Raman tensor analysis of ultra-high molecular weight polyethylene and its application to study retrieved hip joint components

3D Orientation Imaging of Polymer Chains with Polarization-Controlled Coherent Raman Microscopy

Despite the ubiquity of three-dimensional (3D) anisotropic materials, their 3D molecular alignment cannot be measured using conventional two-dimensional (2D) polarization imaging. Here, we present images of the 3D angles of molecular orientations with submicrometer spatial resolution acquired through polarization-controlled coherent anti-Stokes Raman scattering microscopy. The hyperspectral Raman data of a polyethylene (PE) film were converted into images, showing the polymer chains' 3D angles and order parameters. The 3D orientation images of PE chains in ring-banded spherulites show that the azimuthal angles of the chains are perpendicular to the crystal growth direction, while the out-of-plane angles display limited-range oscillations synchronous with ring banding. The prevailing crystal growth model of fully twisting lamellae is inconsistent with the observed restricted oscillations of the out-of-plane direction, which are unobservable through conventional 2D projected imaging. This high-resolution, label-free, quantitative imaging of 3D molecular orientation can become a standard measurement tool for the microscopic structures of complex synthetic and biological materials.

Probing fibrin's molecular response to shear and tensile deformation with coherent Raman microscopy

Blood clots are essential biomaterials that prevent blood loss and provide a temporary scaffold for tissue repair. In their function, these materials must be capable of resisting mechanical forces from hemodynamic shear and contractile tension without rupture. Fibrin networks, the primary load-bearing element in blood clots, have unique nonlinear mechanical properties resulting from fibrin's hierarchical structure. This structure provides multiscale load bearing from fiber deformation to protein unfolding. Here, we study the fiber and molecular scale response of fibrin under shear and tensile loads in situ using a combination of fluorescence and vibrational (molecular) microscopy. Imaging protein fiber orientation and molecular vibrations, we find that fiber alignment and molecular unfolding in fibrin appear at much larger strains under shear compared to uniaxial tension. Alignment levels reached at 150% shear strain were reached already at 60% tensile strain, and molecular unfolding of fibrin was only detected at shear strains above 300%, whereas fibrin unfolding began already at 20% tensile strain. Moreover, shear deformation caused progressive changes in vibrational modes consistent with increased protofibril and fiber packing that were already present even at very low tensile deformation. Together with a bioinformatic analysis of the primary fibrinogen structure, we propose a scheme for the molecular response of fibrin from low to high deformation, which may relate to the teleological origin of fibrin's resistance to shear and tensile forces.

Wear Simulation of Ceramic-on-Crosslinked Polyethylene Hip Prostheses: A New Non-Oxide Silicon Nitride versus the Gold Standard Composite Oxide Ceramic Femoral Heads

The purpose of the present study was to compare the wear behavior of ceramic-on-vitamin-E-diffused crosslinked polyethylene (Vit-E XLPE) hip bearings employing the gold standard oxide ceramic, zirconia (ZrO₂)-toughened alumina (Al₂O₃) (ZTA, BIOLOX^®delta) and a new non-oxide ceramic, silicon nitride (Si₃N₄, MC^2®). In vitro wear test was performed using a 12-station hip joint simulator. The test was carried out by applying the kinematic inputs and outputs as recommended by ISO 14242-1:2012. Vitamin-E-diffused crosslinked polyethylene (Vit-E XLPE) acetabular liners (E1^®) were coupled with Ø28-mm ZTA and Si₃N₄ femoral heads. XLPE liner weight loss over 5 million cycles (Mc) of testing was compared between the two different bearing couples. Surface topography, phase contents, and residual stresses were analyzed by contact profilometer and Raman microspectroscopy. Vit-E XLPE liners coupled with Si₃N₄ heads produced slightly lower wear rates than identical liners with ZTA heads. The mean wear rates (corrected for fluid absorption) of liners coupled with ZTA and Si₃N₄ heads were 0.53 ± 0.24 and 0.49 ± 0.23 mg/Mc after 5 Mc of simulated gait, respectively. However, after wear testing, the ZTA heads retained a smoother topography and showed fewer surface stresses than the Si₃N₄ ones. Note that no statistically significant differences were found in the above comparisons. This study suggests that the tribochemically formed soft silica layer on the Si₃N₄ heads may have reduced friction and slightly lowered the wear of the Vit-E XLPE liners. Considering also that the toughness of Si₃N₄ is superior to ZTA, the present wear data represent positive news in the future development of long-lasting hip components.

