Tim Desmet

Nonthermal Plasma Technology as a Versatile Strategy for Polymeric Biomaterials Surface Modification: A Review

In modern technology, there is a constant need to solve very complex problems and to fine-tune existing solutions. This is definitely the case in modern medicine with emerging fields such as regenerative medicine and tissue engineering. The problems, which are studied in these fields, set very high demands on the applied materials. In most cases, it is impossible to find a single material that meets all demands such as biocompatibility, mechanical strength, biodegradability (if required), and promotion of cell-adhesion, proliferation, and differentiation. A common strategy to circumvent this problem is the application of composite materials, which combine the properties of the different constituents. Another possible strategy is to selectively modify the surface of a material using different modification techniques. In the past decade, the use of nonthermal plasmas for selective surface modification has been a rapidly growing research field. This will be the highlight of this review. In a first part of this paper, a general introduction in the field of surface engineering will be given. Thereafter, we will focus on plasma-based strategies for surface modification. The purpose of the present review is twofold. First, we wish to provide a tutorial-type review that allows a fast introduction for researchers into the field. Second, we aim to give a comprehensive overview of recent work on surface modification of polymeric biomaterials, with a focus on plasma-based strategies. Some recent trends will be exemplified. On the basis of this literature study, we will conclude with some future trends for research.

Synergistic effect of surface modification and scaffold design of bioplotted 3-D poly-ε-caprolactone scaffolds in osteogenic tissue engineering.

The hydrophobic nature and the regular scaffold architecture of bioplotted poly(ε-caprolactone) (PCL) scaffolds present some hurdles for homogeneous tissue formation and differentiation. The current hypothesis is that a synergistic effect of applied surface modification and scaffold design enhances colonization and osteogenic differentiation. First, PCL scaffolds with a 0/90° lay-down pattern (0/90) were plotted and subjected to an oxygen plasma (O2) or multistep surface modification, including post-argon 2-amino-ethylmethacrylate grafting (AEMA), followed by immobilization of gelatin type B (gelB) and physisorption of fibronectin (gelB Fn). Secondly, scaffolds of different designs were plotted (0/90° shift (0/90 S), 0/45° and 0/90° with narrow pores (0/90 NP)) and subjected to the double protein coating. Preosteoblasts were cultured on the scaffolds and the seeding efficiency, colonization and differentiation were studied. The data revealed that a biomimetic surface modification improved colonization (gelB Fn>gelB>AEMA>O2). Compact scaffold architectures (0/90 NP, 0/45, 0/90 S>0/90) positively influenced the seeding efficiency and differentiation. Interestingly, the applied surface modification had a greater impact on colonization than the scaffold design. In conclusion, the combination of a double protein coating with a compact design enhances tissue formation in the plotted PCL scaffolds.

In vitro cell-biological performance and structural characterization of selective laser sintered and plasma surface functionalized polycaprolactone scaffolds for bone regeneration

In the present study a structural characterization and in vitro cell-biological evaluation was performed on polycaprolactone (PCL) scaffolds that were produced by the additive manufacturing technique selective laser sintering (SLS), followed by a plasma-based surface modification technique, either non-thermal oxygen plasma or double protein coating, to functionalize the PCL scaffold surfaces. In the first part of this study pore morphology by means of 2D optical microscopy, surface chemistry by means of hydrophilicity measurement and X-ray photoelectron spectroscopy, strut surface roughness by means of 3D micro-computed tomography (CT) imaging and scaffold mechanical properties by means of compression testing were evaluated before and after the surface modifications. The results showed that both surface modifications increased the PCL scaffold hydrophilicity without altering the morphological and mechanical properties. In the second part of this study the in vitro cell proliferation and differentiation of human osteoprogenitor cells, over 14 days of culture in osteogenic and growth medium were investigated. The O2 plasma modification gave rise to a significant lower in vitro cell proliferation compared to the untreated and double protein coated scaffolds. Furthermore the double protein coating increased in vitro cell metabolic activity and cell differentiation compared to the untreated and O2 plasma PCL scaffolds when OM was used.

Double protein-coated poly-ε-caprolactone scaffolds: successful 2D to 3D transfer.

In the past decade, tissue engineering has evolved from a promising technology to an established scientific field. Large attention has focussed on developing scaffolds from both biodegradable and nondegradable polymers to be cultivated with cells, to replace human body defects. The major drawback of most polymers is however their limited cell-interactive properties. An additional complication when developing a surface modification protocol for those materials is the transferability of protocols from 2D substrates to 3D scaffolds. In the present work, we therefore report on possible biological effects originating from the transfer of a double protein coating protocol, involving gelatin type B and fibronectin, from 2D poly-ε-caprolactone (PCL) films to 3D PCL scaffolds produced by rapid prototyping. A variety of techniques including scanning electron microscopy, X-ray photoelectron spectroscopy and confocal fluorescence microscopy confirmed a successful and homogeneous protein-coating on both 2D and 3D substrates. Interestingly, the biological performance of the double protein-coated PCL substrates, reflected by the initial cell adhesion, proliferation, and colonization was superior compared to the other surface modification steps, independent of the material dimension.

