Innovating in the medical device industry - challenges & opportunities ESB 2015 translational research symposium.

The European Society for Biomaterials 2015 Translational Research Symposium focused on 'Innovating in the Medical Device Industry - Challenges & Opportunities' from different perspectives, i.e., from a non-profit research organisation to a syndicate of small and medium-sized companies and large companies. Lecturers from regulatory consultants, industry and research institutions described the innovation process and regulatory processes (e.g., 510K, PMA, combination product) towards market approval. The aim of the present article is to summarise and explain the main statements made during the symposium, in terms of challenges and opportunities for medical device industries, in a constantly changing customer and regulatory environment.

Progressing innovation in biomaterials. From the bench to the bed of patients.

A Translational Research Symposium was organized at the 2014 annual meeting of the European society for biomaterials. This brought together leading Tier one companies in clinical biomaterials and medical device markets, small and medium enterprises and entrepreneurial academics who shared their experiences on taking biomaterials technologies to commercial endpoints, in the clinics. The symposium focused on "Progressing Innovation in Biomaterials. From the Bench to the Bed of Patients". The aim of the present document is to illustrate the content of the symposium and to highlight the key lessons from selected lectures.

Influence of physico-chemical material characteristics on staphylococcal biofilm formation--a qualitative and quantitative in vitro analysis of five different calcium phosphate bone grafts.

Various compositions of synthetic calcium phosphates (CaP) have been proposed and their use has considerably increased over the past decades. Besides differences in physico-chemical properties, resorption and osseointegration, artificial CaP bone graft might differ in their resistance against biofilm formation. We investigated standardised cylinders of 5 different CaP bone grafts (cyclOS, chronOS (both ß-TCP (tricalcium phosphate)), dicalcium phosphate (DCP), calcium-deficient hydroxyapatite (CDHA) and α-TCP). Various physico-chemical characterisations e.g., geometrical density, porosity, and specific surface area were investigated. Biofilm formation was carried out in tryptic soy broth (TSB) and human serum (SE) using Staphylococcus aureus (ATCC 29213) and S. epidermidis RP62A (ATCC 35984). The amount of biofilm was analysed by an established protocol using sonication and microcalorimetry. Physico-chemical characterisation showed marked differences concerning macro- and micropore size, specific surface area and porosity accessible to bacteria between the 5 scaffolds. Biofilm formation was found on all scaffolds and was comparable for α-TCP, chronOS, CDHA and DCP at corresponding time points when the scaffolds were incubated with the same germ and/or growth media, but much lower for cyclOS. This is peculiar because cyclOS had an intermediate porosity, mean pore size, specific surface area, and porosity accessible to bacteria. Our results suggest that biofilm formation is not influenced by a single physico-chemical parameter alone but is a multi-step process influenced by several factors in parallel. Transfer from in vitro data to clinical situations is difficult; thus, advocating the use of cyclOS scaffolds over the four other CaP bone grafts in clinical situations with a high risk of infection cannot be clearly supported based on our data.

Effect of grain size and microporosity on the in vivo behaviour of ß-tricalcium phosphate scaffolds.

Defining the most adequate architecture of a bone substitute scaffold is a topic that has received much attention over the last 40 years. However, contradictory results exist on the effect of grain size and microporosity. Therefore, the aim of this study was to determine the effect of these two factors on the in vivo behaviour of ß-tricalcium phosphate (ß-TCP) scaffolds. For that purpose, ß-TCP scaffolds were produced with roughly the same macropore size (≈ 150 µm), and porosity (≈ 80 %), but two levels of microporosity (low: 10 % / high: ≈ 25 %) and grain size (small: 1.3 µm /large: ≈ 3.3 µm). The sample architecture was characterised extensively using materialography, Hg porosimetry, micro-computed tomography (µCT), and nitrogen adsorption. The scaffolds were implanted for 2, 4 and 8 weeks in a cylindrical 5-wall cancellous bone defect in sheep. The histological, histomorphometrical and µCT analysis of the samples revealed that all four scaffold types were almost completely resorbed within 8 weeks and replaced by new bone. Despite the three-fold difference in microporosity and grain size, very few biological differences were observed. The only significant effect at p < 0.01 was a slightly faster resorption rate and soft tissue formation between 4 and 8 weeks of implantation when microporosity was increased. Past and present results suggest that the biological response of this particular defect is not very sensitive towards physico-chemical differences of resorbable bone graft substitutes. As bone formed not only in the macropores but also in the micropores, a closer study at the microscopic and localised effects is necessary.