Novel Polyethylene Fibers of Very High Thermal Conductivity Enabled by Amorphous Restructuring

High-thermal-conductivity polymers are very sought after for applications in various thermal management systems. Although improving crystallinity is a common way for increasing the thermal conductivity (k) of polymers, it has very limited capacity when the crystallinity is already high. In this work, by heat-stretching a highly crystalline microfiber, a significant k enhancement is observed. More interestingly, it coincides with a reduction in crystallinity. The sample is a Spectra S-900 ultrahigh-molecular-weight polyethylene (UHMW-PE) microfiber of 92% crystallinity and high degree of orientation. The optimum stretching condition is 131.5 °C, with a strain rate of 0.0129 s-1 to a low strain ratio (∼6.6) followed by air quenching. The k enhancement is from 21 to 51 W/(m·K), the highest value for UHMW-PE microfibers reported to date. X-ray diffraction study finds that the crystallinity reduces to 83% after stretching, whereas the crystallite size and crystallite orientation are not changed. Cryogenic thermal characterization shows a reduced level of phonon-defect scattering near 30 K. Polarization Raman spectroscopy finds enhanced alignment of amorphous chains, which could be the main reason for the k enhancement. A possible relocation of amorphous phase is also discussed and indirectly supported by a bending test.

Raman spectroscopy of biomedical polyethylenes

With the development of three-dimensional Raman algorithms for local mapping of oxidation and plastic strain, and the ability to resolve molecular orientation patterns with microscopic spatial resolution, there is an opportunity to re-examine many of the foundations on which our understanding of biomedical grade ultra-high molecular weight polyethylenes (UHMWPEs) are based. By implementing polarized Raman spectroscopy into an automatized tool with an improved precision in non-destructively resolving Euler angles, oxidation levels, and microscopic strain, we become capable to make accurate and traceable measurements of the in vitro and in vivo tribological responses of a variety of commercially available UHMWPE bearings for artificial hip and knee joints. In this paper, we first review the foundations and the main algorithms for Raman analyses of oxidation and strain of biomedical polyethylene. Then, we critically re-examine a large body of Raman data previously collected on different polyethylene joint components after in vitro testing or in vivo service, in order to shed new light on an area of particular importance to joint orthopedics: the microscopic nature of UHMWPE surface degradation in the human body. A complex scenario of physical chemistry appears from the Raman analyses, which highlights the importance of molecular-scale phenomena besides mere microstructural changes. The availability of the Raman microscopic probe for visualizing oxidation patterns unveiled striking findings related to the chemical contribution to wear degradation: chain-breaking and subsequent formation of carboxylic acid sites preferentially occur in correspondence of third-phase regions, and they are triggered by emission of dehydroxylated oxygen from ceramic oxide counterparts. These findings profoundly differ from more popular (and simplistic) notions of mechanistic tribology adopted in analyzing joint simulator data. Statement of Significance This review was dedicated to the theoretical and experimental evaluation of the commercially available biomedical polyethylene samples by Raman spectroscopy with regard to their molecular textures, oxidative patterns, and plastic strain at the microscopic level in the three dimensions of the Euclidean space. The main achievements could be listed, as follow: (i) visualization of molecular patterns at the surface of UHMWPE bearings operating against metallic components; (ii) differentiation between wear and creep deformation in retrievals; (iii) non-destructive mapping of oxidative patterns; and, (iv) the clarification of chemical interactions between oxide/non-oxide ceramic heads and advanced UHMWPE liners.

