Karlis Gross

Riga Technical University | RTU · Institute of Biomaterials and Biomechanics

Ice temperature is commonly thought to be the most influential on sliding over ice, but ambient conditions are continually changing. A complete description of the ambient conditions needs more than the ice temperature. For example, in skeleton competition, where weather conditions significantly influence runners' treatment, it would be essential to...

The skeleton is one of the winter Olympics games sports and it is the only sport where is possible to alter the degree of contact with ice by altering the runner stiffness. Stiffness increased by compressing the ends of the runners resulting in less contact with the ice. Beginners prefer a runner setting with a lower stiffness for greater stability...

Hydroxyapatite coatings need similarly shaped splats as building blocks and then a homogeneous microstructure to unravel the structural and chemical hierarchy for more refined improvements to implant surfaces. Coatings were thermally sprayed with differently sized powders (20-40, 40-63 and 63-80 µm) to produce flattened homogeneous splats. The surf...

Vanadium drugs have been known for more than a century, but clinical translation has been hampered by unfavorable pharmacokinetics and lack of reliable delivery systems. Because of considerable interest in hydroxyapatite (HAP; Ca10(PO4)6(OH)2) nanoparticle-based drug delivery vehicles, particularly for the treatment of bone disease, and the well-kn...

Brushite cement has advantages such as fast setting, high reactivity and good injectability over apatitic cements. To induce the bioactivity of brushite cements, the goal was to convert it into a bone-like low crystalline carbonate apatite. To achieve this induced transformation, potassium and magnesium were used as dopants which were claimed to be...

The largest comparison of analytical techniques, useful for calcium phosphates, has been conducted, presenting recommendations for quality control and research of calcium phosphates. Results from three classical quantification methods (gravimetry, titramety, photometry) and three instrumental measurement methods (XRF, FAAS ICP-OES) are reported to...

The effect of surface topography in combination with chemical composition on hydrophobicity and static coefficient-of-friction between steel and ice was studied. Polished stainless steel blocks were etched to introduce a roughness, and further treated with octadecanethiol. Carbon rich inclusions, identified by energy-dispersive X-ray spectroscopy,...

The growing demand for functionality of implants is necessary to cater for the growing elderly population in need of repair for damaged or diseased tissues and organs. Radiofrequency magnetron sputtered crystalline hydroxyapatite thin films with preferred crystal orientation on polished titanium substrates were produced without additional substrate...

The incorporation of protein based polymeric materials with calcium phosphate (CaP) coatings can provide excellent biological characteristics and biocompatibility with its bone-like protein-CaP composition. Calcium chloride-ethanol catalysed hydrolysis reaction and wet precipitation method was used to prepare silk solution and to precipitate brushi...

Winter forces people to adapt to snow and ice conditions and understand the regularities in nature in the cold regions. The understanding of ice and snow regularities can help to improve the safety of moving over ice. The objective of this work was to investigate the impact of ice texture on the static coefficient of friction for stainless steel sa...

Many studies use an inclined ice plane to determine the effect of different materials and processing conditions on sliding over an ice surface. Experiments measure the sliding time at different stages on the track to determine the sliding speed and the coefficient-of-friction. Here, the sliding body vibrations and, the oscillations in axial directi...

Hydroxyapatite (HAp) has shown the ability to store a charge induced by electrical polarization a method potential for improved bone ingrowth. In this work electrical polarization was carried out on HAp ceramics with three different microstructures, obtained at three different sintering temperatures. Microstructure of the sintered HAp was evaluated...

Scientists and sport athletes are constantly seeking for the methods which could improve surface sliding ability on ice. Modifications of contact area and surface roughness are relatively easy but not yet fully understood methods for sliding ability improvements. This research contains information of how one can perform on-field experiments with sk...

Switching adsorption-desorption by visible light would provide the possibility for a wide range of applications that require controlled release-on-demand. Here we demonstrate a visible-light controlled desorption behavior in aqueous suspensions for the first time. We observed cationic dye adsorption on amphoteric goethite α-FeOOH in the dark and re...

Strontium substituted hydroxyapatite is a biomaterial with high level of biocompatibility with improved cell proliferation from Sr ions in hydroxyapatite. The aim of this research is to study the effect of saturated steam pressure at elevated temperature on the transitions within amorphous strontium hydroxyapatite powder. Processing temperatures an...

