Since one hundred years it is known that some scarab beetles reflect elliptically and near-circular polarized light as demonstrated by Michelson for the beetle Chrysina resplendens. The handedness of the polarization is in a majority of cases left-handed but also right-handed polarization has been found. In addition, brilliant colors with metallic shine are observed. The polarization and color effects are generated in the beetle exoskeleton, the so-called cuticle. The objective of this work is to demonstrate that structural parameters and materials optical functions of these photonic structures can be extracted by advanced modeling of spectral multi-angle Mueller-matrix data recorded from beetle cuticles. A dual-rotating compensator ellipsometer is used to record normalized Mueller-matrix data in the spectral range 400 – 800 nm at angles of incidence in the range 25–75°. Analysis of data measured on the scarab beetle Cetonia aurata are presented in detail. The model used in the analysis mimics a chiral nanostructure and is based on a twisted layered structure. Given the complexity of the nanostructure, an excellent fit between experimental and model data is achieved. The obtained model parameters are the spectral variation of the refractive indices of the cuticle layers and structural parameters of the chiral structure.

Total joint replacements currently have relatively high success rates at 10–15 years; however, increasing ageing and an active population places higher demands on the longevity of the implants. A wear resistant configuration with wear particles that resorb in vivo can potentially increase the lifetime of an implant. In this study, silicon nitride (Si_xN_y) and silicon carbon nitride (Si_xC_yN_z) coatings were produced for this purpose using reactive high power impulse magnetron sputtering (HiPIMS). The coatings are intended for hard bearing surfaces on implants. Hardness and elastic modulus of the coatings were evaluated by nanoindentation, cohesive, and adhesive properties were assessed by micro-scratching and the tribological performance was investigated in a ball-on-disc setup run in a serum solution. The majority of the Si_xN_y coatings showed a hardness close to that of sintered silicon nitride (∼18 GPa), and an elastic modulus close to that of cobalt chromium (∼200 GPa). Furthermore, all except one of the Si_xN_y coatings offered a wear resistance similar to that of bulk silicon nitride and significantly higher than that of cobalt chromium. In contrast, the Si_xC_yN_z coatings did not show as high level of wear resistance.

SiN_x and SiC_xN_y coatings were fabricated with high power impulse magnetron sputtering (HiPIMS). The coatings microstructure, growth pattern, surface morphology, composition, and bonding structure were investigated by AFM, SEM, GIXRD, TEM, EDS as well as XPS, and related to the deposition parameters target powers and substrate temperature. Cross-sections of SiC_xN_y coatings showed either dense and laminar, or columnar structures. These coatings varied in roughness (Ra between 0.2 and 3.8 nm) and contained up to 35 at.% C. All coatings were substoichiometric (with an N/Si ratio from 0.27 to 0.65) and contained incorporated particles (so called droplets). The SiN_x coatings, in particular those deposited at the lower power on the silicon target, demonstrated a dense microstructure and low surface roughness (Ra between 0.2 and 0.3 nm). They were dominated by an (X-ray) amorphous structure and consisted mainly of Si–N bonds. The usefulness of these coatings is discussed for bearing surfaces for hip joint arthroplasty in order to prolong their life-time. The long-term aim is to obtain a coating that reduces wear and metal ion release, that is biocompatible, and with wear debris that can dissolve in vivo.

Many of the failures of total joint replacements are related to tribology, i.e., wear of the cup, head and liner. Accumulation of wear particles at the implants can be linked to osteolysis which leads to bone loss and in the end aseptic implant loosening. Therefore it is highly desirable to reduce the generation of wear particles from the implant surfaces.

Silicon nitride (Si₃N₄) has shown to be biocompatible and have a low wear rate when sliding against itself and is therefore a good candidate as a hip joint material. Furthermore, wear particles of Si₃N₄ are predicted to slowly dissolve in polar liquids and they therefore have the potential to be resorbed in vivo, potentially reducing the risk for aseptic loosening.

In this study, it was shown that α-Si₃N₄-powder dissolves in PBS. Adsorption of blood plasma indicated a good acceptance of Si₃N₄ in the body with relatively low immune response. Si₃N₄ sliding against Si₃N₄ showed low wear rates both in bovine serum and PBS compared with the other tested wear couples. Tribofilms were built up on the Si₃N₄ surfaces both in PBS and in bovine serum, controlling the friction and wear characteristics.