Vitamin E-stabilized highly crosslinked polyethylenes: The role and effectiveness in total hip arthroplasty

Morphology and design of ultra-high molecular weight polyethylene (UHMWPE or simply PE) acetabular components used in total hip arthroplasty (THA) have been evolving for more than half a century. Since the late-1990s, there were two major technological innovations in PE emerged from necessity to overcome the wear-induced periprosthetic osteolysis, i.e., the development of highly crosslinked PEs (HXLPEs). There are many literature reporting that radiation crosslinked and remelted/annealed (first-generation) HXLPEs markedly reduced the incidence of osteolysis and aseptic loosening. Regardless of such clinical success in the first-generation technologies, there were some recent shifts in Japan toward the use of new second-generation HXLPEs subjected to sequential irradiation/annealing or antioxidant vitamin E (α-tocopherol) incorporation. Although the selection rate of first-generation liners still account for more than half of all the PE THAs (∼58% in 2015), the use of vitamin E-stabilized liners has been steadily growing each year since their clinical introduction in 2010. In these contexts, it is of great importance to evaluate and understand the real clinical benefits of using the new second-generation liners as compared to the first generation. This article first summarizes structural evolution and characteristic features of first-generation HXLPEs, and then provides a detailed description of second-generation antioxidant HXLPEs in regard to the role of vitamin E incorporation on their chemical and mechanical performances in THA.

Raman spectroscopic investigation on the molecular structure of apatite and collagen in osteoporotic cortical bone

This study employed highly spectrally resolved Raman spectroscopy to examine the molecular composition of cortical bone tissue obtained from murine females in their healthy and ovariectomy- (OVX-) induced osteoporotic states. The aim of the study was to identify structural differences at the molecular scale both in apatite mineral and collagen fibrils between the two groups of samples. Raman spectroscopy was used to determine the chemical composition of cortical bone in regions including characteristic bands of both bone mineral and bone matrix. The results demonstrated that the mineral apatite of bone did not undergo significant amorphization in its diseased state, with the Raman microprobe also failing in recognizing a direct role of carbonate content in the embrittlement of OVX-diseased bone. On the other hand, complex off-stoichiometry variations could be detected in the columnar Ca-structure of the bony hydroxyapatite according to morphological variations of the Raman band belonging to the symmetric phosphate stretching (A₁) band at ~959cm^-1. A fundamental role was also recognized for collagen quality on the process of bone embrittlement. The so-called matrix maturity ratio, as systematically measured on Raman spectra in the Amide I region, increased with statistical significance in OVX-treated samples as compared to control samples. An 8% increase could be associated to a 115% increase in elastic stress intensification in the mineral phase of OVX-diseased tissue as compared to the control one, thus proving a degradation in the (elastic) energy-dissipative capacity of a diseased bone matrix.

Effects of vitamin E blending on plastic deformation mechanisms of highly crosslinked ultrahigh molecular weight polyethylene (HXL-UHMWPE) in total hip arthroplasty

The molecular mobility and crystalline texture development in highly crosslinked ultrahigh molecular weight polyethylene (HXL-UHMWPE) blended with antioxidant vitamin E (VE, dl-α-tocopherol) were studied via uniaxial compression at room temperature by means of confocal/polarized Raman spectroscopy. The results were compared to morphological analyses under the same compression conditions performed on HXL-UHMWPE prepared in exactly the same way but blending VE into the polyethylene resin (VE-free HXL-UHMWPE). These comparative analyses allow us to evaluate the physical role of VE in morphological alterations of HXL-UHMWPE induced by compression deformation, which can greatly affect its micromechanical behavior. Molecular rearrangement and phase transitions in crystalline and non-crystalline phase, i.e. amorphous and intermediate (third) phase, were found to be part of a reconstruction process after plastic deformation in the samples. Although VE-blended HXL-UHMWPE exhibited more pronounced molecular mobility, as evidenced by its significant deformation-induced texturing, crystallinity change was totally inhibited by the presence of VE during deformation. On the other hand, amorphous-to-intermediate phase transition was confirmed. VE-free HXL-UHMWPE also presented significant crystallization after deformation, but its surface texture evolution occurred to a much lesser extent. This study suggests that the addition of VE induced earlier activation of compression deformation modes in crystalline and non-crystalline phases (e.g. chain slip, interlamellar shear and rotation) due to an increase in polyethylene chain mobility.

