Browsing by Subject "MECHANICAL-PROPERTIES"

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  • Poojari, Chetan; Zak, Agata; Dzieciuch-Rojek, Monika; Bunker, Alex; Kepczynski, Mariusz; Rog, Tomasz (2020)
    Cholesterol plays a crucial role in modulating the physicochemical properties of biomembranes, both increasing mechanical strength and decreasing permeability. Cholesterol is also a common component of vesicle-based delivery systems, including liposome-based drug delivery systems (LDSs). However, its effect on the partitioning of drug molecules to lipid membranes is very poorly recognized. Herein, we performed a combined experimental/computational study of the potential for the use of the LDS formulation for the delivery of the antifungal drug itraconazole (ITZ). We consider the addition of cholesterol to the lipid membrane. Since ITZ is only weakly soluble in water, its bioavailability is limited. Use of an LDS has thus been proposed. We studied lipid membranes composed of cholesterol, 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine (POPC), and ITZ using a combination of computational molecular dynamics (MD) simulations of lipid bilayers and Brewster angle microscopy (BAM) experiments of monolayers. Both experimental and computational results show separation of cholesterol and ITZ. Cholesterol has a strong preference to orient parallel to the bilayer normal. However, ITZ, a long and relatively rigid molecule with weakly hydrophilic groups along the backbone, predominantly locates below the interface between the hydrocarbon chain region and the polar region of the membrane, with its backbone oriented parallel to the membrane surface; the orthogonal orientation in the membrane could be the cause of the observed separation. In addition, fluorescence measurements demonstrated that the affinity of ITZ for the lipid membrane is decreased by the presence of cholesterol, which is thus probably not a suitable formulation component of an LDS designed for ITZ delivery.
  • Akmal, Jan Sher; Salmi, Mika; Hemming, Björn; Teir, Linus; Suomalainen, Anni; Kortesniemi, Mika; Partanen, Jouni; Lassila, Antti (2020)
    Featured Application Accuracy of additively manufactured implants for clinical surgery. Abstract In craniomaxillofacial surgical procedures, an emerging practice adopts the preoperative virtual planning that uses medical imaging (computed tomography), 3D thresholding (segmentation), 3D modeling (digital design), and additive manufacturing (3D printing) for the procurement of an end-use implant. The objective of this case study was to evaluate the cumulative spatial inaccuracies arising from each step of the process chain when various computed tomography protocols and thresholding values were independently changed. A custom-made quality assurance instrument (Phantom) was used to evaluate the medical imaging error. A sus domesticus (domestic pig) head was analyzed to determine the 3D thresholding error. The 3D modeling error was estimated from the computer-aided design software. Finally, the end-use implant was used to evaluate the additive manufacturing error. The results were verified using accurate measurement instruments and techniques. A worst-case cumulative error of 1.7 mm (3.0%) was estimated for one boundary condition and 2.3 mm (4.1%) for two boundary conditions considering the maximum length (56.9 mm) of the end-use implant. Uncertainty from the clinical imaging to the end-use implant was 0.8 mm (1.4%). This study helps practitioners establish and corroborate surgical practices that are within the bounds of an appropriate accuracy for clinical treatment and restoration.
  • Veira Canle, Daniel; Mäkinen, Joni Mikko Kristian; Blomqvist, Richard; Gritsevich, M.; Salmi, Ari; Haeggström, Edward (2021)
    The primary goal of this study is to localize a defect (cavity) in a curved geometry. Curved topologies exhibit multiple resonances and the presence of hotspots for acoustic waves. Launching acoustic waves along a specific direction e.g. by means of an extended laser source reduces the complexity of the scattering problem. We performed experiments to demonstrate the use of a laser line source and verified the experimental results in FEM simulations. In both cases, we could locate and determine the size of a pit in a steel hemisphere which allowed us to visualize the defect on a 3D model of the sample. Such an approach could benefit patients by enabling contactless inspection of acetabular cups.