Winter brings snow and ice to Northern Europe and with it the need to move safely over ice. It has been customary to select an appropriate material surface for movement over ice, but another way to influence the interaction with ice is to appropriately prepare the ice surface. The objective of the research is to investigate the influence of ice tex...

While single compound semiconductors were initially used for photocatalysis, combining two compounds to form a heterojunction significantly increases the photocatalysis performance. This review will outline how heterojunctions are superior, explain the different heterostructure architectures assembled from nanoparticles, and discuss the importance...

The skeleton is a high-speed sport achieving speeds up to 130 km·h-1 on an ice track, but conditions for faster sliding have not been documented. This paper describes a theoretical model, an experiment and numerical modelling to evaluate the effect of air drag and runner stiffness. A mathematical model was determined for the forward motion of the s...

The amount and alignment of hydroxyl ions influence the bioactivity of hydroxyapatite. Hydroxyl ions in hydroxyapatite are the most mobile and upon heating are the first to leave the lattice to form oxyhydroxyapatite. This work describes a method for producing hydroxyapatite with different amounts of hydroxyl ions, and reports on the changes in Fou...

Thermal spraying is commercially used to produce hydroxyapatite coatings, but the high temperature depletes hydroxyl ions in the structure. To return hydroxyapatite to its original state, it is necessary to restore the hydroxyl ion content in the structure. In this work, the effect of hydrothermal treatment on the hydroxyl ions was investigated in...

The ability to slide on ice has previously focused on the measurement of friction coefficient rather than the actual sliding velocity that is affected by it. The performance can only be directly measured by the sliding velocity, and therefore the objective was to design and setup a facility to measure velo-city, and determine how experimental condi...

Scratch detection and the location of the scratch on the surface is important in the quality control of multilayered, functional and polished surfaces. Visual examination in good lighting conditions has been previously used to detect scratches, but interest to see finer scratches and their location should consider use of the optical microscope, and...

Bioactive calcium phosphate coatings were deposited by radio-frequency magnetron sputtering from biphasic targets of hydroxyapatite and tricalcium phosphate, sintered at different mass % ratios. According to Raman scattering and X-ray diffraction data, the deposited hydroxyapatite coatings have a disordered structure. High-temperature treatment of...

This study demonstrates the significant potential of cantilever-enhanced Fourier transform infrared photoacoustic spectroscopy (FTIR PAS) principles. The improved sensitivity and reproducibility of this method presents a potent tool in the study of biomaterials. The article discusses aspects of the application of cantilever-enhanced FTIR PAS in the...

The incorporation of biomaterials in human tissue requires methods to study the interface of the implant with the biological setting. We set out to study whether Photoacoustic spectroscopy with a higher level of sensitivity from the cantilever detector could simultaneously detect amorphous calcium phosphate and the bacteria. The calcium phosphate w...

While considerable work has been done on chemically functionalizing hydroxyapatite, little has been done on tailoring the electrical surface potential. This has been due to limitations in the available methods to impart a surface charge. Work to date has charged conventionally manufactured hydroxyapatite exhibiting a random crystal orientation. At...

The stability of silver halide colloids is reported to be important for the toxicological outcome. This study shows a well-suited and cheap condensation reaction to obtain negatively charged silver iodide (AgI) nanoparticles without additional stabilization agents. Charged AgI colloids were synthesized from silver nitrate and potassium iodide solut...

The current research focuses on peroxide ion incorporation in the structure of strontium hydroxyapatite (SrHAp) by exposure to H2O2 ina closed hydrothermal vessel under mild conditions at 130 to 170 °C. Peroxide enriched strontium apatite (SrPerAp) was characterized for structure by X-Ray diffraction, for peroxide content- by permanganometric titra...

Nano-sized pure α-tricalcium phosphate (α-TCP) fabricated by a novel synthesis approach shows great potential for a faster transformation into calcium-deficient hydroxyapatite (CDHA) than conventionally prepared α-TCP. In this work, amorphous tricalcium phosphate precursors were precipitated and treated with a solvent (water or ethanol), and dried...

New insight on the conversion of amorphous calcium phosphate (ACP) to nano-sized alpha tricalcium phosphate (α-TCP) provides a faster pathway to calcium phosphate bone cements. In this work, synthesized ACP powders were treated with either water or ethanol, dried, crystallized between 700 and 800 °C, and then cooled at different cooling rates. Part...