Vibrational algorithms for quantitative crystallographic analyses of hydroxyapatite-based biomaterials: I, theoretical foundations

The Raman spectroscopic method has quantitatively been applied to the analysis of local crystallographic orientation in both single-crystal hydroxyapatite and human teeth. Raman selection rules for all the vibrational modes of the hexagonal structure were expanded into explicit functions of Euler angles in space and six Raman tensor elements (RTE). A theoretical treatment has also been put forward according to the orientation distribution function (ODF) formalism, which allows one to resolve the statistical orientation patterns of the nm-sized hydroxyapatite crystallite comprised in the Raman microprobe. Close-form solutions could be obtained for the Euler angles and their statistical distributions resolved with respect to the direction of the average texture axis. Polarized Raman spectra from single-crystalline hydroxyapatite and textured polycrystalline (teeth enamel) samples were compared, and a validation of the proposed Raman method could be obtained through confirming the agreement between RTE values obtained from different samples.

Mechanisms of plastic deformation in highly cross-linked UHMWPE for total hip components--the molecular physics viewpoint

Plastic deformation is an unavoidable event in biomedical polymeric implants for load-bearing application during long-term in-vivo service life, which involves a mass transfer process, irreversible chain motion, and molecular reorganization. Deformation-induced microstructural alterations greatly affect mechanical properties and durability of implant devices. The present research focused on evaluating, from a molecular physics viewpoint, the impact of externally applied strain (or stress) in ultra-high molecular weight polyethylene (UHMWPE) prostheses, subjected to radiation cross-linking and subsequent remelting for application in total hip arthroplasty (THA). Two different types of commercial acetabular liners, which belong to the first-generation highly cross-linked UHMWPE (HXLPE), were investigated by means of confocal/polarized Raman microprobe spectroscopy. The amount of crystalline region and the spatial distribution of molecular chain orientation were quantitatively analyzed according to a combined theory including Raman selection rules for the polyethylene orthorhombic structure and the orientation distribution function (ODF) statistical approach. The structurally important finding was that pronounced recrystallization and molecular reorientation increasingly appeared in the near-surface regions of HXLPE liners with increasing the amount of plastic (compressive) deformation stored in the microstructure. Such molecular rearrangements, occurred in response to external strains, locally increase surface cross-shear (CS) stresses, which in turn trigger microscopic wear processes in HXLPE acetabular liners. Thus, on the basis of the results obtained at the molecular scale, we emphasize here the importance of minimizing the development of irrecoverable deformation strain in order to retain the pristine and intrinsically high wear performance of HXLPE components.

Highly cross-linked polyethylene in total hip and knee replacement: spatial distribution of molecular orientation and shape recovery behavior

The present study investigated effects of processing procedures on morphology of highly cross-linked and re-melted UHMWPE (XLPE) in total hip and knee arthroplasty (THA, TKA). The shape recovery behavior was also monitored via uniaxial compression test at room temperature after non-destructive characterizations of the in-depth microstructure by confocal/polarized Raman spectroscopy. The goal of this study was to relate the manufacturing-induced morphology to the in vivo micromechanical performance, and ultimately to explore an optimal structure in each alternative joint bearing. It was clearly confirmed that the investigated XLPE hip and knee implants, which were produced from different orthopaedic grade resins (GUR 1050 and GUR 1020), consisted of two structural regions in the as-received states: the near-surface transitional anisotropic layer (≈100 μm thickness) and the bulk isotropic structural region. These XLPEs exhibited a different crystalline anisotropy and molecular texture within the near-surface layers. In addition, the knee insert showed a slightly higher efficiency of shape recovery against the applied strain over the hip liner owing to a markedly higher percentage of the bulk amorphous phase with intermolecular cross-linking. The quantitative data presented in this study might contribute to construct manufacturing strategies for further rationalized structures as alternative bearings in THA and TKA.