Optimization of a tunable process for rapid production of calcium phosphate microparticles using a droplet-based microfluidic platform.

Calcium phosphate (CaP) biomaterials are amongst the most widely used synthetic bone graft substitutes, owing to their chemical similarities to the mineral part of bone matrix and off-the-shelf availability. However, their ability to regenerate bone in critical-sized bone defects has remained inferior to the gold standard autologous bone. Hence, there is a need for methods that can be employed to efficiently produce CaPs with different properties, enabling the screening and consequent fine-tuning of the properties of CaPs towards effective bone regeneration. To this end, we propose the use of droplet microfluidics for rapid production of a variety of CaP microparticles. Particularly, this study aims to optimize the steps of a droplet microfluidic-based production process, including droplet generation, in-droplet CaP synthesis, purification and sintering, in order to obtain a library of CaP microparticles with fine-tuned properties. The results showed that size-controlled, monodisperse water-in-oil microdroplets containing calcium- and phosphate-rich solutions can be produced using a flow-focusing droplet-generator microfluidic chip. We optimized synthesis protocols based on in-droplet mineralization to obtain a range of CaP microparticles without and with inorganic additives. This was achieved by adjusting synthesis parameters, such as precursor concentration, pH value, and aging time, and applying heat treatment. In addition, our results indicated that the synthesis and fabrication parameters of CaPs in this method can alter the microstructure and the degradation behavior of CaPs. Overall, the results highlight the potential of the droplet microfluidic platform for engineering CaP microparticle biomaterials with fine-tuned properties.

Effect of minor amounts of ß-calcium pyrophosphate and hydroxyapatite on the physico-chemical properties and osteoclastic resorption of ß-tricalcium phosphate cylinders.

ß-Tricalcium Phosphate (ß-TCP), one of the most used bone graft substitutes, may contain up to 5 wt% foreign phase according to standards. Typical foreign phases include ß-calcium pyrophosphate (ß-CPP) and hydroxyapatite (HA). Currently, the effect of small amounts of impurities on ß-TCP resorption is unknown. This is surprising since pyrophosphate is a very potent osteoclast inhibitor. The main aim of this study was to assess the effect of small ß-CPP fractions (<1 wt%) on the in vitro osteoclastic resorption of ß-TCP. A minor aim was to examine the effect of ß-CPP and HA impurities on the physico-chemical properties of ß-TCP powders and sintered cylinders. Twenty-six batches of ß-TCP powder were produced with a Ca/P molar ratio varying between 1.440 and 1.550. Fifteen were further processed to obtain dense and polished ß-TCP cylinders. Finally, six of them, with a Ca/P molar ratio varying between 1.496 (1 wt% ß-CPP) and 1.502 (1 wt% HA), were incubated in the presence of osteoclasts. Resorption was quantified by white-light interferometry. Osteoclastic resorption was significantly inhibited by ß-CPP fraction in a linear manner. The presence of 1% ß-CPP reduced ß-TCP resorption by 40%, which underlines the importance of controlling ß-CPP content when assessing ß-TCP biological performance.

Design of ceramic-based cements and putties for bone graft substitution.

In the last 15 years, a large number of commercial ceramic-based cements and putties have been introduced as bone graft substitutes. As a result, large efforts have been made to improve our understanding of the specific properties of these materials, such as injectability, cohesion, setting time (for cements), and in vivo properties. The aim of this manuscript is to summarize our present knowledge in the field. Instead of just looking at scientific aspects, industrial needs are also considered, including mixing and delivery, sterilization, and shelf-life.

Theoretical and experimental approach to test the cohesion of calcium phosphate pastes.

Recent studies have revealed that the ability of a calcium phosphate cement paste to harden in a physiological environment without desintegrating into small particles might be a key property to ensure a safe and reliable clinical use of calcium phosphate cements. However, this property called cohesion is not well understood and has not been studied extensively. The goal of the present study was to better understand which factors affect the cohesion of a calcium phosphate paste using the combination of a theoretical and experimental approach. In the theoretical approach, factors expected to influence the paste cohesion such as Van der Waals forces, electrostatic and steric interactions, as well as osmotic effects were listed and discussed. In the experimental approach, a new method to measure the cohesion of a non-setting calcium phosphate paste was presented and used to assess the effects of various factors on this property. The new method allowed a continuous measurement of cohesion and gave reproducible results. The experimental results confirmed the theoretical predictions: an increase of the liquid-to-powder ratio of the paste and of the powder particle size, as well as the addition of citrate ions and in limited cases dissolved xanthan polymer chains reduced the paste cohesion.