  • Stenroth, Lauri; Sefa, Sandra; Arokoski, Jari; Töyräs, Juha (2019)
    This study investigated the reliability of Achilles and patellar tendon cross-sectional area (CSA) measurement using ultrasound imaging (USI) and magnetic resonance imaging (MRI). Fifteen healthy adults were imaged twice on two occasions, interrupted by a tendon loading protocol. Tendon CSA segmentations were conducted by an experienced and an inexperienced rater blinded to information regarding subject, session and loading status. USI provided good test-retest reliability (intra-class correlation coefficient [ICC] 2,1 > 0.85, standard error of measurement [SEM] 5%-6%), while with MRI it was excellent (ICC 2,1 > 0.92, SEM 4%) for the experienced rater. This study suggests that MRI provides superior reliability for tendon CSA measurements compared with USI. However, the difference in reliability between the methods was small, and the results were inconclusive regarding objectivity and sensitivity to change when assessed based on the effect of loading. We concluded that both methods can be used for reliable CSA measurements of the Achilles and patellar tendons when using a highly standardized measurement protocol and when conducted by an experienced rater. (C) 2019 World Federation for Ultrasound in Medicine & Biology. All rights reserved.
  • Ramos, Flávio James Humberto Tommasini Vieira; Reis, Raphael Henrique Morais; Grafova, Iryna; Grafov, Andriy; Monteiro, Sergio Neves (2020)
    Civil construction wastes have been incorporated into polymers for recycling as novel engineering composites. In the present work eco-friendly composites with recycled polypropylene (rPP) matrix incorporated with geopolymer concrete waste particles, wither plain (GCW) or surface-modified with oleic acid (AGC) were investigated. The geopolymer concrete waste particles were mixed with polymer powder to provide an effective dispersion between the different materials. Composites were produced by an initial reactive extrusion processing followed by injection molding. These novel composites with amount of 20, 40 and 50 wt% of GCW particles, both plain as-received and surface-modified, were technically evaluated by tensile tests, statistically analyzed by ANOVA, as well as by water absorption as per ASTM standards. Surface dispersion of nanoparticles was revealed by atomic force microscopy. Microstructural analysis was performed by scanning electron microscopy. The results indicated that these sustainable GCW particles incorporated into rPP matrix exhibit superior processability and water absorption less than 0.01%. The rPP/AGC composites present relatively higher elastic modulus, 629 MPa, as compared to the neat rPP, with 529 MPa. These properties suggest potential sustainable applications in building construction using waste materials.
  • Sodupe-Ortega, E.; Fraile-Garcia, E.; Ferreiro-Cabello, J.; Sanz-Garcia, A. (2016)
    Waste tire rubber is a promising lightweight aggregate for building products that enhances their thermal and acoustic properties. Even the environmental benefits of its use are evident, higher cost and significant changes in compressive strength and workability hinder its widespread adoption. This article examines the use of crumb rubber (CR) as aggregate in dry-mix mortars to produce rubberized long hollow blocks and bricks using automated brick machines. CR was incorporated over a range of 10-40% with water/ cement ratio varying from 0.7 to 0.9. The production of rubberized bricks exhibited better performance than long hollow blocks in factory trials. Tests showed important deformations and drastic reduction in compressive strength, especially for crumb rubber percentages greater than 20%. Due to this and the high cost of CR, caution must be taken with the design of new rubberized building products to make sure they are profitable. (C) 2015 Elsevier Ltd. All rights reserved.
  • Scarabello Stape, Thiago Henrique; Tjäderhane, Leo; Tezvergil-Mutluay, Arzu; Da Silva, Wagner Gomes; dos Santos Silva, Alan Roger; da Silva, Wander Jose; Marques, Marcelo Rocha (2018)
    Matrix metalloproteinases (MMPs) such as gelatinases are differentially expressed in human tissues. These enzymes cleave specific substrates involved in cell signaling, tissue development and remodeling and tissue breakdown. Recent evidences show that gelatinases are crucial for normal dentin development and their activity is maintained throughout the entire tooth function in the oral cavity. Due to the lack of information about the exact location and activity of gelatinases in mature human dentin, the present study was designed to examine gelatinolytic levels In sound dentin. In situ zymography using confocal microscopy was performed on both mineralized and demineralized dentin samples. Sites presenting gelatinase activity were identified throughout the entire biological tissue pursuing different gelatinolytic levels for distinct areas: predentin and dentinal tubule regions presented higher gelatinolytic activity compared to intertubular dentin. Dentin regions with higher gelatinolytic activity immunohistochemically were partially correlated with MMP-2 expression. The maintenance of gelatinolytic activity in mature dentin may have biological implications related to biomineralization of predentin and tubular/peritubular dentinal regions, as well as regulation of defensive mechanisms of the dentin-pulp complex.
