Processing and characterization of piezoelectric polymers for tissue engineering applications

Tese de doutoramento em Ciências (ramo de conhecimento em Física)In the last decades, the biomaterials and tissue engineering interdisciplinary research fields have been two of the most dynamic ones and have attracted increasing attention by the scientific community. With the advancements in the tissue engineering field the necessity of study and develop a wide variety of biomaterials with different properties has emerged. Among the different types of materials, polymers have proved to be an excellent choice, due to their simple processing, flexibility, physical properties and due to be easy to get in different shapes. In particular, piezoelectric polymers have attracted interest since they respond to electrical and mechanical solicitations, allowing to actively stimulating tissues. Further, interesting for tissue engineering are the polymer structures in the form of micro and nanofibers. In this work, the processing and characterization of two piezoelectric polymers, poly(Llactic acid) (PLLA) and poly(vinylidene fluoride) (PVDF), aiming tissue engineering applications was achieved. Processing was achieved both in the form of films and fibers. PLLA and PVDF electrospun fibers morphology was controlled by changing process parameters such as applied voltage, feed rate and collector system. Regarding PVDF fibers, the processing parameters allows to change the -phase fraction between 50% and 85% and fiber diameter from a few hundreds of nanometers to micrometers. Concerning PLLA fibers, the crystallinity was tailored between 0%, i.e. amorphous fibers, and 50%, by annealing treatment. The PLLA fibers diameter was further reduced by the introduction of poly(ethylene oxide) (PEO) polymer. In this way, electrospun membranes were prepared with tailored fiber diameter from some micrometers for pure PLLA membranes to few hundreds of nanometers by electrospinning of PLLA-PEO solution. PLLA degradation was also studied and it was observed that the samples degradation in phosphate buffer solution (PBS) up to 20 weeks produces only a slight decrease in the sample weight. The local properties of the PLLA and PVDF individual electrospun fibers were studied by piezoelectric force microscopy (PFM) and the piezoelectric response for both polymers have been proved. Additionally, the biological response of PLLA and PVDF membranes was also addressed. In the case of PLLA electrospun membranes, human chondrocytes were used and it was found that proliferation of human chondrocytes cultured in the monolayer substrates is not different on aligned or non-aligned amorphous mats. However, the differentiation rate seems to be higher on the non-aligned amorphous mats. Furthermore, the crystallization of the aligned mats showed nearly suppressed proliferation and the cells had produced higher amounts of aggrecan, characteristic of the extracellular matrix of hyaline cartilage. In relation to the PVDF, the biological response to the polarization state was studied in films. The effect of polarization on fibronectin conformation, cell adhesion and proliferation has been studied. It was observed that polarization of a PVDF modifies the conformation of adsorbed fibronectin on the material surface and therefore cell adhesion on the fibronectin-coated substrates. As a consequence, a higher number of cells on the substrate were observed in poled than in non-poled samples. These results open the possibility of developing active substrates for cell culture and tissue engineering. Further investigations on the piezoelectric effect on cell response were performed by evaluating osteoblast growth in poled and non-poled PVDF (non-coated and coated with thin titanium layer to get a more homogeneous charge distribution) under static and dynamic conditions. The polarization and titanium layer modifies the mean roughness of PVDF films surface and therefore also modifies cell adhesion and proliferation on the samples, also, the positively charge of β-PVDF promotes higher adhesion and proliferation on osteoblast. Finally, dynamic culture with MC3T3-E1 cells showed higher cell proliferation on "poled +" β-PVDF. Thus, results reported in this thesis have demonstrated that varying surface electrical charge (when a mechanical solicitation is applied) influences cell response and confirms the interest of electroactive polymers in cell culture and tissue engineering applications.Nas últimas décadas, os biomateriais e a engenharia de tecidos têm sido dos campos interdisciplinares mais dinâmicos e que têm atraído cada vez mais atenção por parte da