of superparamagnetic iron oxide nanoparticles on neural stem cell (NSC) behavior and differentiation is investigated. Fibers with an average diameter of 1189 ± 274 nm greatly improve neuronal differentiation. The highest relative expression of the neuronal marker Tuj‐1 and the formation of interconnected neuronal networks with neurites aligned along the fiber axis demonstrate this. Incorporating SPIONS into the fibers do not damage the cells, and aligned fibers do much better than random fibers at cell proliferation, migration, and neurite alignment. Further, magnetoresponsive fiber‐based hydrogels are fabricated by embedding SPIONS‐loaded fibers within a collagen matrix, which enables remote alignment of the fibers via a magnetic field. Postcrosslinking maintains this alignment and induces significant neurite orientation within the hydrogels. The potential use of automated, magnetically responsive electrospun fibers as flexible supports for NSC differentiation and neural tissue engineering guidance is demonstrated.

Simultaneous Measurement of Fiber-Matrix Interface Debonding and Tunneling Using a Dual-Vision Experimental Setup

BackgroundFiber-matrix debonding is a precursor for transverse cracking and several other types of damage in fiber composites. However, to date, there are limited experiment-based reports that study the fundamental mechanisms of fiber-matrix debonding.ObjectiveThis work aims to uncover the governing mechanisms of fiber-matrix interface debonding by full-field measurements supplemented by numerical simulations. In particular, the application of a dual-vision image-based characterization approach on single glass macro fiber samples is discussed and proven useful in understanding the in-plane and out-of-plane debonding characteristics at the fiber-matrix interface.MethodsFull-field strain and displacement measurements based on digital image correlation are performed on model single-fiber composites. The use of a dual-vision system allows strain measurements in the vicinity of the fiber-matrix interface, also allowing for the identification of critical strain and stress values corresponding to the initiation and propagation of debonding damage. The experimental data are used to calibrate an inverse identification approach that outputs the shape of the debonded interface along the fiber length.ResultsFull-field measurements allow for establishing correlations between local and global strain fields. Observation of debonding propagation along the fiber axis seems to be representative of the crack tunneling during the early stages of the failure process, i.e., when the crack tip is subjected to opening mode only.ConclusionsSide view measurements are useful as a first-order approximation of the debonding propagation velocity along the fiber axis but fail to provide accurate measurements for the debonding shape, esp. in areas where the crack is under a dominantly shear stress state. This issue can be resolved by full-field measurements coupled with computational simulations.

High sensitivity and directional recognition torsion sensor based on rotating distributed slot long-period fiber grating

In this paper, a rotationally distributed slot type long period fiber grating torsion sensor is proposed. The sensor employs CO2 laser to etch grooves on the fiber surface in a structure helically distributed slot around the fiber axis, which breaks the circular symmetry of the fiber and introduces a residual twisting force, thus introducing a large amount of elliptical birefringence. By investigating the effect of different periods on the torsional sensitivity, the torsional sensitivity is effectively improved. The experimental results indicate that the torsion sensitivity of the sensor is up to 0.42 nm/(rad/m) in the measurement range of −13.08 rad/m to 13.08 rad/m, which is 10 times more sensitive than that of the traditional torsion sensor, and the direction of torsion can be detected. The temperature characteristics of the sensor are also investigated, and the temperature sensitivity in the detection range of 30 °C to 120 °C is 76 pm/°C, with a temperature crosstalk of 0.18(rad/m)/°C. This sensor provides a new solution for LPFG torsion sensing enhancement, which can be used in the field of engineering structure monitoring with high torsion sensitivity requirements.

Revealing the failure mechanism of 2D triaxially braided composites under off-axial tension through mesoscale simulations

The external load angle is known to have a significant influence on the mechanical behavior of two-dimensional triaxially braided composites (2DTBCs). However, the experimental data for 2DTBCs under off-axial loading provide limited information for understanding the failure mechanisms. In this study, a comprehensive mesoscale finite element (FE) model for simulating 2DTBC specimens was established to evaluate the mechanical responses and damage characteristics when off-axial tensile loads are applied. The FE model effectively captured the mechanical response at five distinct angles (0°, 30°, 45°, 60°, and 90°) and revealed the evolving patterns of failure behavior, damage morphology, and out-of-plane deformation mechanisms corresponding to the different loading angles. The findings indicate that, when the external load aligns with the axial fiber bundle direction, the primary failure mechanism involves the fracture of load-bearing fiber bundles. In contrast, deviations from the axial loading direction resulted in failure that was primarily due to the undulation of the bias fiber bundles, resulting in a loading angle–dependent warping at the edge of the specimen due to local shear stress concentration. The findings of this study provide valuable insights that can inform the design of structures with improved application.