This work will consider Fourier transform infrared spectroscopy – diffuse reflectance infrared reflection (FTIR-DRIFT) for collecting the spectra and deconvolution to identify changes in bonding as a means of more powerful detection. Spectra were recorded from amorphous calcium phosphate synthesized by wet precipitation, and from bone. FTIR-DRIFT w...

Transient or metastable phases offer an intermediate phase with additional flexibility for creating the end product. The processing pathway remains unknown when the final product is viewed. Biological processes frequently employ the amorphous phase as the transition phase. This is shown in mineralized tissues: invertebrates, pathological calcified...

In the investigation of mechanical properties, calcium phosphate cements exhibit large sample-to-sample deviation due to its porous nature, possibility of un-homogenous distribution and small specimen size. This situation generates difficulties for obtaining accurate results and creates an obstacle for testing different composition where only a sma...

Silver halides represent a yet unexplored avenue for imparting antimicrobial activity to calcium phosphates. Negtively charged silver halide colloids (AgI, AgBr and AgCl) were added to synthesized amorphous calcium phosphate. Concurrent melting of silver halides and crystallization to carbonated apatite at 700 oC increased the silver halide surface...

Tetracalcium phosphate (TTCP) requires the highest synthesis temperatures of all the calcium phosphates, but now a new process is available at 400 °C lower than previously, at 900 °C. Instead of ball-milling reactants for a homogeneous mix, the reactants for solid-state synthesis were assembled in an amorphous phase. Heating produced hydroxyapatite...

Peroxide ions in apatite provides an additional resource for imparting an antibacterial capability in apatite. A hydrothermal process has been developed for including peroxide ions into the apatite lattice. Three oxygen generating compounds (hydrogen peroxide, ammonium persulphate and peracetic acid) were investigated for generating peroxyapatite....

Amorphous calcium phosphate (ACP) plays an important role in the body and can be used as an intermediate phase for forming calcium phosphates. All ACPs are thermodynamically unstable compounds unless stored in dry conditions and spontaneously undergo transformation to crystalline calcium phosphates (CaP). This work will investigate the influence of...

Silver halides represent a yet unexplored avenue for imparting antimicrobial activity in calcium phosphates. Silver halide colloids were added to calcium phosphate. Concurrent melting of silver halides and crystallization of carbonated apatite was achieved by heating to increase the silver halide surface area available to bacteria.Pseudomonas aerug...

Peroxide ions in apatite provides an additional resource for imparting an antibacterial capability in apatite. A hydrothermal process has been developed for including peroxide ions into the apatite lattice. Three oxygen generation compounds, hydrogen peroxide, ammonium persulphate and paracetic acid were investigated for peroxyapatite generation. H...

Tetracalcium phosphate (TTCP) requires the highest synthesis temperatures of all the calcium phosphates, but now a new process is available at 400 °C lower than previously, at 900 °C. Instead of ball-milling reactants for a homogeneous mix, the reactants were included in an amorphous phase. Heating produced hydroxyapatite, oxyapatite and then TTCP....

Objective: The objective of this in vitro study was to evaluate the rehardening potential of a nano-hydroxyapatite/gamma-polyglutamic acid (nano-HAp/γ-PGA) on surface-etched enamels. Method: Thirty-two coronal-part of human premolars were selected and divided into four groups (N=8). Artificial surface-etched specimens were treated by 1M acetic ac...

Previous synthesis routes created apatites in low crystallinity and high crystallinity states, but a wider range will extend the design capabilities of apatites for hard tissue replacements. While high crystallinity apatites are more conventional, this work investigated lower crystallinity variations from an amorphous state to low crystallinity apa...

Calcium phosphate research in Australia has originated from studies on natural minerals – biological deposits in teeth and in nature. This review surveys the research in the different regions in Australia and shows the most activity in Victoria and New South Wales concentrated on the theme of apatite. While the first research on calcium phosphate i...

The diversity in the structural and chemical state of apatites allows implant manufacturers to finely tune implant properties. This requires suitable manufacturing processes and characterization tools to adjust the amorphous phase and hydroxyl content from the source hydroxylapatite. Hydroxylapatite was processed by high velocity oxy fuel spraying,...

Inclusion of zinc in apatites is not completely understood due to complexes in solution preventing the incorporation or the inclusion of defects. Some work has shown zinc addition by alternative synthesis routes, but the level of incorporation has not been reassessed. This work uses a newly developed approach whereby zinc is included in an amorphou...