comunidade científica. Com os avanços realizados em engenharia de tecidos, surgiu a necessidade de estudar e desenvolver uma ampla variedade de biomateriais com diferentes propriedades. Entre os diferentes tipos de materiais, os polímeros têm provado serem uma excelente escolha devido às suas várias vantagens tais como o seu simples processamento, flexibilidade, propriedades físicas e serem facilmente obtidos em diferentes formas. Em particular, os polímeros piezoelétricos têm atraído cada vez mais interesse uma vez que conseguem responder a solicitações elétricas e mecânicas, permitindo assim o estímulo ativo dos tecidos. Além disso, as estruturas poliméricas na forma de micro e nanofibras mostraram ser interessantes para engenharia de tecidos. Neste trabalho, o processamento e a caracterização de dois polímeros piezoelétricos, o poli(L-ácido láctico) (PLLA) e o poli(fluoreto de vinilideno) (PVDF), foram realizados com o objetivo de serem utilizados para aplicações de engenharia de tecidos. O processamento foi realizado sob a forma de filmes e fibras. A morfologia das fibras de PVDF e PLLA foi controlada pela variação dos parâmetros de processamento tais como a tensão aplicada, o fluxo e o coletor. Em relação às fibras de PVDF, os parâmetros do processamento mostraram influenciar a fração de fase entre 50% e 85% e o diâmetro das fibras desde algumas centenas de nanómetros até micrómetros. Em relação às fibras de PLLA, a cristalinidade pode ser alterada de 0%, isto é, fibras amorfas, até 50%, através de um tratamento térmico. O diâmetro das fibras de PLLA pode ainda ser reduzido através da introdução do polímero poli(óxido de etileno) (PEO). Desta forma, as membranas foram preparadas por electrospinning através de uma solução de PLLAPEO com diâmetros de fibras desde micrómetros até algumas centenas de nanómetros. A degradação do PLLA também foi estudada e verificou-se que a degradação até 20 semanas das amostras numa solução tampão de fosfato (PBS - phosphate buffer solution) produz uma ligeira diminuição de peso na amostra. As propriedades locais das fibras individuais de PLLA e PVDF foram estudadas por microscopia de força piezoelétrica (PFM) e a resposta piezoelétrica para ambos os polímeros foi verificada. Além disso, a resposta biológica das membranas de PLLA e PVDF foi também investigada. No caso das membranas de PLLA, os condrócitos humanos foram utilizados e verificou-se que a sua proliferação, aquando cultivados nos substratos de monocamada, é igual tanto para as membranas amorfas de fibras alinhadas como para as nãoalinhadas. No entanto, a taxa de diferenciação parece ser maior nos substratos amorfos de fibras não-alinhadas. Além disso, a cristalização dos substratos amorfos de fibras alinhadas demonstraram uma supressão da proliferação, sendo que as células produziram elevadas quantidades de agrecano, característica da matriz extracelular da cartilagem hialina. Em relação ao PVDF, a resposta biológica relativamente ao estado de polarização foi estudada em filmes. O efeito da polarização na conformação da fibronectina, adesão e proliferação celular foi estudado. Verificou-se que a polarização do PVDF tem influência na conformação da fibronectina adsorvida na superfície do material e, por conseguinte, na adesão de células em substratos revestidos com fibronectina. Como consequência, foi observado um maior número de células nos substratos polarizados relativamente aos não-polarizados. Estes resultados abrem a possibilidade de desenvolver substratos ativos para cultivo celular e para engenharia de tecidos. Outras investigações sobre o efeito piezoelétrico na resposta celular foram realizadas sob condições estáticas e dinâmicas, através da avaliação do crescimento de osteoblastos em filmes de PVDF polarizados e não-polarizados (não revestidos e revestidos com uma camada fina de titânio para obter uma distribuição mais homogénea de carga). Além disso, verificou-se que a camada de polarização e de titânio modificam a rugosidade média da superfície dos filmes de PVDF e, portanto, também modificam a adesão e proliferação celular. Observou-se ainda que a carga positiva dos filmes - PVDF promove uma maior adesão e proliferação de osteoblastos. Finalmente, um estudo dinâmico realizado com as células MC3T3-E1 mostrou uma maior proliferação celular nos filmes β-PVDF com polarização positiva. Assim, os resultados observados neste trabalho demonstraram que a variação da carga elétrica na superfície (quando é aplicada uma solicitação mecânica) influencia a resposta celular e confirma o interesse dos polímeros eletroativos para cultivos celulares e aplicações em engenharia de tecidos