Numerical Study of Melt-Spinning Dynamic Parameters and Microstructure Development with Ongoing Crystallization.

In response to an investigation on the paths of changes in the crystallization and radial differences during the forming process of nascent fibers, in this study, we conducted numerical simulation and analyzed the changes in crystallization mechanical parameters and tensile properties through a fluid dynamics two-phase model. The model was based on the melt-spinning method focusing on melt spinning, the environment of POLYFLOW, and the method of joint simulation, coupled with Nakamura crystallization kinetics, including the development of process collaborative parameters, stretch-induced crystallization, viscoelasticity, filament cooling, gravity term, inertia, and air resistance. Finally, for nylon 6 BHS and CN9987 resin spinning, the model successfully predicted the distribution changes in temperature, velocity, strain rate tensor, birefringence, and stress tensor along the axial and radial fibers and obtained the variation pattern of fibers' crystallinity along the entire spinning process under different stretching rates. Furthermore, we also explored the effects of spinning conditions, including inlet flow rate, winding speeds, and the extrusion temperature, on the fibers' crystallization process and obtained the influence rules of different spinning conditions on fiber crystallization. Knowing the paths of changes in mechanical performance can provide important guidance and optimization strategies for the future industrial preparation of high-performance fibers.

Efficient numerical analysis of in-plane compression-induced defects in thick multi-ply woven textile preforms during double diaphragm forming

To gain adoption in the composites manufacturing sector, forming process models must be assessed for their suitability in analysing representative preform architectures processed in industrial production environments. A computationally efficient, mutually constrained Shell-Membrane (S-M) finite element approach is developed to predict the anisotropic, non-linear deformation of a woven textile preform in a Double Diaphragm Forming (DDF) process. Formability of a thick, multi-ply aerostructures preform is numerically investigated and experimentally correlated using a high-volume, industrial forming process. The effects of fibre orientation, preform thickness, and inter-ply friction on defect formation are investigated. The process model identifies in-plane axial fibre compression as the primary mechanism leading to macroscopic buckling defects and infers the underlying defect morphology. Defect formation is shown to be weakly correlated with the state of the fabric shear strain field. The study concludes that buckling defects are mitigated by reducing tangential contact friction between preform plies.

Emergence of Amphiphilicity on Surfaces of Pure Cellulose Nanofibrils Directly Generated by Aqueous Counter Collision Process.

The present paper describes a downsizing mechanism of an aqueous counter collision (ACC) process that enables the rapid preparation of cellulose nanofibrils (CNFs) as an aqueous dispersion solely by impinging a pair of water jets containing the raw materials. Extensive studies have revealed that the resulting CNFs by ACC have amphiphilic fiber surfaces, in which two kinds of faces with different natures are present along the entire fiber axis. They therefore have superior adsorption to surfaces of various conventional polymer plastics. These characteristic adsorption behaviors, which are totally different from those for other CNFs prepared by other means, are attributable to their hydrophobic surfaces. In the present study, high-resolution microscopy, including atomic force microscopy, confocal laser scanning microscopy, and scanning electron microscopy with broad argon ion beam milling, was used to determine how the emergence of such hydrophobic characteristics in a nanofibril face occurs in relation to the ACC nanopulverization mechanism due to the collision of the pair of water jets.

Multi-Tissue Integrated Tissue-Engineered Trachea Regeneration Based on 3D Printed Bioelastomer Scaffolds.

Functional segmental trachea reconstruction is a critical concern in thoracic surgery, and tissue-engineered trachea (TET) holds promise as a potential solution. However, current TET falls short in fully restoring physiological function due to the lack of the intricate multi-tissue structure found in natural trachea. In this research, a multi-tissue integrated tissue-engineered trachea (MI-TET) is successfully developed by orderly assembling various cells (chondrocytes, fibroblasts and epithelial cells) on 3D-printed PGS bioelastomer scaffolds. The MI-TET closely resembles the complex structures of natural trachea and achieves the integrated regeneration of four essential tracheal components: C-shaped cartilage ring, O-shaped vascularized fiber ring, axial fiber bundle, and airway epithelium. Overall, the MI-TET demonstrates highly similar multi-tissue structures and physiological functions to natural trachea, showing promise for future clinical advancements in functional TETs.