Technological issues for the development of more efficient calcium phosphate bone cements: a critical assessment.

The first calcium phosphate cements (CPCs) were discovered in the 1980s. Two decades later, the interest for these materials is still rising. The goal of the present document is to review the most recent achievements in the field and to analyze future directions in research and development.

Correlating crystallinity and reactivity in an alpha-tricalcium phosphate.

In this study, the effect of how variant milling time affects material characteristics of alpha phase tricalcium phosphate powder (alpha-TCP) was studied. Two alpha-TCP batches were separated in small lots and milled for various times for up to 4 h. The resulting milled lots were characterized by measuring their crystallinity, particle size, specific surface area, thermal stability, and heat released during hydration. Mechanical treatment was seen to greatly increase the alpha-TCP X-ray amorphous fraction and heat release during hydration, almost independently of alpha-TCP particle size and specific surface area. Therefore, the results suggest that the formation and presence of an X-ray amorphous phase in the alpha-TCP powder greatly contribute to its reactivity. The exotherm of the powders increases from 103 to 238 kJ/mol after milling.

Synthesis and characterization of porous beta-tricalcium phosphate blocks.

Porous beta-tricalcium phosphate (beta-TCP) blocks with four different macropore sizes (pore larger than 50 microm were synthesized using "calcium phosphate emulsions", and characterized by optical, geometrical, gravimetric, and radiological methods. The reproducibility of the synthesis method was excellent. Moreover, the macropore size could be easily controlled without modifying the microporosity (pore smaller than 50 microm) or the total porosity (microporosity+macroporosity). Based on the initial composition of the blocks and their final apparent density, the microporosity, macroporosity, and the total block porosity were calculated to be close to 21%, 54%, and 75%, respectively. These values were confirmed by microcomputed tomography (microCT). The mean macropore diameters were close to 150, 260, 510 and 1220 microm, as measured optically. Consistently lower values (25% lower) were obtained by microCT, but the linear correlation between microCT and optical method was high (r(2)>0.97). The macropore size distribution calculated from microCT scans appears to be narrow and normally distributed. The very good correlation between the results of the various methods and the possibility to determine the pore size distribution suggest that microCT is an ideal tool to non-destructively characterize macroporous calcium phosphate bone substitutes.

Rheological characterization of concentrated aqueous beta-tricalcium phosphate suspensions: the effect of liquid-to-powder ratio, milling time, and additives.

The field of injectable calcium phosphate suspensions and cements is experiencing vigorous research activity. This is stimulated by their importance for the cement augmentation procedure (vertebroplasty), which is an emerging procedure to treat osteoporotic fragility fractures. The rheological properties such as the yield stress and viscosity play an important role in the process of cement delivery and infiltration into the cancellous bone cavities. However, the number of studies relating to their rheological properties is very limited. The objective of this first study was to examine the effects of the following three variables on the rheological properties of a non-setting beta-tricalcium phosphate suspension: liquid-to-powder ratio, milling of powder particles, and additives. The broad finding is that all the variables affect the rheological properties remarkably. The more specific salient finding is the large variation in viscosity and in the yield stress. The viscosity spanned three orders of magnitude and the yield stress spanned five orders of magnitude. It appears that the rheological properties can be altered at will. However, one has to exercise extreme caution because these changes are not without cost to other important properties such as the cohesiveness and mechanical properties of the cement. Another important finding is that a linear correlation between the yield stress and the viscosity was found. Measurement of one of these variables might be enough to determine the other.

[Investigation about the clinical use of brushite- and hydroxylapatite-cement in sheep]. / Untersuchungen uber den klinischen Einsatz von Brushite- und Hydroxylapatit-Zement beim Schaf.

For future clinical use as synthetic bone replacement, an injectable brushite-(chronOS-Inject) and hydroxylapatite-(Biobon) cement were compared in a drill hole model in 10 sheep over time at 2, 4, 6, 8, 16 and 24 weeks. Results were compared regarding their practical use, biocompatibiliy, resorption mechanism and subsequent new bone formation. The cements were filled into drill holes (psi 8 x 13mm) of the proximal and distal humerus, and femur and the samples evaluated macroscopically, radiologically and microscopically including histomorphometrical quantification of percentages of new bone, fibrous tissue and remnants of cements. The cement area decreased continuously from 2 to 24 weeks with chronOS-Inject, as well as the area of granules. Inversely, the subsequent new bone formation increased from 2-24 weeks accordingly. With Biobon the cement area decreased slower between 2 and 24 weeks, and the new bone formation was less. Both cements were well integrated into the bone in long bones. chronOS-Inject demonstrated good biocompatibility and was almost completely replaced through bone within 24 weeks. Biobon was resorbed considerably slower and initially a slight inflammatory reaction including bone resorption was observed within the adjacent host bone.