  • Pooch, Fabian; Sliepen, Marjolein; Svedström, Kirsi J.; Korpi, Antti; Winnik, Francoise M.; Tenhu, Heikki (2018)
    Miscible block copolymers (BCPs) are rarely studied. When one or both components of such BCPs are semi-crystalline polymers, strong effects on the crystallization behavior can be expected. We present a study of 18 miscible BCPs comprised of poly(lactide) (PLLA, semi-crystalline and PDLLA, amorphous) and poly(2-isopropyl-2-oxazoline) (PiPOx, semi-crystalline) with PiPOx volume fractions of 0.14 <phi(PiPOx) <0.82. All BCPs exhibit a single glass transition and form a homogeneous melt. Mixing has a plasticizing effect on PiPOx and increases its crystallization rates (DSC). In contrast, the crystallization rates of PLLA are dramatically reduced, or in most cases entirely prevented. During isothermal crystallization at 130 degrees C, the crystallization rates of the BCPs were inverted in comparison with those of the parent homopolymers. Crystallization drives the BCPs to phase separate and the formed crystalline structure is that of the parent homopolymers. The fast crystallization of PiPOx confines the observed superstructure. The BCPs were studied on multiple length scales - from the atomic level (WAXS, IR spectroscopy) to the meso level (AFM, SAXS) and the macroscopic superstructure (polarized optical microscopy). A mechanism of the structure evolution is presented.
  • Carvalho, Tiago; Guedes, Gabriela; Sousa, Filipa L.; Freire, Carmen S. R.; Santos, Hélder A. (2019)
    Bacterial cellulose (BC) is a nanocellulose form produced by some nonpathogenic bacteria. BC presents unique physical, chemical, and biological properties that make it a very versatile material and has found application in several fields, namely in food industry, cosmetics, and biomedicine. This review overviews the latest state-of-the-art usage of BC on three important areas of the biomedical field, namely delivery systems, wound dressing and healing materials, and tissue engineering for regenerative medicine. BC will be reviewed as a promising biopolymer for the design and development of innovative materials for the mentioned applications. Overall, BC is shown to be an effective and versatile carrier for delivery systems, a safe and multicustomizable patch or graft for wound dressing and healing applications, and a material that can be further tuned to better adjust for each tissue engineering application, by using different methods.
  • Tjäderhane, Leo; Rabelo Buzalaf, Marilia Afonso; Carrilho, Marcela; Chaussain, Catherine (2015)
    Dentin organic matrix, with type I collagen as the main component, is exposed after demineralization in dentinal caries, erosion or acidic conditioning during adhesive composite restorative treatment. This exposed matrix is prone to slow hydrolytic degradation by host collagenolytic enzymes, matrix metalloproteinases (MMPs) and cysteine cathepsins. Here we review the recent findings demonstrating that inhibition of salivary or dentin endogenous collagenolytic enzymes may provide preventive means against progression of caries or erosion, just as they have been shown to retain the integrity and improve the longevity of resin composite filling bonding to dentin. This paper also presents the case that the organic matrix in caries-affected dentin may not be preserved as intact as previously considered. In partially demineralized dentin, MMPs and cysteine cathepsins with the ability to cleave off the terminal non-helical ends of collagen molecules (telopeptides) may lead to the gradual loss of intramolecular gap areas. This would seriously compromise the matrix ability for intrafibrillar remineralization, which is considered essential in restoring the dentin's mechanical properties. More detailed data of the enzymes responsible and their detailed function in dentin-destructive conditions may not only help to find new and better preventive means, but better preservation of demineralized dentin collagenous matrix may also facilitate true biological remineralization for the better restoration of tooth structural and mechanical integrity and mechanical properties. (C) 2015 S. Karger AG, Basel
  • Wiikinkoski, Elmo W.; Xu, Junhua; Zhang, Wenzhong; Hietala, Sami; Koivula, Risto T. (2018)
    The aim of an ongoing study is to develop ion exchange materials to be used in low pH in mineral acids for the uptake and separation of trivalent lanthanides and actinides. We present material development of alpha-zirconium phosphate (ZrP), namely the effects of its crystallinity and acidity on other material properties and on Eu(III) and Am(III) trace ion exchange. One-pot synthesis for three ZrP products having a time for (re)crystallization of 1 hour, 5 hours and 25 hours, is reported. Characterization techniques include X-ray diffraction, solid-state nuclear magnetic resonance, infrared spectroscopy, thermal analysis, electron microscopy, pK(a1) determination, and Eu-152(III) and Am-241(III) ion exchange studies, i.e. distribution-, selectivity- and metal binding coefficient determinations. As crystallinity and crystallite size increase, so do Eu(III) and Am(III) metal binding coefficients. The acidity and Eu(III) and Am(III) distribution- and selectivity coefficients increase in the reverse order. Findings are discussed with separation in mind. Promising separation factors (Eu:Am) of up to 90 were achieved in pH less than 1 in nitric acid.