Nonreciprocal response of electrically biased graphene-coated fiber.

Here, we theoretically investigate the nonreciprocal response of an electrically biased graphene-coated dielectric fiber. By electrically biasing the graphene coating along the fiber axis, the dynamic conductivity of graphene exhibits a nonsymmetric response with respect to the longitudinal component of guided-mode wave vectors. Consequently, the guided waves propagating in two opposite directions may encounter distinct propagation features. In this work, the electromagnetic properties, such as modal dispersion and some field distributions, are presented, and the strength of nonreciprocity is discussed for different parameters of graphene, such as its chemical potential and material loss. Furthermore, the influence of the radius of the fiber on the nonreciprocal response is investigated. We envision that such nonreciprocal optical fibers may find various potential applications in the THz regime.

Collagen composite fibers with various functionalized carbon nanotubes fabricated via microfluidic spinning

AbstractCollagen (Col) composite fibers containing various functionalized multiwalled carbon nanotubes (MWNTs) were prepared via microfluidic spinning, and the influences of MWNT content and type on the performances of the composite fibers were investigated by transmission electron microscope, UV–vis, SEM, Fourier Transform Infrared Spectrometer, differential scanning calorimetry, X‐ray diffraction, and tensile testing. The Col composite fibers showed tightly packed bundle structures on surfaces originated from the high shear rates in the microfluidic channels, and MWNT facilitates the self‐assembly of Col molecules, leading to the ordered arrangement of Col fibrils along the fiber axis. When the loading of carboxylated MWNT (cMWNT) was 0.5 wt%, the tensile strength of Col/cMWNT achieved the maximum of 1.94 cN/dtex owing to the excellent hydrogen bond and electrostatic interactions between the carboxyl groups in cMWNT and amino groups in Col molecules, which is significantly higher than those composite fibers made from unfunctionalized and hydroxylated MWNT. Moreover, with the incorporation of MWNT the thermal stability and water resistance of Col fibers were improved due to the enhanced interfacial interactions between Col and MWNT. The fabrication method in this work enables the controlled formation of Col fibers and demonstrates huge potential for use in Col‐based biomaterials.Highlights Novel collagen (Col)/multiwalled carbon nanotube (MWNT) composite fibers were prepared via microfluidic spinning. The Col composite fibers showed tightly packed bundle surface structures. Col/carboxylated MWNT achieved the maximum tensile strength of 1.94 cN/dtex. MWNT enhanced thermal stability and water resistance of Col fibers.

Three-dimensional fiber orientation mapping of the human brain at micrometer resolution.

The accurate measurement of three-dimensional (3D) fiber orientation in the brain is crucial for reconstructing fiber pathways and studying their involvement in neurological diseases. Comprehensive reconstruction of axonal tracts and small fascicles requires high-resolution technology beyond the ability of current in vivo imaging (e.g. diffusion magnetic resonance imaging). Optical imaging methods such as polarization-sensitive optical coherence tomography (PS-OCT) and polarization microscopy can quantify fiber orientation at micrometer resolution but have been limited to two-dimensional in-plane orientation or thin slices, preventing the comprehensive study of connectivity in 3D. In this work we present a novel method to quantify volumetric 3D orientation in full angular space with PS-OCT. We measure the polarization contrasts of the brain sample from two illumination angles of 0 and 15 degrees and apply a computational method that yields the 3D optic axis orientation and true birefringence. We further present 3D fiber orientation maps of entire coronal cerebrum sections and brainstem with 10 μm in-plane resolution, revealing unprecedented details of fiber configurations. We envision that our method will open a promising avenue towards large-scale 3D fiber axis mapping in the human brain as well as other complex fibrous tissues at microscopic level.

Myopia and Other Refractive Error and Their Relationships to Glaucoma Screening.