Polarization effects in reflection from the cuticle of scarab beetles studied by spectroscopic Mueller-matrix ellipsometry

H. Arwin^*, T. Berlind, J. Birch, L. Fernandez Del Rio, J. Gustafson, J. Landin,

R. Magnusson, C. Åkerlind, and K. Järrendahl

Department of Physics, Chemistry and Biology, Linköping University, Sweden

^*corresponding author: han@ifm.liu.se

Abstract- Many scarab beetles exhibit structural colors and complex polarization phenomena in reflection. These effects are characterized with spectroscopic Mueller-matrix ellipsometry in our work. The polarization ellipse of reflected light as well as the degree of polarization is presented including variations with angle of incidence and wavelength. Emphasis is on beetles showing chiral effects and structural modeling of cuticle nanostructure is discussed.

Background Since one hundred years it is known that some scarab beetles reflect elliptically polarized light as demonstrated by Michelson for the beetle Chrysina resplendens [1]. The handedness of the polarization is in a majority of the cases left-handed but also right-handed polarization has been found [2,3]. The ellipticity varies with wavelength and viewing angle but can be close to +1 or -1 (right or left circular polarization, respectively) and in addition these beetles may exhibit beautiful structural colors. The polarization and color effects are generated in the outer part of the exoskeleton, the cuticle. These natural photonic structures are often multifunctional and play important roles for survival of beetles, e.g. for hiding from or scaring predators, for intraspecies communication, etc. [4]. However, such structures may find use in many commercial applications and a major motivation for detailed studies of natural photonic structures is that they inspire to biomimetic applications [5,6].

Approach Our objective is to use spectral Mueller-matrix data on scarab beetles to parameterize reflection properties in terms of polarization parameters and degree of polarization. The studied beetles all are phytophagous and include species from the Cetoniinae subfamily (e.g. Cetonia aurata and Coptomia laevis,), the Rutelinae subfamily (e.g. Chrysina argenteola and Chrysina resplendens) and the Melolonthinae subfamily (Cyphochilus insulanus). Furthermore, structural modeling is presented on Cetonia aurata and a few more beetles to demonstrate that structural parameters can be extracted by advanced modeling of Mueller-matrix data.

Experimental A dual rotating compensator ellipsometer (RC2, J. A. Woollam Co., Inc.) is used to record all 16 Mueller-matrix elements m_ij (i,j=1..4) in the spectral range 300 – 900 nm at angles of incidence in the range 20-70º. The elements are normalized to m₁₁ and thus have values between -1 and +1. All measurements are performed on the scutellum (a small triangular part on the dorsal side of the beetles) with focusing optics resulting in a spot size of the order of 50-100 mm. The software CompleteEASE (J. A. Woollam Co., Inc.) is used for analysis.

Results and discussion As an example, Fig. 1 shows contour plots of Mueller-matrix data measured on Cetonia aurata. This beetle has a metallic shine and if illuminated with unpolarized white light it reflects left-handed polarized green light as revealed by the non-zero Mueller-matrix elements m₁₄ and m₄₁ in the green spectral region for angles of incidence below about 45º. This is clearly seen in the graph to the right in Fig. 1 which shows a spectrum for Mueller-matrix element m₄₁at 20º as well as fitted model data. A model based on a twisted lamella structure, also called Bouligand structure, is used to model the chiral nanostructure [4]. Given the complexity of the nanostructure, an excellent model fit is achieved. The obtained model parameters are the spectral variation of the refractive index of the birefringent lamellas and the pitch. The model also includes a dielectric surface layer.

Fig.1. Left: Mueller-matrix data on Cetonia aurata. Each contour plot shows m_ij, where i and j correspond to the row and column, respectively. m₁₁ =1 and is not shown but is replaced with a photo of the beetle. Right: Experimental and model-generated Mueller-matrix element m₄₁at an angle of incidence of 20º.

From the Mueller-matrix data one can also determine so called derived parameters including azimuth and ellipticity of the polarization ellipse and the degree of polarization. The variations of these parameters with angle of incidence are presented for a selection of scarab beetles. Examples of both left-handed and right-handed polarization effects are shown and the importance of degree of polarization will be discussed.

Concluding remarks Mueller-matrix spectra at oblique incidence are very rich in information about reflection properties and allows parameterization of polarization parameters of the reflected light. Both left-handed and right-handed reflected light is found in scarab beetles. Mueller-matrix data can also be used for a detailed modeling of the nanostructure of the cuticle of beetles.

AcknowledgementsFinancial support was obtained from the Knut and Alice Wallenberg foundation and the Swedish Research Council. The Museum of Natural History in Stockholm, the National Museum of Natural Science in Madrid, the Berlin Museum of Natural History and the Natural History Museum in London are acknowledged for loan of beetles.