Surface charge mediated cell-surface interaction on piezoelectric materials

Cell–material interactions play an essential role in the development of scaffold-based tissue engineering strategies. Cell therapies are still limited in treating injuries when severe damage causes irreversible loss of muscle cells. Electroactive biomaterials and, in particular, piezoelectric materials offer new opportunities for skeletal muscle tissue engineering since these materials have demonstrated suitable electroactive microenvironments for tissue development. In this study, the influence of the surface charge of piezoelectric poly(vinylidene fluoride) (PVDF) on cell adhesion was investigated. The cytoskeletal organization of C2C12 myoblast cells grown on different PVDF samples was studied by immunofluorescence staining, and the interactions between single live cells and PVDF were analyzed using an atomic force microscopy (AFM) technique termed single-cell force spectroscopy. It was demonstrated that C2C12 myoblast cells seeded on samples with net surface charge present a more elongated morphology, this effect being dependent on the surface charge but independent of the poling direction (negative or positive surface charge). It was further shown that the cell deadhesion forces of individual C2C12 cells were higher on PVDF samples with an overall negative surface charge (8.92 ± 0.45 nN) compared to those on nonpoled substrates (zero overall surface charge) (4.06 ± 0.20 nN). These findings explicitly demonstrate that the polarization/surface charge is an important parameter to determine cell fate as it affects C2C12 cell adhesion, which in turn will influence cell behavior, namely, cell proliferation and differentiationPortuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2019, UID/BIA/04050/2013, UID/BIO/04469, project POCI-01-0145-FEDER-028237 and under BioTecNorte operation (NORTE-01-0145-FEDER-000004). The authors also thank the FCT for the SFRH/BD/111478/2015 (S.R.) and SFRH/BPD/90870/2012 (C.R.) grants. Funds provided by FCT in the framework of EuroNanoMed 2016 call, Project LungChek ENMed/0049/2016 are also gratefully acknowledged. The authors acknowledge funding by the Spanish Ministry of Economy and Competitiveness (MINECO) through the project MAT2016-76039-C4-3-R (AEI/FEDER, UE) and from the Basque Government Industry and Education Department under the ELKARTEK, HAZITEK and PIBA (PIBA-2018-06

Piezoelectric polymers as biomaterials for tissue engineering applications

Tissue engineering often rely on scaffolds for supporting cell differentiation and growth. Novel paradigms for tissue engineering include the need of active or smart scaffolds in order to properly regenerate specific tissues. In particular, as electrical and electromechanical clues are among the most relevant ones in determining tissue functionality in tissues such as muscle and bone, among others, electroactive materials and, in particular, piezoelectric ones, show strong potential for novel tissue engineering strategies, in particular taking also into account the existence of these phenomena within some specific tissues, indicating their requirement also during tissue regeneration. This referee reports on piezoelectric materials used for tissue engineering applications. The most used materials for tissue engineering strategies are reported together with the main achievements, challenges and future needs for research and actual therapies. This review provides thus a compilation of the most relevant results and strategies and a start point for novel research pathways in the most relevant and challenging open questions.This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2013 and by the project Matepro – Optimizing Materials and Processes”, ref. NORTE-07-0124-FEDER-000037”, co-funded by the “Programa Operacional Regional do Norte” (ON.2 – O Novo Norte), under the “Quadro de Referência Estratégico Nacional” (QREN), through the “Fundo Europeu de Desenvolvimento Regional” (FEDER). CR, VS and DMC would like to acknowledge the FCT for the SFRH/BPD/90870/2012, SFRH/BD/64901/2009 and SFRH/BD/82411/2011 grants respectively

Natural based reusable materials for microfluidic substrates: The silk road towards sustainable portable analytical systems

