The effect of simvastatin on bone formation and ceramic resorption in a peri-implant defect model

Experimental use of statins as stimulators of bone formation suggests they may have widespread applicability in the field of orthopaedics. With their combined effects on osteoblasts and osteoclasts, statins have the potential to enhance resorption of synthetic materials and improve bone ingrowth. In this study, the effect of oral and local administration of simvastatin to a 0 tricalcium phosphate (beta TCP)-filled defect around an implant was compared with recombinant human bone morphogenetic protein 2 (rhBMP2). On hundred and sixty-two Sprague-Dawley rats were assigned to treatment groups: local application of 0.1, 0.9 or 1.7 mg of simvastatin, oral simvastatin at 5, 10 or 50 mg kg(-1) day(-1) for 20 days, local delivery of I or 10 mu g of rhBMP2, or control. At 6 weeks rhBMP2 increased serum tartrate-resistant acid phosphatase 5b levels and reduced PTCP area fraction, particle size and number compared with control, suggesting increased osteoclast activity. There was reduced stiffness and increased mechanical strength with this treatment. Local simvastatin resulted in a decreased mineral apposition rate at 6 weeks and increased fibrous area fraction, PTCP area fraction, particle size and number at 26 weeks. Oral simvastatin had no effect compared with control. Local application of rhBMP2 increased resorption and improved mechanical strength whereas simvastatin was detrimental to healing. Oral simvastatin was ineffective at promoting either ceramic resorption or bone formation. The effect of statins on the repair of bone defects with graft substitute materials is influenced by its bioavailability. Thus, further studies on the optimal delivery system are needed. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The effect of osteogenic growth factors on bone growth into a ceramic filled defect around an implant

Currently available synthetic bone substitutes perform poorly compared to autograft. It is hoped that by adding osteogenic growth factors to the materials, new bone formation could be increased and the clinical outcome improved. In this study, IGF-1, bFGF and TGFbeta1, alone and in combination, were absorbed onto a carrier of P-tricalcium phosphate (PTCP) and implanted into a defect around a hydroxyapatite-coated, stainless steel implant in the proximal tibia of rat in a model of revision arthroplasty. Animals were sacrificed at 6 and 26 weeks for routine histology and histomorphometry and mechanical push out tests. The results show that only bFGF had a significant effect on ceramic resorption. The groups that received bFGF and bFGF in combination with TGFbeta1 had smaller and fewer betaTCP particles remaining in the defect at 6 and 26 weeks. No growth factor combination significantly enhanced new bone formation or the mechanical strength of the implant. These results indicate that, of the growth factors tested, only bFGF had any beneficial effect on the host response to the implant, perhaps by delaying osteoblast differentiation and thereby prolonging osteoclast access to the ceramic. (C) 2004 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved.

Laser-engineered dissolving microneedles for active transdermal delivery of nadroparin calcium

There is an urgent need to replace the injection currently used for low molecular weight heparin (LMWH) multidose therapy with a non- or minimally invasive delivery approach. In this study, laser-engineered dissolving microneedle (DMN) arrays fabricated from aqueous blends of 15% w/w poly(methylvinylether-co-maleic anhydride) were used for the first time in active transdermal delivery of the LMWH nadroparin calcium (NC). Importantly, an array loading of 630 IU of NC was achieved without compromising the array mechanical strength or drug bioactivity. Application of NC-DMNs to dermatomed human skin (DHS) using the single-step 'poke and release' approach allowed permeation of approximately 10.6% of the total NC load over a 48-h study period. The cumulative amount of NC that permeated DHS at 24 h and 48 h attained 12.28 ± 4.23 IU/cm and 164.84 ± 8.47 IU/cm , respectively. Skin permeation of NC could be modulated by controlling the DMN array variables, such as MN length and array density as well as application force to meet various clinical requirements including adjustment for body mass and renal function. NC-loaded DMN offers great potential as a relatively low-cost functional delivery system for enhanced transdermal delivery of LMWH and other macromolecules. © 2012 Elsevier B.V. All rights reserved.