Double protein functionalized poly-ε-caprolactone surfaces: in depth ToF–SIMS and XPS characterization

In biomaterial research, great attention has focussed on the immobilization of biomolecules with the aim to increase cell-adhesive properties of materials. Many different strategies can be applied. In previously published work, our group focussed on the treatment of poly-ε-caprolactone (PCL) films by an Ar-plasma, followed by the grafting of 2-aminoethyl methacrylate (AEMA) under UV-irradiation. The functional groups introduced, enabled the subsequent covalent immobilisation of gelatin. The obtained coating was finally applied for the physisorption of fibronectin. The successful PCL surface functionalization was preliminary confirmed using XPS, wettability studies, AFM and SEM. In the present article, we report on an in-depth characterization of the materials developed using ToF–SIMS and XPS analysis. The homogeneous AEMA grafting and the subsequent protein coating steps could be confirmed by both XPS and ToF–SIMS. Using ToF–SIMS, it was possible to demonstrate the presence of polymethacrylates on the surface. From peak deconvoluted XPS results (C- and N-peak), the presence of proteins could be confirmed. Using ToF–SIMS, different positive ions, correlating to specific amino-acids could be identified. Importantly, the gelatin and the fibronectin coatings could be qualitatively distinguished. Interestingly for biomedical applications, ethylene oxide sterilization did not affect the surface chemical composition. This research clearly demonstrates the complementarities of XPS and ToF–SIMS in biomedical surface modification research.

Visualization of the Penetration Depth of Plasma in Three-Dimensional Porous PCL Scaffolds

A dielectric barrier discharge (DBD) discharge is used to modify the surface properties of 3-D porous polycaprolactone (PCL) scaffolds. After plasma treatment, the penetration of blue ink into the samples was used to determine the effectiveness of the plasma treatment inside the structures. It was found that the ink could penetrate deeper into the scaffolds after plasma treatment.

Radiolabeled gelatin type B analogues can be used for non-invasive visualisation and quantification of protein coatings on 3D porous implants.

This study covers the quantification of the covalent attachment of gelatin type B (GelB) and the subsequent adsorption of Fibronectin (Fn) on poly-ε-caprolactone (PCL) surfaces, functionalised with 2-aminoethyl methacrylate (AEMA) by means of post-plasma UV-irradiation grafting. As typical surface characterisation tools do not allow quantification of deposited amounts of GelB or Fn, radiolabeled analogues were used for direct measurement of the amount of immobilized material. Bolton-Hunter GelB (BHG) and Fn were radioiodinated with 131I and 125I respectively and S-Hynic GelB (SHG) was labeled with 99mTc. Immobilisation of 131I-BHG or 99mTc-SHG on both PCL and PCL-AEMA scaffolds was performed in analogy with earlier work. SPECT images on scaffolds coated with 99mTc-SHG conjugates were acquired on a U-SPECT II camera. There was a clear difference in the amount of deposited 131I-BHG between blanco and AEMA-grafted PCL on 2D samples. No significant differences in immobilization behaviour were observed between 99mTc-SHG and 131I-BHG. Subsequent immobilisation of Fn was successful and depended on the amounts of deposited GelB. SPECT imaging on cylindrical 3D scaffolds confirmed these findings and showed that the amount of immobilized 99mTc-SHG was depth dependant. The architecture of the scaffolds strongly influences the distribution of GelB within these structures. Furthermore, there is a clear difference in the homogeneity of the protein coating when different GelB immobilization protocols were applied. This study shows that radiolabeled compounds are a rapid and accurate tool in the quantitative and qualitative evaluation of the biofunctionalisation of AEMA grafted PCL scaffolds.

The effect of medium pressure plasma treatment on thin poly-ϵ-caprolactone layers

Abstract In this work, the effect of medium pressure plasma treatment on thin poly-ϵ-caprolactone (PCL) layers on glass plates is investigated. PCL is a biocompatible and biodegradable polymer which potentially can be used for bone repair, tissue engineering and other biomedical applications. However, cell adhesion and proliferation are inadequate due to its low surface energy and a surface modification is required in most applications. To enhance the surface properties of thin PCL layers spin coated on glass plates, a dielectric barrier discharge (DBD) at medium pressure operating in different atmospheres (dry air, argon, helium) was used. After plasma treatment, water contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to examine the PCL samples. These measurements show that the medium pressure plasma treatment is able to increase the hydrophilic character of the samples, due to an incorporation of oxygen groups at the surface and that the surface roughness is significantly decreased after plasma treatment.

Plasma surface treatment of biomedical polymers to improve cell adhesion

Summary form only given. Biomedical polymers have a great potential in medicine due to their biocompatible nature and versatility. However, their low surface energy leads to a cell adhesion and proliferation that is far less then optimal. To make them excellent candidates for implants and tissue engineering scaffolds, a surface modification is required.