A large disk, a large parapapillary delta zone and a long axial length may be used as screening criteria to detect glaucomatous optic neuropathy in highly myopic eyes. To describe aspects for screening of glaucomatous optic neuropathy in dependence of refractive error, under special consideration of high myopia. Studies on the anatomy of the myopic optic nerve head and results of investigations on the relationship between glaucomatous optic neuropathy and axial myopia were included. In the range from hyperopia to moderate myopia, refractive error is not a strong glaucoma risk factor and may not be included in glaucoma screening strategies. Care should be taken, that in moderate myopia, a shift of Bruch´s membrane opening usually into the temporal direction leads to parapapillary gamma zone and a corresponding shortening of the horizontal disk diameter. In these moderately myopic eyes, a secondarily small optic disk with a correspondingly small optic cup should not lead to an overlooking of intrapapillary glaucomatous changes. Prevalence of glaucomatous or glaucoma-like optic nerve atrophy (GOA) steeply increases with longer axial length in highly myopic eyes (cutoff approximately -8 diopters/axial length 26.5mm), with prevalences higher than 50% in extremely high myopia. Besides longer axial length, morphological parameters associated with GOA in highly myopic eyes are a secondarily enlarged disk and large parapapillary delta zone. Both parameters, together with long axial length, may be used as screening criteria in high myopia for GOA. The latter is characterized by an abnormal neuroretinal rim shape, that is, vessel kinking close to the intrapapillary disk border. Factors associated with nonglaucomatous optic neuropathy are larger gamma zone and longer axial length, potentially due to an axial elongation-related retinal nerve fiber stretching.

Crashworthiness analysis and optimization design of special-shaped thin-walled tubes by experiments and numerical simulation

Thin-walled structures are widely used as typical high-efficiency energy-absorbing structures in crash safety protection. This paper aims to combine the characteristic of the square tube which concentrates energy dissipation through angle elements with the stable fold deformation of the circular tube, and innovatively propose a special-shaped thin-walled tube that contains both angle elements and arc elements. The failure behavior and energy absorption characteristics of the special-shaped tube were investigated by compression test and numerical simulation. The simulation results showed that the special-shaped thin-walled tubes have a significant advantage in energy absorption, and the specific energy absorption of the special-shaped tubes is 31.57% and 75.2% higher than that of the equal-mass circular and square tubes, respectively. Next, through finite element analysis, the effects of various parameters and initial imperfections on the crashworthiness of the special-shaped tubes were compared. The results indicated that adjustments to geometrical parameters significantly impact the energy absorption properties and crushing behavior of thin-walled tubes. Moreover, the tube with induced circular holes exhibited an optimal deformation mode. Further, a multi-objective optimal design of the metal special-shaped tube was carried out to obtain the best parameter configuration of the metal special-shaped tube. Finally, the special-shaped structure was extended to CFRP/AL hybrid thin-walled tubes. The failure behavior and energy absorption enhancement mechanism of the hybrid special-shaped tubes were investigated with experiments, and the absorbed energy of the hybrid tube was about 20% higher than the sum of the absorbed energy of the single constituent tubes. The effect of the CFRP lay-up pattern on the crashworthiness of the hybrid tube was investigated by numerical simulation, and it was found that the energy absorption capacity of the hybrid tube could be improved by appropriately increasing the number of layers and the proportion of axial fibers.

Conducting Electrospun Poly(3-hexylthiophene-2,5-diyl) Nanofibers: New Strategies for Effective Chemical Doping and its Assessment Using Infrared Spectroscopy.

Vibrational spectroscopy allows the investigation of structural properties of pristine and doped poly(3-hexylthiophene-2,5-diyl) (P3HT) in highly anisotropic materials, such as electrospun micro- and nanofibers. Here, we compare several approaches for doping P3HT fibers. We have selected two different electron acceptor molecules as dopants, namely iodine and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). In the case of iodine, we have explored the doping of the fibers according to several different procedures, i.e., by sequential doping both in vapors and in solution, and with a novel promising one-step method, which exploits the mixing of the dopant to the electrospinning feed solution. Polarized infrared (IR) spectroscopy experiments prove the orientation of P3HT chains, with the polymer backbone mainly running parallel to the fiber axis. After doping, P3HT fibers show very strong and polarized doping-induced IR active vibrations (IRAVs), which are the spectroscopic signature of the structure relaxation induced by the charged defects (polarons), thus providing an unambiguous proof of the effective doping. Raman spectroscopy complements the IR evidence: The Raman spectrum shows a clearly recognizable shift of the main band, the so-called effective conjugation coordinate band, in the doped samples. A simple protocol, which quantifies the evolution of the IRAV bands with time, allows monitoring of the doping stability over time and confirms that F4TCNQ is by far superior to iodine.