Liquid marble formation using hydrophobic powders

This work aims to investigate and quantitatively measure “liquid marble” phenomena using hydrophobic powders (granules). The hydrophobic powders based on a copper substrate were prepared by a silver deposition technique of particle sizes 9 µm, 20 µm and 320 µm and of contact angle with water approaching 160°. The hydrophobic powder poly-methylmethacralate (PMMA) particle size 42 µm and contact angle of 120° was also used to determine the effect of powder density on liquid marble stability. The experimental investigations indicated that for successful formation of liquid marbles a number of variables in addition to hydrophobicity need to be considered, namely: powder density; powder particle size; powder shape; liquid marble formation technique. It was found that liquid marbles were formed using all four powders to varying extents, with a low powder particle size forming more stable liquid marbles. In a series of gravimetric tests, adhered powder mass on liquid marbles was found to be directly proportional to the water droplet surface area. A more complete coverage of the water drops were found with PMMA powder than the hydrophobic granules. Moreover, a further procedure was developed to increase the mechanical strength of the liquid marble, by polymerising methylmethacrylate (MMA) on the surface of a PMMA powder – liquid marble, with the aim of maintaining water within a more robust PMMA – liquid marble shell. This technique may prove to be a novel way of encapsulating drug compounds, such as gentamicin sulphate, for PMMA bone cement.

Structural reinforcement of microvascular networks using electrostatic layer-by-layer assembly with halloysite nanotubes

We demonstrate a method for tailoring local mechanical properties near channel surfaces of vascular structural polymers in order to achieve high structural performance in microvascular systems. While synthetic vascularized materials have been created by a variety of manufacturing techniques, unreinforced microchannels act as stress concentrators and lead to the initiation of premature failure. Taking inspiration from biological tissues such as dentin and bone, these mechanical deficiencies can be mitigated by complex hierarchical structural features near to channel surfaces. By employing electrostatic layer-by-layer assembly (ELbL) to deposit films containing halloysite nanotubes onto scaffold surfaces followed by matrix infiltration and scaffold removal, we are able to controllably deposit nanoscale reinforcement onto 200 micron diameter channel surface interiors in microvascular networks. High resolution strain measurements on reinforced networks under load verify that the halloysite reduces strain concentrations and improves mechanical performance.

Degradation of poly-L-lactide. Part 1: in vitro and in vivo physiological temperature degradation

Poly-L-Lactide is a bioresorbable polymer which degrades through hydrolysis of its ester linkage influenced by initial molecular weight and degree of crystallinity. Polymers belonging to the aliphatic polyester family currently represent the most attractive group of polymers that meet the medical and physical demands for safe clinical applications. Compression moulded PLLA pellets were produced as rods, sterilized and degraded both in vitro and in vivo (sub-dermal implantation model). The material molecular weight, crystallinity, mechanical strength and thermal properties were evaluated. In both in vitro and in vivo environments, degradation proceeded at the same rate and followed the general sequence of aliphatic polyester degradation, ruling out enzymes accelerating the degradation rate in vivo. By 44 weeks duration of implantation the PLLA rods were still biocompatible, before any mass loss was observed.

Degradation of poly-L-lactide. Part2 : increased temperature accelerated degradation

Poly-L-lactide (PLLA) is one of the most significant members of a group of polymers regarded as bioresorbable. The degradation of PLLA proceeds through hydrolysis of the ester linkages in the polymer's backbone; however, the time for the complete resorption of orthopaedic devices manufactured from PLLA is known to be in excess of five years in a normal physiological environment. To evaluate the degradation of PLLA in an accelerated time period, PLLA pellets were processed by compression moulding into tensile test specimens, prior to being sterilized by ethylene oxide gas (EtO) and degraded in a phosphate-buffered solution (PBS) at both 50°C and 70°C. On retrieval, at predetermined time intervals, procedures were used to evaluate the material's molecular weight, crystallinity, mechanical strength, and thermal properties. The results from this study suggest that at both 50°C and 70°C, degradation proceeds by a very similar mechanism to that observed at 37°C in vitro and in vivo. The degradation models developed also confirmed the dependence of mass loss, melting temperature, and glass transition temperature (Tg) on the polymer's molecular weight throughout degradation. Although increased temperature appears to be a suitable method for accelerating the degradation of PLLA, relative to its physiological degradation rate, concerns still remain over the validity of testing above the polymer's Tg and the significance of autocatalysis at increased temperatures.