New hydraulic cements based on alpha-tricalcium phosphate-calcium sulfate dihydrate mixtures.

Calcium sulfate dihydrate (CSD) powder was added to a cement consisting of alpha-tricalcium phosphate (alpha-TCP) and water. The changes of the physico-chemical properties of the cement were investigated as a function of the CSD amount, the phosphate concentration in the mixing solution, and the solution volume. An increase of the phosphate concentration in the mixing liquid and small additions of CSD powder strongly reduced the cement setting time. Simultaneously, the fraction of unreacted alpha-TCP powder present after 1 day of incubation increased, indicating that alpha-TCP hydrolysis was inhibited. The effects of the CSD amount and the phosphate concentration were synergetic, i.e. the effect of CSD powder was increased with an increase of the phosphate concentration and vice versa. Interestingly, none of the factors affected the cement diametral tensile strength. The present results were explained based on solubility calculations. The present study shows that the use of CSD crystals in combination with phosphate ions is an easy and interesting way to control the setting time of alpha-TCP-water mixtures, in particular, because the mechanical properties of the cement are not modified.

Theoretical model to determine the effects of geometrical factors on the resorption of calcium phosphate bone substitutes.

A theoretical approach was used to determine the effect of geometrical factors on the resorption rate of calcium phosphate bone substitutes that are either dense, microporous, and/or contain spherical macropores. Two cases were considered: (a) macroporous blocks that can be invaded by resorbing cells either directly because the structure is fully open-porous, or indirectly after some resorption of the macropores walls and/or interconnections. (b) Microporous or dense blocks/granules that cannot be invaded by resorbing cells, i.e. can only be resorbed from the outside to the inside, layer by layer. The theoretical approach was based on five assumptions: (i) the pores are spherical; (ii) the pores are ordered according to a face-centered cubic packing; (iii) the resorption is surface-controlled; (iv) the resorption is only possible if the surface can be accessed by blood vessels of 50 microm in diameter; and (v) the resorption time of a given amount of calcium phosphate is proportional to the net amount of material. Based on these assumptions, the calculations showed that the resorption time of a macroporous block could be minimized at a specific pore radius. This pore radius depended (i) on the size of the bone substitute and (ii) on the interpore distance. Typical radii were in the range of 100-400 microm. These values are similar to the numerous pore size optima mentioned in the scientific literature. For microporous or dense blocks/granules, the model suggested that a relatively small radius should be preferred. Such a radius leads to an optimum combination of a high surface area favorizing resorption and the presence of large intergranular gaps favorizing blood vessel ingrowth. In that case, the optimum of granule radius is around 100-200 microm. Finally, a very good agreement was found between the predictions of the model and experimental data, i.e. the model explained in all but two cases the results with an accuracy superior to 80%. In conclusion, the model appears to be a useful tool to better understand in vivo results, and possibly better define the geometry and distribution of the pores as well as the size of a bone substitute.

Theoretical and experimental model to describe the injection of a polymethylmethacrylate cement into a porous structure.

A theoretical approach was used to determine the distribution of a poly(methylmethacrylate) cement after its injection into a porous structure. The predictions of the model were then compared to experimental results obtained by injecting a polymethylmethacrylate cement into an open-porous ceramic filter. The goal was to define a model that could predict what factors affect the risk of cement extravasation and hence how the risk of cement extravasation can be minimized. The calculations were based on two important rheological laws: the law of Hagen-Poiseuille and the law of Darcy. The law of Hagen-Poiseuille describes the flow of a fluid in a cylindrical tube. The law of Darcy describes the flow of a fluid through a porous media. The model predicted that the extravasation risk was decreased when the cement viscosity, the bone pore size, the bone permeability and the bone porosity were increased, and when the diameter of the extravasation path and the viscosity of the marrow were decreased. Experimentally, the effect of the marrow viscosity and extravasation path could be evidenced. Therefore, the model was believed to be an adequate approximation of the experimental behavior. In conclusion, the experimental results demonstrated that the model was adequate and that the best practical way to decrease the risk of extravasation is to increase the cement viscosity.