Portable analytical systems are versatile tools for application in areas including biomedicine, biosecurity, food safety and environmental monitoring. This work contributes to the increasing demand for low-cost, environmentally friendly substrates for portable analytical systems by using natural Bombyx mori cocoons. Further, silk fibroin is also extracted from these cocoons and electrospun into oriented and randomly oriented fiber substrates. Oxygen plasma treatment is applied to improve their hydrophilicity. Fiber morphology, mechanical properties, porosity, thermal characteristics and surface contact angle are extensively characterized and the ability of the samples for passive capillary flows demonstrated. Plasma treated pressed cocoons show superhydrophilicity, capillary flow rates of 44.8 ± 3.75 mm.min-1, and high mechanical resistance with Young's modulus values up to 592.13 ± 19.83 MPa. The developed materials are used as substrates for the colorimetric quantification of three commonly scrutinized clinical analytes. Hydrophobic barriers are first wax-printed on all samples with a proper design and albumin assays are performed on all substrates. Further assays for uric acid and glucose quantification are successfully accomplished on the pressed cocoons after a simple in between washing step, with overall high coefficient of determination, proving the suitability of the developed materials as low-cost, sustainable and reusable microfluidic substrates.This work was supported by the Portuguese Foundation for Science and Technology (FCT) under strategic funding UID/FIS/04650/2020, UIDB/04436/2020, UIDP/04436/2020 and project PTDC/EMD-EMD/28159/2017 (POCI-01–0145-FEDER-028159). The authors also thank FCT for financial support under grants SFRH/BD/140698/2018 (R.B. P.), 2020.09218.BD (A.S.M.), 2020.04163.CEECIND (C.R.) and 2020.02304.CEECIND (V.F.C.). Finally, the authors acknowledge fund ing by Spanish State Research Agency (AEI) and the European Regional Development Fund (ERFD) through the project PID2019–106099RB C43/AEI/10.13039/501100011033 and from the Basque Government Industry Departments under the ELKARTEK program. Finally, the au thors also thank Dr. J. Borges and Prof. F. Vaz for experimental support.info:eu-repo/semantics/publishedVersio

Environmentally friendlier wireless energy power systems: the coil on a paper approach

Paper is ubiquitous in everyday life and a low-cost environmentally friendly material. Thus, the printing of advanced conductive/magnetic nanomaterials on paper will allow the scalable production of flexible smart electronics, including energy-storage devices, sensors, inductors or antennas, among others, contributing towards more sustainable electronics. Particularly, wireless charging technologies are becoming essential for internet-of-things (IoT)-related electronic devices due to the ever-decreasing dimensions of portable/mobile devices that limits the quantity of energy that can be stored. Here, screen-printed paper-based coils and inductors operating on the 1MHz - 20MHz range are presented based on Poly(vynil alcohol)/Fe3O4 and Ag inks. The ability of the printed cores and inductors to be incorporated on flexible wireless power transfer modules (WPTM) is technologically demonstrated by wireless powering light-emitting diodes (LEDs). The achieved induction efficiency of 94% is the highest reported on printed WPTM. The printed coils are also characterized by mechanical, hydrophobic and electrical properties that are suitable for IoT and industry 4.0 applications.All authors thank the FCT- Fundação para a Ciência e Tecnologia the financial support in the context of the Strategic Funding UID/FIS/04650/2019 and under projects PTDC/EEI-SII/5582/2014, PTDC/BTM-MAT/28237/2017 and PTDC/EMD-EMD/28159/2017. Pedro Martins and Clarisse Ribeiro thank the FCT for the contracts under the Stimulus of Scientific Employment, Individual Support: CEECIND/03975/2017, and 2020.04163.CEECIND, respectively. Ricardo Brito-Pereira also acknowledges the FCT for the SFRH/BD/140698/2018 grant. Finally, the authors acknowledge funding by the Basque Government Industry and Education Department under the ELKARTEK, and PIBA (PIBA 2018-06) programs, respectively. Finally, funding from European Union’s Horizon 2020 Program for Research, ICT-02-2018 - Flexible and Wearable Electronics, Grant agreement no. 824339 – WEARPLEX is also acknowledgedinfo:eu-repo/semantics/publishedVersio

Piezoelectric and magnetically responsive biodegradable composites with tailored porous morphology for biotechnological applications