  • Tanner, Timo; Antikainen, Osmo; Pollet, Arne; Räikönen, Heikki; Ehlers, Henrik; Juppo, Anne; Yliruusi, Jouko (2019)
    In the present study, a model was developed to estimate tablet tensile strength utilizing the gravitation-based high-velocity (G-HVC) method introduced earlier. Three different formulations consisting of microcrystalline cellulose (MCC), dicalcium phosphate dihydrate (DCP), hydroxypropyl methylcellulose (HPMC), theophylline and magnesium stearate were prepared. The formulations were granulated using fluid bed granulation and the granules were compacted with the G-HVC method and an eccentric tableting machine. Compaction energy values defined from G-HVC data predicted tensile strength of the tablets surprisingly well. It was also shown, that fluid bed granulation improved the compaction energy intake of the granules in comparison to respective physical mixtures. In addition, general mechanical properties and elastic recovery were also examined for all samples. In this study it was finally concluded, that the data obtained by the method was of practical relevance in pharmaceutical formulation development.
  • Sattary, Mansoureh; Rafienia, Mohammad; Kazemi, Mohammad; Salehi, Hossein; Mahmoudzadeh, Mohammad (2019)
    Tissue engineering knowledge is a step towards the treatment of irreversible damages to human beings. In the present study, PCL/Gel, PCL/Gel/nHA, PCL/Gel/Vit D3 and PCL/Gel/nHA/Vit D3 (Polycaprolactone/Gelatin/Nanohydroxyapatite/Vitamin D3) composite scaffolds were successfully constructed using electrospinning method. The proliferation and differentiation of hADSCs into the bone phenotype were determined using MTT method, ALP activity, Von Kossa and Alizarin red staining, and qRT-PCR test. The simultaneous presence of nHA and vitamin D3 led to the increased activity of ALP in the early stages (on the 14th day) and increased mineralization in the late stages (on the 21st day) in differentiated hADSCs. Further, it was found that the use of nHA and vitamin D3 resulted in increased expression of BGLAP and COLL I and reduced expression of ALP and RUNX2 in hADSCs for 21 days. The results indicated that nHA and vitamin D3 have a synergistic effect on the osteogenic differentiation of hADSCs.
  • Vienna, University; Renk, Oliver; Ghosh, Sanyukta; Mallik, Ramesh Chandra; Grytsiv, Andriy; Bursik, Jiri; Schafler, Erhard; Tuomisto, Filip; Bauer, Ernst; Rogl, Peter Franz (2021)
    The influence of shear strain on the microstructural, physical, and mechanical properties was studied on large bulk samples (diameter: 30 mm, thickness: 1 or 8 mm), which were consolidated by high-pressure torsion (HPT) from a commercial powder DD0.7Fe3CoSb12. Particularly, the thick sample (mass similar to 53 g) allowed measuring the thermoelectric (TE) properties with respect to various orientations of the specimen in the sample. All data were compared with those of a hot-pressed (HP) reference sample, prepared with the same powder. Transmission electron microscopy, as well as X-ray powder diffraction profile analyses, Hall measurements, and positron annihilation spectroscopy, supported these investigations. Furthermore, synchrotron data for the temperature range from 300 to 825 K were used to evaluate the changes in the grain size and dislocation density as well as the thermal expansion coefficient via the change in the lattice parameter during heating. In addition, hardness and direct thermal expansion measurements of the HPT samples were performed and compared with the HP reference sample's values. With the increase of the shear strain from the center to the rim of the sample, the electrical resistivity becomes higher, whereas the thermal conductivity becomes lower, but the Seebeck coefficient remained almost unchanged. For the thin as well as thick samples, the enhanced electrical resistivity was balanced out by a decreased thermal conductivity such that the maximum ZT values (ZT = 1.3-1.35 at 856 K) do not vary much as a function of the shear strain throughout the sample, however, all ZTs are higher than that of the HP sample. The thermal-electric conversion efficiencies are in the range of 14-15% (for 423-823 K). With similar high ZT values for the n-type skutterudites, fabricated in the same fast and sustainable way, these p- and n-type skutterudites may serve as legs for TE generators, directly cut from the big HPT bulks.