Microcomputed tomography imaging in a rat model of delayed union/non-union fracture

We aimed to develop a clinically relevant delayed union/non-union fracture model to evaluate a cell therapy intervention repair strategy. Histology, three-dimensional (3D) micro-computed tomography (micro-CT) imaging and mechanical testing were utilized to develop an analytical protocol for qualitative and quantitative assessment of fracture repair. An open femoral diaphyseal osteotomy, combined with periosteal diathermy and endosteal excision, was held in compression by a four pin unilateral external fixator. Three delayed union/non-union fracture groups established at 6 weeks-(a) a control group, (b) a cell therapy group, and (c) a group receiving phosphate-buffered saline (PBS) injection alone-were examined subsequently at 8 and 14 weeks. The histological response was combined fibrous and cartilaginous non-unions in groups A and B with fibrous non-unions in group C. Mineralized callus volume/total volume percentage showed no statistically significant differences between groups. Endosteal calcified tissue volume/endosteal tissue volume, at the center of the fracture site, displayed statistically significant differences between 8 and 14 weeks for cell and PBS intervention groups but not for the control group. The percentage load to failure was significantly lower in the control and cell treatment groups than in the PBS alone group. High-resolution micro-CT imaging provides a powerful tool to augment characterization of repair in delayed union/non-union fractures together with outcomes such as histology and mechanical strength measurement. Accurate, nondestructive, 3D identification of mineralization progression in repairing fractures is enabled in the presence or absence of intervention strategies. (c) 2007 Orthopaedic Research Society.

Hydrogels as drug-delivery platforms:Physicochemical barriers and solutions

The properties of hydrogels, in particular their high biocompatibility and water sorption uptake, make hydrogels very attractive in drug delivery and biomedical devices. These favorable features of hydrogels are compromised by certain structural limitations such as those associated with their low mechanical strength in the swollen state. This review highlights the most important challenges that may seriously affect the practical implementation of hydrogels in clinical practice and the solutions that may be applied to overcome these limitations. © 2012 Future Science Ltd.

Hydrogel-forming microneedle arrays exhibit antimicrobial properties:potential for enhanced patient safety

We describe, for the first time, the microbial characterisation of hydrogel-forming polymeric microneedle arrays and the potential for passage of microorganisms into skin following microneedle penetration. Uniquely, we also present insights into the storage stability of these hydroscopic formulations, from physical and microbiological viewpoints, and examine clinical performance and safety in human volunteers. Experiments employing excised porcine skin and radiolabelled microorganisms showed that microorganisms can penetrate skin beyond the stratum corneum following microneedle puncture. Indeed, the numbers of microorganisms crossing the stratum corneum following microneedle puncture were greater than 105 cfu in each case. However, no microorganisms crossed the epidermal skin. When using a 21G hypodermic needle, more than 104 microorganisms penetrated into the viable tissue and 106 cfu of Candida albicans and Staphylococcus epidermidis completely crossed the epidermal skin in 24 h. The hydrogel-forming materials contained no microorganisms following de-moulding and exhibited no microbial growth during storage, while also maintaining their mechanical strength, apart from when stored at relative humidities of 86%. No microbial penetration through the swelling microneedles was detectable, while human volunteer studies confirmed that skin or systemic infection is highly unlikely when polymeric microneedles are used for transdermal drug delivery. Since no pharmacopoeial standards currently exist for microneedle-based products, the exact requirements for a proprietary product based on hydrogel-forming microneedles are at present unclear. However, we are currently working towards a comprehensive specification set for this microneedle system that may inform future developments in this regard.