The biomedical area in the scope of tissue regeneration pursues the development of advanced materials that can target biomimetic approaches and, ideally, have an active role in the environment they are placed in. This active role can be related to or driven by morphological, mechanical, electrical, or magnetic stimuli, among others. This work reports on the development of active biomaterials based on poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid), PHBV, a piezoelectric and biodegradable polymer, for tissue regeneration application by tailoring its morphology and functional response. PHBV films with different porosities were obtained using the solvent casting method, resorting to high-boiling-point solvents, as N,N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO), and the combination of chloroform (CF) and DMF for polymer dissolution. Furthermore, magnetoelectric biomaterials were obtained through the combination of the piezoelectric PHBV with magnetostrictive iron oxide (Fe3O4) nanoparticles. Independently of the morphology or filler content, all biomaterials proved to be suitable for biomedical applications.This work was supported by national funds through the Fundação para a Ciência e Tecnologia (FCT) and by ERDF through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI) in the framework of the Strategic Programs UID/FIS/04650/2020, and project PTDC/BTM-MAT/28237/2017. TMA thank FCT for the research grant: SFRH/BD/141136/2018, VC for the junior researcher contract (DL57/2016) and CR for the contract under the Stimulus of Scientific Employment, Individual Support (CEECIND) – 3 rd Edition (2020.04163.CEECIND). Finally, the authors acknowledge funding by Spanish State Research Agency (AEI) and the European Regional Development Fund (ERFD) through the project PID2019-106099RBC43/AEI/10.13039/501100011033 and from the Basque Government Industry Departments under the ELKARTEK program

Morphology dependence degradation of electro- and magnetoactive poly(3-hydroxybutyrate-co-hydroxyvalerate) for tissue engineering applications

Poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) is a piezoelectric biodegradable and biocompatible polymer suitable for tissue engineering applications. The incorporation of magnetostrictive cobalt ferrites (CFO) into PHBV matrix enables the production of magnetically responsive composites, which proved to be effective in the differentiation of a variety of cells and tissues. In this work, PHBV and PHBV with CFO nanoparticles were produced in the form of films, fibers and porous scaffolds and subjected to an experimental program allowing to evaluate the degradation process under biological conditions for a period up to 8 weeks. The morphology, physical, chemical and thermal properties were evaluated, together with the weight loss of the samples during the in vitro degradation assays. No major changes in the mentioned properties were found, thus proving its applicability for tissue engineering applications. Degradation was apparent from week 4 and onwards, leading to the conclusion that the degradation ratio of the material is suitable for a large range of tissue engineering applications. Further, it was found that the degradation of the samples maintain the biocompatibility of the materials for the pristine polymer, but can lead to cytotoxic effects when the magnetic CFO nanoparticles are exposed, being therefore needed, for magnetoactive applications, to substitute them by biocompatible ferrites, such as an iron oxide (Fe3O4).This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2020, UID/BIO/04469/2020 and UID/QUI/00686/2020, and projects PTDC/BTM-MAT/28237/2017 and PTDC/EMD-EMD/28159/2017 and Associate Laboratory for Green Chemistry—LAQV, financed by national funds from FCT/MCTES (UIDB/50006/2020). The authors also thank the FCT for the SFRH/BPD/121526/2016 (DMC) grant. The authors acknowledge funding by the Spanish Ministry of Economy and Competitiveness (MINECO) through the project MAT2016-76039-C4-3-R (AEI/FEDER, UE) and from the Basque Government Industry and Education Departments under the ELKARTEK, HAZITEK and PIBA (PIBA-2018-06) programs, respectively.info:eu-repo/semantics/publishedVersio

Electrospun magnetic ionic liquid based electroactive materials for tissue engineering applications