  • Harjumaki, Riina; Nugroho, Robertus Wahyu N.; Zhang, Xue; Lou, Yan-Ru; Yliperttula, Marjo; Valle-Delgado, Juan Jose; Österberg, Monika (2019)
    In vitro cell culture or tissue models that mimic in vivo cellular response have potential in tissue engineering and regenerative medicine, and are a more economical and accurate option for drug toxicity tests than animal experimentation. The design of in vivo-like cell culture models should take into account how the cells interact with the surrounding materials and how these interactions affect the cell behavior. Cell-material interactions are furthermore important in cancer metastasis and tumor progression, so deeper understanding of them can support the development of new cancer treatments. Herein, the colloidal probe microscopy technique was used to quantify the interactions of two cell lines (human pluripotent stem cell line WA07 and human hepatocellular carcinoma cell line HepG2) with natural, xeno-free biomaterials of different chemistry, morphology, and origin. Key components of extracellular matrices -human collagens I and IV, and human recombinant laminin-521-, as well as wood-derived, cellulose nanofibrils -with evidenced potential for 3D cell culture and tissue engineering- were analysed. Both strength of adhesion and force curve profiles depended on biomaterial nature and cell characteristics. The successful growth of the cells on a particular biomaterial required cell-biomaterial adhesion energies above 0.23 nJ/m. The information obtained in this work supports the development of new materials or hybrid scaffolds with tuned cell adhesion properties for tissue engineering, and provides a better understanding of the interactions of normal and cancerous cells with biomaterials in the human body.
  • Nugroho, Robertus Wahyu N.; Harjumäki, Riina; Zhang, Xue; Lou, Yan-Ru; Yliperttula, Marjo; Valle-Delgado, Juan Jose; Österberg, Monika (2019)
    Biomaterials of different nature have been and are widely studied for various biomedical applications. In many cases, biomaterial assemblies are designed to mimic biological systems. Although biomaterials have been thoroughly characterized in many aspects, not much quantitative information on the molecular level interactions between different biomaterials is available. That information is very important, on the one hand, to understand the properties of biological systems and, on the other hand, to develop new composite biomaterials for special applications. This work presents a systematic, quantitative analysis of self- and cross-interactions between films of collagen I (Col I), collagen IV (Col IV), laminin (LN-521), and cellulose nanofibrils (CNF), that is, biomaterials of different nature and structure that either exist in biological systems (e.g., extracellular matrices) or have shown potential for 3D cell culture and tissue engineering. Direct surface forces and adhesion between biomaterials-coated spherical micro-particles and flat substrates were measured in phosphate-buffered saline using an atomic force microscope and the colloidal probe technique. Different methods (Langmuir-Schaefer deposition, spin-coating, or adsorption) were applied to completely coat the flat substrates and the spherical micro particles with homogeneous biomaterial films. The adhesion between biomaterials films increased with the time that the films were kept in contact. The strongest adhesion was observed between Col IV films, and between Col IV and LN-521 films after 30 s contact time. In contrast, low adhesion was measured between CNF films, as well as between CNF and LN-521 films. Nevertheless, a good adhesion between CNF and collagen films (especially Col I) was observed. These results increase our understanding of the structure of biological systems and can support the design of new matrices or scaffolds where different biomaterials are combined for diverse biological or medical applications.