Control of morphological properties of porous biodegradable scaffolds processed by supercritical CO2 foaming

A low cost supercritical CO foaming rig with a novel design has been used to prepare fully interconnected and highly porous biodegradable scaffolds with controllable pore size and structure that can promote cancellous bone regeneration. Porous polymer scaffolds have been produced by plasticising the polymer with high pressure CO and by the formation of a porous structure following the escape of CO from the polymer. Although, control over pore size and structure has been previously reported as difficult with this process, the current study shows that control is possible. The effects of processing parameters such as CO saturation pressure, time and temperature and depressurisation rate on the morphological properties, namely porosity, pore interconnectivity, pore size and wall thickness- of the scaffolds have been investigated. Poly(d,l)lactic acid was used as the biodegradable polymer. The surfaces and internal morphologies of the poly(d,l)lactic acid scaffolds were examined using optical microscope and micro computed tomography. Preosteoblast human bone cells were seeded on the porous scaffolds in vitro to assess cell attachment and viability. The scaffolds showed a good support for cell attachment, and maintained cell viability throughout 7 days in culture. This study demonstrated that the morphology of the porous structure can be controlled by varying the foaming conditions, allowing the porous scaffolds to be used in various tissue engineering applications.

Rotational moulding of thermoplastic polyester urethanes

Various grades of Thermoplastic Polyurethane (TPU) supplied by Bayer were studied to determine their suitability for the rotational moulding process. Following grinding, parts were produced using a variety of peak internal air temperatures and cooling rates. The tensile and impact properties of these parts were then analysed and it was found that both the grade and moulding conditions had a large bearing on the quality and mechanical strength of the part produced.

Characterisation of Bioresorbable Composite Performance for Validation of New On-Line Process Monitoring Technology

There is an increasing interest in the biomedical field to create implantable medical devices to provide a temporary mechanical function for use inside the human body. In many of these applications bioresorbable polymer composites using PLLA with β-TCP , are increasingly being used due to their biocompatability, biodegradability and mechanical strength.1,3 These medical devices can be manufactured using conventional plastics processing methods such as injection moulding and extrusion, however there is great need to understand and control the process due to a lack of knowledge on the influence of processing on material properties. With the addition of biocompatible additives there is also a requirement to be able to predict the quality and level of dispersion within the polymer matrix. On-line UV-Vis spectroscopy has been shown to monitor the quality of fillers in polymers. This can eliminate time consuming and costly post-process evaluation of additive dispersion. The aim of this work was to identify process and performance relationships of PLLA/β-TCP composites with respect to melt-extrusion conditions. This is part of a wider study into on-line process monitoring of bioresorbable polymers as used in the medical industry.
These results show that final properties of the PLLA/ β-TCP composite are highly influenced by the particle size and loading. UV-Vis spectroscopy can be used on-line to monitor the final product and this can be utilised as a valuable tool for quality control in an application where consistent performance is of paramount importance.

A facile system to evaluate in vitro drug release from dissolving microneedle arrays

The use of biological tissues in the in vitro assessments of dissolving (?) microneedle (MN) array mechanical strength and subsequent drug release profiles presents some fundamental difficulties, in part due to inherent variability of the biological tissues employed. As a result, these biological materials are not appropriate for routine used in industrial formulation development or quality control (QC) tests. In the present work a facile system using Parafilm M® (PF) to test drug permeation performance using dissolving MN arrays is proposed. Dissolving MN arrays containing 196 needles (600 μm needle height) were inserted into a single layer of PF and a hermetic “pouch” was created including the array inside. The resulting system was placed in a dissolution bath and the release of model molecules was evaluated. Different MN formulations were tested using this novel setup, releasing between 40 and 180 µg of their cargos after 6 hours. The proposed system is a more realistic approach for MN testing than the typical performance test described in the literature for conventional transdermal patches. Additionally, the use of PF membrane was tested either in the hermetic “pouch” and using Franz Cell methodology yielding comparable release curves. Microscopy was used in order to ascertain the insertion of the different MN arrays in the PF layer. The proposed system appears to be a good alternative to the use of Franz cells in order to compare different MN formulations. Given the increasing industrial interest in MN technology, the proposed system has potential as a standardised drug/active agent release test for quality control purposes.