Functional electrospun fibers incorporating ionic liquids (ILs) present a novel approach in the development of active microenviroments due to their ability to respond to external magnetic fields without the addition of magnetic particles. In this context, this work reports on the development of magnetically responsive magneto-ionic fibers based on the electroactive polymer poly(vinylidene fluoride) and the magnetic IL (MIL), bis(1-butyl-3-methylimidazolium) tetrathiocyanatocobaltate ([Bmim]2[(SCN)4Co]). The PVDF/MIL electrospun fibers were prepared incorporating 5, 10 and 15 wt.% of the MIL, showing that the inclusion of the MIL increases the polar β-phase content of the polymer from 79% to 94% and decreases the crystallinity of the fibers from 47% to 36%. Furthermore, the thermal stability of the fibers decreases with the incorporation of the MIL. The magnetization of the PVDF/MIL composite fibers is proportional to the MIL content and decreases with temperature. Finally, cytotoxicity assays show a decrease in cell viability with increasing the MIL content.This research was funded by FCT—Fundação para a Ciência e Tecnologia (FCT) under the scope of the strategic funding of UID/FIS/04650/2020, and project PTDC/BTM-MAT/28237/2017. Moreover, the authors thank FCT for the research grant SFRH/BD/145345/2019 (LMC), SFRH/BD/148655/2019 (RMM), and D.M.C. and CR thank the FCT for the contract under the Stimulus of Scientific Employment 2020.02915.CEECIND and 2020.04163.CEECIND, respectively.The authors acknowledge funding by Spanish State Research Agency (AEI) and the European Regional Development Fund (ERFD) through the project PID2019-106099RB-C43/AEI/10.13039/501100011033 and from the Basque Government Industry Departments under the ELKARTEK program. Technical and human support provided by IZO-SGI, SGIker (UPV/EHU, MICINN, GV/EJ, ERDF and ESF) is gratefully acknowledged

Dynamic piezoelectric stimulation enhances osteogenic differentiation of human adipose stem cells

This work reports on the influence of the substrate polarization of electroactive β-PVDF on human adipose stem cells (hASCs) differentiation under static and dynamic conditions. hASCs were cultured on different β-PVDF surfaces (non-poled and “poled -”) adsorbed with fibronectin and osteogenic differentiation was determined using a quantitative alkaline phosphatase assay. “Poled -” β-PVDF samples promote higher osteogenic differentiation, which is even higher under dynamic conditions. It is thus demonstrated that electroactive membranes can provide the necessary electromechanical stimuli for the differentiation of specific cells and therefore will support the design of suitable tissue engineering strategies, such as bone tissue engineering.This work is funded by FEDER funds through the "Programa Operacional Fatores de Competitividade – COMPETE" and by national funds arranged by FCT- Fundação para a Ciência e a Tecnologia, project references PTDC/CTM-NAN/112574/2009 and PEST-C/FIS/UI607/2014. The authors also thank funding from Matepro –Optimizing Materials and Processes”, ref. NORTE-07-0124-FEDER-000037”, co-funded by the “Programa Operacional Regional do Norte” (ON.2 – O Novo Norte), under the “Quadro de Referência Estratégico Nacional” (QREN), through the “Fundo Europeu de Desenvolvimento Regional” (FEDER). CR, VS and VC thank the FCT for the SFRH/BPD/90870/2012, SFRH/BPD/64958/2009 and SFRH/BPD/97739/2013 grants, respectively. Academy of Finland is acknowledged for research funding (projects 136288 (VH) and 256931 (JP))

Beeswax multifunctional composites with thermal-healing capability and recyclability

Natural beeswax reinforced with conductive nanofillers allows solvent-free processing and presents remarkable functional response as piezoresistive and thermoresistive sensors with thermal healing capability. The low melting temperature of the composites, around 60 °C, allows additive manufacturing of conductive patterns with a high electrical conductivity of 50 S/m. Further, the graphene/beeswax composites show suitable deformation and temperature sensing characteristics based on the piezoresistive and thermoresistive sensitivities, around GF≈9 and S≈ 120 %/°C, respectively. Natural beeswax is a food and drug administration approved substance, and all graphene/beeswax composites present no cytotoxic behavior, demonstrating their potential use for biomedical applications. Proofs-of-concept demonstrate the conductive and thermal-healing properties of the screen-printed sensors developed both on paper and Kapton substrates, proving the applicability and multifunctionality of the developed materials. Finally, the multifunctional composites can be recycled and reused without losing their electrical and functional performances