REFERENCES

1. Michelson, A. A. “On Metallic Colouring in Birds and Insects,” Phil. Mag., 21, 554-567, 1911.
2. Goldstein, D. H. “Polarization properties of Scarabaeidae,” Appl. Opt., 45, 7944-7950, 2006.
3. Hodgkinson, I., Lowrey, S., Bourke, L., Parker, A. and McCall, M. W. “Mueller-matrix characterization of beetle cuticle polarized and unpolarized reflections from representative architectures,” Appl. Opt., 49, 4558-4567, 2010.
4. Vukusic, P. and Sambles, J. R. “Photonic structures in biology,” Nature, 424, 852-855, 2003.
5. Lenau, T. and Barfoed, M. “Colours and Metallic Sheen in Beetle Shells - A Biomimetic Search for Material Structuring Principles Causing Light Interference,” Adv. Eng. Mat., 10, 299-314. 2008.
6. Parker, A. R. and Townley, H. E “Biomimetics of photonic nanostructures,” Nature Nanotech., 2, 347-351, 2007.

The Mueller matrix elements m_ij representing the polarization response from a nanostructured materialis determined by the constituent materials optical properties and the superstructure. Here, we investigate how chiral structures in form of helicoidally stacked uniaxial layers determine m_ij as a functionof polarization state, wavelength, incidence angle and azimuthal angle of the incoming light. The studied parameters include the layer materials ordinary/extraordinary optical properties, Euler angle values, and layer thickness as well as the thickness and pitch of the helicoidal superstructure. Sub- and superstructure inhomogeneity is also introduced. From the Fresnel-based calculations, m_ij aswell as the degree of polarization, ellipticity and azimuth of the polarization ellipse are obtained and presented as contour and trace plots to give a complete view of the polarization behavior. The results from the calculations are compared with Mueller matrix spectroscopic ellipsometry measurements of both natural and synthesized helicoidal structures. The measurements were performed with a dualrotating compensator system (RC2, J.A. Woollam Co., Inc.) for wavelengths in the range from 245 to 1000 nm and incident angles from 20 to 75°. For some measurements the azimuthal angle of the incident light was varied. The investigated natural chiral structures were exoskeletons from several beetles in the scarab subfamilies Cetoniinae and Rutelinae. As predicted from the calculations it isobserved that the reflection from these beetles can have a high degree of polarization and high ellipticity (near-circular polarization). Both left- and right-polarization was observed. The synthesized structures are helicoidal nanorods of Al_1−xIn_xN grown on sapphire substrates with metal-nitride seedlayers using UHV magnetron sputtering. Due to an internal composition gradient (a variation of x) in the crystalline structure, the nanorods will tilt away from the substrate normal. Helicoidal structures can thus be obtained by rotating the substrate around its normal during deposition. Samples with different pitch and layer thickness with right-handed as well as left-handed chirality were grown. Also for these structures both left and right near-circular polarized light is observed. By combining calculations, ellipsometry measurements and scanning electron microscopy characterization we get agood input to build layered models of the natural and synthetic samples. After regression fitting agood agreement between calculated and measured optical data were obtained.

Amorphous carbon and amorphous, graphitic and fullerene-like carbon nitride thin filmswere deposited by reactive magnetron sputtering and optically characterized withspectroscopic ellipsometry. The films were exposed to human serum albumin and theadsorption was monitored in situ using dynamic ellipsometry. From the ellipsometric data theadsorbed amount of proteins was quantified in terms of surface mass density using de Feijter'smodel. The results indicated larger adsorption of proteins onto the amorphous films comparedto the films with a more ordered microstructure. Complementary studies with labeled HSAusing radioimmunoassay showed up to 6 times higher protein adsorption compared to theellipsometry measurement which partly might be explained by differences in surfaceroughness (from 0.3 to 13 nm) among the films. The elutability of adsorbed labeled HSAusing unlabeled HSA and sodium dodecyl sulphate was low compared to a silicon reference.In addition, the four types of films were incubated in blood plasma followed by antifibrinogen,anti-HMWK or anti-C3c revealing the materials response to complement andcontact activation. Three of the films indicated immunoactivity, whereas the amorphouscarbon showed less immunoactivity compared to a titanium reference. All films showedindications of a stronger ability to initiate the intrinsic pathway of coagulation, compared tothe reference. Finally, the surfaces bone bonding ability was investigated by examination oftheir ability to form calcium phosphate (CaP) crystals in a simulated body fluid, with a-CN_xdepositing most CaP after 21 days of incubation.