Comparison of human bone marrow stromal cells seeded on calcium-deficient hydroxyapatite, beta-tricalcium phosphate and demineralized bone matrix.

The aim of this study was to compare three resorbable biomaterials regarding seeding efficacy with human bone marrow stromal cells (BMSCs), cell penetration into the matrix, cell proliferation and osteogenic differentiation. Calcium-deficient hydroxyapatite (CDHA), beta-tricalcium phosphate (beta-TCP), and demineralized bone matrix (DBM) were seeded with human BMSCs and kept in human serum and osteogenic supplements for 3 weeks. Morphologic and biochemical evaluations were performed on day 1, 7, 14 and 21. The allograft DBM and CDHA exhibited both an excellent seeding efficacy while the performance of beta-TCP was lower when compared. The total protein content and the values for specific alkaline phosphatase (ALP) increased on all matrices and no significant difference was found for these two markers. BMSCs in monolayer had a significant increase of protein, but not of ALP. Osteocalcin (OC) values increased significantly higher for BMSC in cultures on DBM when compared to CDHA and beta-TCP. The OC levels decreased significantly in the BMSC monolayer culture. BMSCs were found inconsistently within the synthetic materials, whereas in DBM they were found more homogeneously distributed throughout the matrix. All three matrices promoted BMSC proliferation and differentiation to osteogenic cells. DBM allografts seem to be more favorable with respect to cell ingrowth tested by histology, and osteogenic differentiation ascertained by an increase of OC. CDHA with its high specific surface area showed more favorable properties than beta-TCP regarding reproducibility of the seeding efficacy.

Compositional changes of a dicalcium phosphate dihydrate cement after implantation in sheep.

A hydraulic calcium phosphate cement having dicalcium phosphate dihydrate (DCPD) as end-product of the setting reaction was implanted in a cylindrical defect in the diaphysis of sheep for up to 6 months. The composition of the cement was investigated as a function of time. After setting, the cement composition consisted essentially of a mixture of DCPD and beta-tricalcium phosphate (beta-TCP). In the first few weeks of implantation, the edges of the cement samples became depleted in DCPD, suggesting a selective dissolution of DCPD, possibly due to low pH conditions. The cement resorption at this stage was high. After 8 weeks, the resorption rate slowed down. Simultaneously, a change of the color and density of the cement center was observed. These changes were due to the conversion of DCPD into a poorly crystalline apatite. Precipitation started after 6-8 weeks and progressed rapidly. At 9 weeks, the colored central zone reached its maximal size. The fraction of beta-TCP in the cement was constant at all time. Therefore, this study demonstrates that the resorption rate of DCPD cement is more pronounced as long as DCPD is not transformed in vivo.

In vivo behavior of three different injectable hydraulic calcium phosphate cements.

Two dicalcium phosphate dihydrate (DCPD) hydraulic cements and one apatite hydraulic cement were implanted in epiphyseal and metaphyseal, cylindrical bone defects of sheep. The in vivo study was performed to assess the biocompatibility of the DCPD cements, using the apatite cement as control. After time periods of 2, 4 and 6 months the cement samples were clinically and histologically evaluated. Histomorphometrically the amount of new bone formation, fibrous tissue and the area of remaining cement were measured over time. In all specimens, no signs of inflammation were detectable either macroscopically or microscopically. All cements were replaced by different amounts of new bone. The two DCPD-cements showed the highest new bone formation and least cement remnants at 6 months, whereas the apatite was almost unchanged over all time periods.

Resorption patterns of calcium-phosphate cements in bone.

Two calcium phosphate cements, one monophasic and the other biphasic, have been used as bone void filler in a sheep model. The cements were injected into a slot defect in the proximal tibia and into a cylindrical defect in the distal femur. In this study, we focused on the resorption pattern of the two cement formulations and the subsequent biologic reaction. Bone remodeling occurred synchronously with the resorption of the implant material in a creeping substitution process. Cracks and pores in the monophasic cement were filled with osseous tissues. The biphasic cement showed faster resorption of the matrix. The more slowly resorbing granules were surrounded by newly grown bone, thus providing an inverse scaffold for cancellous bone regeneration. In highly loaded areas, the long-term support function of the fixation appears to be critical. Because cortical bridging of the defects was seen in only one case, it can be concluded that calcium-phosphate cements are preferentially suitable as cancellous bone substitute materials.