  • Reyes, Guillermo; Borghei, Maryam; King, Alistair W. T.; Lahti, Johanna; Rojas, Orlando J. (2019)
    Cellulose nanofiber films (CNFF) were treated via a welding process using ionic liquids (ILs). Acid base-conjugated ILs derived from 1,5-diazabicyclo[4.3.0]non-5-ene [DBN] and 1-ethyl-3-methylimidazolium acetate ([emim][OAc]) were utilized. The removal efficiency of ILs from welded CNFF was assessed using liquid-state nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared spectroscopy (FTIR). The mechanical and physical properties of CNFF indicated surface plasticization of CNFF, which improved transparency. Upon treatment, the average CNFF toughness increased by 27%, and the films reached a Young's modulus of similar to 5.8 GPa. These first attempts for IL "welding" show promise to tune the surfaces of biobased films, expanding the scope of properties for the production of new biobased materials in a green chemistry context. The results of this work are highly relevant to the fabrication of CNFFs using ionic liquids and related solvents.
  • Tiainen, Tony; Lobanova, Marina; Karjalainen, Erno; Tenhu, Heikki; Hietala, Sami (2020)
    Nanodiamonds (NDs) can considerably improve the mechanical and thermal properties of polymeric composites. However, the tendency of NDs to aggregate limits the potential of these non-toxic, mechanically- and chemically-robust nanofillers. In this work, tough, flexible, and stimuli-responsive polyelectrolyte films composed of cross-linked poly(butyl acrylate-co-dimethylaminoethyl methacrylate) (P(BA-co-DMAEMA)) were prepared by photopolymerization. The effects of the added carboxylate-functionalized NDs on their mechanical and stimuli-responsive properties were studied. When the negatively charged NDs were added to the polymerization media directly, the mechanical properties of the films changed only slightly, because of the uneven distribution of the aggregated NDs in the films. In order to disperse and distribute the NDs more evenly, a prepolymerized polycation block copolymer complexing agent was used during the photopolymerization process. This approach improved the mechanical properties of the films and enhanced their thermally-induced, reversible phase-transition behavior.
  • Bertula, Kia; Martikainen, Lahja; Munne, Pauliina; Hietala, Sami; Klefström, Juha; Ikkala, Olli; Nonappa, Dr. (2019)
    Strain-stiffening is one of the characteristic properties of biological hydrogels and extracellular matrices, where the stiffness increases upon increased deformation. Whereas strain-stiffening is ubiquitous in protein-based materials, it has been less observed for polysaccharide and synthetic polymer gels. Here we show that agarose, that is, a common linear polysaccharide, forms helical fibrillar bundles upon cooling from aqueous solution. The hydrogels with these semiflexible fibrils show pronounced strain-stiffening. However, to reveal strain-stiffening, suppressing wall slippage turned as untrivial. Upon exploring different sample preparation techniques and rheological architectures, the cross-hatched parallel plate geometries and in situ gelation in the rheometer successfully prevented the slippage and resolved the strain-stiffening behavior. Combining with microscopy, we conclude that strain-stiffening is due to the semiflexible nature of the agarose fibrils and their geometrical connectivity, which is below the central-force isostatic critical connectivity. The biocompatibility and the observed strain-stiffening suggest the potential of agarose hydrogels in biomedical applications.
  • Liu, Jian; Muinos, Henrique Vazquez; Nordlund, Kai; Djurabekova, Flyura (2019)
    In spite of the versatility of electronic properties of graphene, its fragility and low resistance to damage and external deformations reduce the practical value of this material for many applications. Coating of graphene with a thin layer of hard amorphous carbon is considered as a viable solution to protect the 2D material against accidental scratches and other external damaging impacts. In this study, we investigate the relationship between the deposition condition and quality of diamond-like-carbon (DLC) on top of multilayer graphene by means of molecular dynamics simulations. Deposition of carbon atoms with 70 eV incident energy at 100 K resulted in the highest content of sp(3)-bonded C atoms. An increase of the number of dangling bonds at the interface between the top graphene layer and the DLC film indicates that decrease of the incident energy reduces the adhesion quality of DLC thin film on graphene. Analysis of radial distribution function indicates that sp(3) hybridized carbon atoms tend to grow near already existing sp(3) -atoms. This explains why the quality of the DLC structures grown on graphene have generally a lower content of sp(3) C atoms compared to those grown directly on diamond. Ring analysis further shows that a DLC structure grown on the sp(2) -rich structures like graphene contains a higher fraction of disordered ring structures.