The difficulty of beta tricalcium phosphate (b-TCP) crystallization in aqueous media opens the question whether b-TCP can be produced using an alternative pathway. Amorphous calcium phosphate (ACP) is metastable in an aqueous environment and prefers a more stable apatite phase. Others have transformed a crystallized calcium deficient hydroxyapatite...

Removal of bacteria is important not only at implantation, but after long-term implant/prosthesis use. This requires strategies that employ different approaches for combating bacteria. Halides have the potential of an additional mechanism, and together with silver may provide a more powerful antibacterial strategy. Silver iodide was synthesized as...

Zinc offers many benefits for bone growth and an antimicrobial action for implants. Nearly all studies have synthesized crystals in solution by precipitation, but this produces low crystallinity and poor thermal stability. This work addresses a new approach of incorporating zinc in an amorphous calcium phosphate followed by crystallization at an el...

Objective: The aim of this laboratory study was to investigate the effect of three commercial bleaching agents and Tooth Mousse containing 10% w/w casein phosphopeptide –amorphous calcium phosphate on the hardness of tooth enamel. Methods: Sixteen human enamel specimens were exposed to one of three commercial bleaching agents with or without subseq...

Fatigue testing of load-bearing coated implants is usually very time-consuming and so a new contact nanofatigue test using a nanoindenter has been evaluated. A cube corner indenter provided the fastest indication of failure, through crack formation, compared to a spherical indenter. Contact nanofatigue was performed on a sintered hydroxyapatite and...

Droplet processing technologies and many biological processes use disk-like or hemispherical shapes for construction or the design of surfaces. The ability to tune the characteristics and properties of a surface is important at the micro- and nano-scale. The influence of size on the mechanical properties is presently unknown. This work set out to p...

Oxyhydroxyapatite and oxyapatite impact the fields of medicine, energy generation, sensing, and photonics. While oxyapatite may be stabilized with trivalent cations, oxyhydroxyapatite is inherently metastable and represents a pathway from hydroxyapatite to oxyapatite that subsequently decomposes to tricalcium phosphate and tetracalcium phosphate. H...

In this study, nanostructured Ce- and Sm-substituted Ca10(PO4)6(OH)2 samples have been synthesized using an aqueous sol-gel chemistry route. For the characterization of obtained specimens, the XRD, SEM, UV-visible reflection and IR spectroscopy and luminescence measurements were recorded. It was shown that phase purity of the end products highly de...

Incorporation of specific elements into calcium phosphates offers the combination of a bioactive material and a therapeutic effect. This is important for improving the integration of implants as well as treating medical conditions. Strontium is a suitable candidate and displays the ability to stimulate bone growth and reducing bone resorption. This...

Coating characteristics such as composition, crystallite features and topography collectively impact the cell response. The influence from splats has not yet been assessed for hydroxyapatite (HAp) thermal spray coatings. The objective of this work is to (a) survey the topography on commercial implants, (b) ascertain topography formation from single...

Drug delivery in biomaterials requires consideration of the pore network to store and deliver greater concentrations over an extended period of time. The porous structure in wood is used for delivery of water and therefore presents a possible pore architecture for storing fluids. The goal was to determine the pore characteristics of dried and pyrol...

High temperature processing is essential for the preparation of apatites for biomaterials, lighting, waste removal and other applications. This requires a good understanding of the thermal stability and transitions upon heating. The most widely used is hydroxyapatite (HAp), but increasing interest is being directed to fluorapatite (FAp) and chlorap...

Splitting problems at HA-coated implants are generally due to biological reasons. Bond-coatings were used to prevent the splitting problem of zirconia ceramics; this method can be widely seen in industrial applications. Two main groups were used; the first group consisted of spraying a bond layer of titania onto commercially pure titanium. This fol...

Amorphous calcium phosphate (ACP) is found in biological organisms and coated implants, used in calcium phosphate cements, and has been recently confirmed as a precursor in mineralized tissue; however, nothing is known about crack initiation in ACP or its fracture toughness. The objective of this study was to determine the crack resistance of ACP t...

Mechanical compatibility between a coating and a substrate is important for the longevity of implant materials. While previous studies have utilized the entire coating for analysis of mechanical compatibility of the surface, this study focuses on the nanoindentation of a uniformly thermally sprayed splat. Hydroxyapatite was thermally sprayed to cre...