Bioceramics for Hip Joints: The Physical Chemistry Viewpoint

Which intrinsic biomaterial parameter governs and, if quantitatively monitored, could reveal to us the actual lifetime potential of advanced hip joint bearing materials? An answer to this crucial question is searched for in this paper, which identifies ceramic bearings as the most innovative biomaterials in hip arthroplasty. It is shown that, if in vivo exposures comparable to human lifetimes are actually searched for, then fundamental issues should lie in the physical chemistry aspects of biomaterial surfaces. Besides searching for improvements in the phenomenological response of biomaterials to engineering protocols, hip joint components should also be designed to satisfy precise stability requirements in the stoichiometric behavior of their surfaces when exposed to extreme chemical and micromechanical conditions. New spectroscopic protocols have enabled us to visualize surface stoichiometry at the molecular scale, which is shown to be the key for assessing bioceramics with elongated lifetimes with respect to the primitive alumina biomaterials used in the past.

Vitamin-E blended and infused highly cross-linked polyethylene for total hip arthroplasty: a comparison of three-dimensional crystalline morphology and strain recovery behavior

Vitamin-E (α-tocopherol) is now recognized worldwide as one of the most promising antioxidant agents for highly cross-linked polyethylene (HXLPE) used in total joint replacements. In the contemporary manufacturing processes, two alternative methods are currently accepted to incorporate this antioxidant into polyethylene microstructure: (i) blending vitamin-E before consolidation and radiation crosslinking; (ii) infusing vitamin-E via a homogenizing heat treatment after radiation crosslinking. However, the effects of these technological differences on crystalline morphology and mechanical behavior of polyethylene remains to be fully elucidated. The aim of this paper is to quantitatively evaluate the microstructural differences of commercially available vitamin-E blended and infused HXLPE liner (referred to as Liner BL and IF, respectively). For this purpose, confocal/polarized Raman spectroscopy was used to systematically examine the three-phase percentages (amorphous (αa), crystalline (αc), and intermediate third phase (αt)), preferential molecular orientation (θp), and degree of crystalline anisotropy (〈P2(cosβ)〉). Additionally, we compared the time-dependent deformation of Liner BL and IF as obtained by uniaxial stress relaxation tests followed by strain recovery. Distinctive features of the near-surface αc, θp, and〈P2(cosβ)〉 were clearly observed within the first 35μm in the two studied liners. Despite the equivalent level of the bulk αc and 〈P2(cosβ)〉, higher restoring force against a uniaxial strain was observed in Liner IF, which reflects a higher crosslink density in its amorphous phase. On the other hands, a higher degree of surface orientational randomness was detected in Liner BL, which is structurally more beneficial for minimizing the in-vivo occurrence of strain-softening-assisted wear.

Raman spectroscopic study of remelting and annealing-induced effects on microstructure and compressive deformation behavior of highly crosslinked UHMWPE for total hip arthroplasty

Three-dimensional crystallographic morphologies were studied by means of confocal/polarized Raman spectroscopy as developed upon manufacturing in three different types of first and second generation highly crosslinked UHMWPE (HXLPE) acetabular liners. The impact of such microstructural characteristics on the deformation behavior of the liners was also evaluated and discussed from the viewpoint of molecular chain mobility. All the investigated liners showed similar microstructural transitions within the first 35 μm below their surfaces in terms of crystallinity, molecular orientation, and crystalline anisotropy. Interestingly, different postirradiation heat treatments (remelting or annealing in single step or in sequential steps) led to clear differences in the subsurface microstructure among the three liners. Remelted liner possessed both lower bulk crystallinity and degree of molecular orientation as compared to the annealed liners. Sequentially, irradiated/annealed liner showed the highest degree of crystallinity and orientation among the studied liners. The peculiar microstructure of this latter liner exhibited the highest restoring (shape-recovery) force against the applied uniaxial strain. Accordingly, the present study suggests that the sequential irradiation and annealing offers an efficient way to obtain microstructure quite suitable for attaining high creep resistance. However, all the investigated liners exhibited the significantly low values of surface anisotropy, which could be equally efficient in minimizing strain-softening-assisted wear phenomena.