Thick matrices of fibrinogen with incorporation of a matrix metalloproteinaseinhibitor were covalently bonded on functionalized silicon surfaces using an ethyl-3-dimethyl-aminopropyl-carbodiimide and N-hydroxy-succinimide affinity ligand couplingchemistry. The growth of the structure was followed in situ using dynamic ellipsometryand characterized at steady-state with spectroscopic ellipsometry. The growth wascompared with earlier work on ex situ growth of fibrinogen layers studied by singlewavelength ellipsometry. It is found that in situ growth and ex situ growth yield differentstructural properties of the formed protein matrix. Fibrinogen matrices with thicknessesup to 58 nm and surface mass densities of 1.6 μg/cm² have been produced.

This thesis concerns synthesis and characterization of carbon-based materials and theinvestigation of the possible use, of a selection of these materials, in biomedicalapplications. Protein adsorption and blood plasma tests were used for this purposeutilizing a surface sensitive technique called spectroscopic ellipsometry.

The materials were synthesized by physical vapor deposition and characterizedregarding microstructure, mechanical properties and optical properties. The ternaries BC-N and Si-C-N as well as carbon and carbon nitrides (CN_x) of different microstructureshave been examined. In the B-C-N work, the intention was to investigate the possibilityto combine the two materials CN_x and BN, interesting on their own regarding highhardness and extreme elasticity, to produce a material with even better properties.Theoretical calculations were performed to elucidate the different element substitutionsand defect arrangements in the basal planes promoting curvature in the fullerene-likemicrostructure. The Si-C-N ternary was investigated with the consideration of finding away to control the surface energy for certain applications. Amorphous carbon and threemicrostructures of CN_x were analyzed by spectroscopic ellipsometry in the UV-VIS-NIRand IR spectral ranges in order to get further insight into the bonding structure of thematerial.

In the second part of this work focus was held on studies of macromolecularinteractions on silicon, carbon and CN_x film surfaces using ellipsometry. One purposewas to find relevance (or not) for these materials in biological environments. Materials for bone replacement used today, e.g. stainless steel, cobalt-chromium alloys andtitanium alloys suffer from corrosion in body fluids, generation of wear particles inarticulating systems, infections and blood coagulation and cellular damage leading toimpaired functionality and ultimately to implant failure. Artificial heart valves made ofpyrolytic carbon are used today, with friction and wear problems. Thus, there is still aneed to improve biomaterials. The aim of the fourth paper was to investigate theinteraction between carbon-based materials and proteins. Therefore, amorphous carbon(a-C), amorphous (a), graphitic (g) and fullerene-like (FL) CN_x thin films were exposedto human serum albumin and blood plasma and the amount of protein was measured insitu using spectroscopic ellipsometry. Surface located and accessible proteins after blood plasma incubations were eventually identified through incubations in antibody solutions.

Antibody exposures gave indications of surface response to blood coagulation,complement activation and clotting. The a-C and FL-CN_x films might according to theresults have a future in soft tissue applications due to the low immuno-activity, whereasthe g-CN_x film possibly might be a candidate for bone replacement applications.

"Layered" structures of fibrinogen, a fibrous but soft protein involved in manyprocesses in our body, were grown in situ and dynamically monitored by ellipsometry inorder to understand the adsorption process and molecule arrangement onto a siliconsurface.

In the last paper of this thesis, the effects of ion concentration and proteinconcentration on the refractive index of water-based solutions used in in situ ellipsometrymeasurements were demonstrated and spectral refractive index data for water solutionswith different ionic strengths and protein concentrations have been provided.

Carbon nitride (CNx) and amorphous carbon (a-C) thin films are deposited by reactive magnetron sputtering onto silicon (001) wafers under controlled conditions to achieve amorphous, graphitic and fullerene-like microstructures. As-deposited films are analyzed by Spectroscopic Ellipsometry in the UV–VIS–NIR and IR spectral ranges in order to get further insight into the bonding structure of the material. Additional characterization is performed by High Resolution Transmission Electron Microscopy, X-ray Photoelectron Spectroscopy, and Atomic Force Microscopy. Between eight and eleven resonances are observed and modeled in the ellipsometrically determined optical spectra of the films. The largest or the second largest resonance for all films is a feature associated with C–N or C–C modes. This feature is generally associated with sp3 C–N or sp³ C–C bonds, which for the nitrogen-containing films instead should be identified as a three-fold or two-fold sp² hybridization of N, either substituted in a graphite site or in a pyridine-like configuration, respectively. The π→πlow asterisk electronic transition associated with sp² C bonds in carbon films and with sp² N bonds (as N bonded in pyridine-like manner) in CN_x films is also present, but not as strong. Another feature present in all CN_x films is a resonance associated with nitrile often observed in carbon nitrides. Additional resonances are identified and discussed and moreover, several new, unidentified resonances are observed in the ellipsometric spectra.