New processing routes and characterization techniques underpin further growth of biomaterials for improved performance and multifunctionality. This study investigates the characteristics and solubility of amorphous calcium phosphate (ACP) printed splats. Splats made from 20 to 60 μm molten hydroxyapatite particles were classified for shape (rounded...

The structural, microstructural and morphological, as well as electric and dielectric, properties of nickel–zinc ferrite (Ni0.3Zn0.7Fe2O4) derived from sol–gel auto-combustion have been studied after sintering from 900 to 1300 °C. The effect of heating rate has not been previously investigated and is reported here. X-ray diffraction showed a pure c...

The objective of this work was to characterize the deposits of calcium phosphate produced by thermal printing in terms of structure, topography and mechanical properties. Hydroxyapatite was molten and directed to (a) a titanium target in relative motion and (b) stationary titanium substrates preheated to 100 �C and 350 �C. Scanning electron microsc...

The performance of biomedical implants relies on the ability to assess and refine the microstructure of biomaterials. Instrumented nanoindentation was applied to determine the mechanical properties of plasma sprayed hydroxyapatite-coated implants from different commercial vendors. All biomedical devices contained both amorphous and crystalline phas...

Indentation on a smooth surface, perpendicular to the indenter tip, is critical to obtaining accurate mechanical property values with nanoindentation. However, for some materials, achieving such a scenario may not always be feasible. To investigate the effect this may have, angled indentations were made on flat, sintered hydroxyapatite samples indi...

The ability to quantify surface mechanical properties is valuable for assessing the quality of thermal spray coatings. This is especially important for prostheses where loading is placed directly on the surface. Hydroxyapatite was classified to small (20–40 μm), medium (40–60 μm) and large (60–80 μm) particle sizes and thermal sprayed to produce a...

Calcium phosphate ceramics are widely used as coating materials to orthopedic implants and are found to enhance initial bony ingrowth by stimulating osseous apposition to the implant surface. In this study, two novel calcium orthophosphate materials were selected for coating onto the commonly used orthopedic implant material Ti-6Al- 4V. One was cal...

Solution and suspension thermal spraying is providing a more economic approach to the production of thin coatings. Advances in this new promising technology require accompanying tools to assess micro and nanosized areas within these deposits. Hydroxyapatite was deposited in an r.f. plasma using a powder and a suspension. The powder feedstock produc...

Mechanical integrity is a key requirement of all thermal spray coatings for their successful application. It has been customary to obtain this information from bond strength tests, indirect assessment from the microstructure or a range of other mechanical tests that also include the resistance to crack propagation. Since the building block of therm...

Nanoindentation offers a unique capability to assess the mechanical properties of polished cross-sections of thermal spray coatings. This study set out to investigate the suitability of nanoindentation to extend the analysis of the cross-section to multiple points for a more detailed insight into variations through the thickness of the coating. Hyd...

Knowledge of the intrinsic properties of hydroxyapatite (HAp) single crystals is important for the design of natural systems and will assist further improvements of manufactured biomaterials. Nanoindentation provides a useful tool for determining mechanical properties such as the hardness, elastic modulus and fracture toughness of small samples. A...

Knowledge of the intrinsic properties of hydroxyapatite (HAp) single crystals is important for the design of natural systems and will assist further improvements of manufactured biomaterials. Nanoindentation provides a useful tool for determining mechanical properties such as the hardness, elastic modulus and fracture toughness of small samples. A...

Modification of hydroxyapatite surfaces can improve the properties of biomedical devices. The objective of this work was to encapsulate hydroxyapatite particles with a fluorapatite layer. A suspension of hydroxyapatite microspheres was prepared in a solution at different pH and treated with ammonium fluoride. pH, calcium, and fluoride were monitore...

There is considerable literature suggesting that silica (opal) phytoliths cause dental enamel microwear in mammals. Much of this literature cites a single study from 1959 as evidence that silica phytoliths are harder than mammalian tooth enamel and so have the potential to cause dental microwear. No other studies using similar methodology have actu...

Bisphosphonates (BP) are drugs currently administered orally to treat diseases characterised by an excessive bone resorption. Alternative and more efficient delivery routes and more potent compounds are being investigated. Three implantable delivery systems, which allow the controlled release of therapeutic agents from the device core, are examined...

Hydroxyapatite (HAp) is a common implant material choice for orthopaedic implants and is mostly used as a coating to improve the biocompatibility of implants and stimulate tissue growth. This is a desirable material for implants, but the thermal instability requires strict control of the processing conditions to produce a crystalline coating. A coa...