Characterization of network parameters for UHMWPE by plane strain compression

Ultra-high molecular weight polyethylene (PE) is used as a bearing material for total joint replacement prostheses since it is a tough, wear-resistant semicrystalline polymer. Despite its high resistance to wear, PE components have shown measureable wear in vivo, which can cause wear-particle induced osteolysis. Crosslinking of PE using ionizing radiation has been shown to increase wear resistance since both chemical crosslinks and physical entanglements provide high resistance to wear. Molecular characterization of crosslinked PEs is usually conducted using equilibrium swelling or by quantifying gel content. In this study, we compared crosslink densities and molecular weight between crosslinks derived from equilibrium swelling to those obtained by applying the Gaussian and Eight-Chain model to describe plane strain compression of the PE melt. The latter approach has the advantage of accounting for contributions of entanglements to the overall crosslink density, which solvent-based techniques largely neglect. As expected, the crosslink density calculated from model fitting increased monotonically with increase in radiation dose in a 0-200kGy dose range, with a corresponding monotonic decrease in molecular weight between crosslinks, but provided higher values of crosslink density and correspondingly lower values of molecular weight between crosslinks compared to the equilibrium swelling technique.

Strain-induced microstructural rearrangement in ultra-high molecular weight polyethylene for hip joints: A comparison between conventional and vitamin E-infused highly-crosslinked liners

Infusion of vitamin E (α-tocopherol) in highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) liners has been conceived to achieve superior oxidation stability while preserving enhanced mechanical properties as compared to post-irradiation remelted liners. However, the presence of an antioxidant in the material microstructure brings an issue of concern in whether a "foreign substance" might reduce radiation crosslinking efficiency and/or change microstructural characteristics by diffusing into UHMWPE. The key to clarify this fundamental issue resides in performing a quantitative evaluation of the obtained material structure and its polymeric chain mobility on the molecular scale. In this paper, a Raman spectroscopic examination is presented of molecular orientation and phase fractions in as-processed vitamin E-infused UHMWPE acetabular liners in comparison with a model (undoped and unirradiated/uncrosslinked) and a conventional (undoped and 33kGy-sterilized by gamma-irradiation) UHMWPE liners. The microstructural responses of structurally different liners to externally applied compressive strain were also monitored. The main results of the spectroscopic analyses can be summarized as follows: (i) preliminary gamma irradiation reduced the fraction of amorphous phase and increased the degree of molecular alignment, the vitamin E-infused liner preserving the crystallinity level achieved by the 100-kGy irradiation injected before infusion; (ii) the presence of vitamin E significantly promoted orientational randomness, which increased with increasing compressive strain magnitude, a phenomenon beneficial to minimize strain-softening-assisted wear phenomena.

In situ Raman spectrometric analysis of crystallinity and crystal polymorphism of fat in porcine adipose tissue

From the adipose tissues of pork carcasses stored in a refrigerator, Raman spectra were observed in situ by a portable Raman spectrometer. The observed Raman spectra, which were almost completely due to fat, showed clear dependence on the refrigeration time and carcass temperature. This dependence reflected an increase in the crystallinity of the fat and a change in the fraction of the β' polymorph. Evidence of changes in the packing order of the aliphatic chains of acylglycerol molecules was obtained, and the changes lasted for a long time after the temperature reached the lowest point (4.3 °C). Possibilities of using Raman spectrometry as a tool for routine monitoring of the conditions of carcasses as well as for research on the improvement of the mechanical strength of the adipose tissue are discussed.