Eriyik yığma modelleme ile üretilen polimerik hibrit stent tasarımı ve mekanik karakterizasyonu
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2022
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info:eu-repo/semantics/openAccess
Özet
Günümüze kadar yapılan sayısal ve deneysel analiz çalışmaları, biyobozunur polimer esaslı kardiyovasküler stent tasarımları eğrisel strat formunda ve kalın strat kalınlığında olması kabulüne dayanarak gerçekleştirilmiştir. Son beş yılda bu konuda hazırlanan geometrik modellerin, klinik uygulamalarda karşılık bulması ve metalik stentlere alternatif olması amacıyla yeni nesil stent kavramı ortaya çıkarılmıştır. Buna paralel olarak, polimer esaslı stentlerin ticari olarak kullanımıyla ilgili onay alınmasına rağmen, ön klinik çalışmalardan alınan geri bildirimler neticesinde bazı dezavantajları ve eksiklikleri bulunmaktadır. Dolayısıyla, biyobozunur polimer esaslı stentlerin tamamen bozunmaya uğramasına kadar arteri desteklemesi ve performans parametrelerinin etkinliği önem arz etmektedir. Bu çalışmada, klinik geri bildirimlerde sunulan dezavantajlı durumların geometrik faktörler kapsamında ele alınarak, yeni birim hücre geometrisine sahip kardiyovasküler stent tasarımı gerçekleştirilmiştir. Stent performans parametrelerinin klinik uygulamalardan alınan sonuçlarla karşılaştırılması ve geometrik etkilerin değerlendirilmesiyle tasarım iyileştirmesi, sayısal ve analitik yöntemlerle yorumlanmıştır. Stent performans parametrelerinin birbirine ters orantılı veya paralel etkileri belirlenerek ortaya çıkan sonuçlar önem önceliğine göre yorumlanmıştır. Polimer esaslı stentlerin gerçek koşullarda sıkıştırılması ve arter içerisinde genişletilmesi, sıcaklık altında yapılmaktadır. Bu temel çerçevesinde, sayısal analiz modeli oluşturularak, ısıtıcılı özel bir balon kateter sisteminde camsı geçiş ve vücut sıcaklığı arasında eş eksenli genişletme esasına uygun olan çözümler yapılmıştır. Literatürde yer alan biyobozunur polimer esaslı stent tasarımlarından farklı olarak, kapalı birim hücre geometrisinde ve performans parametrelerinin önem durumuna göre yapılan tasarımlarda; kısalma miktarının sıfıra yakın olması sayesinde stendin zamanla malpozisyonu ve kayması engellenmiştir. Bu çalışmada, strat kalınlığının % 50 oranında azaltılması ve strat bağlantı bölgelerinde plastik deformasyonun artmasıyla akut dönemde geri daralma miktarının azalması sağlanmıştır. Yeni birim hücre geometrisindeki stent tasarımıyla, çevresel doğrultuda birim hücre sayısının azalması, damar kaplama oranını da düşürmüştür ve mevcut metalik stentlerin kaplama oranına yakın değer elde edilmiştir. Bu güne kadar üzerinde çeşitli çalışmalar yapılan eğrisel strat formundaki tasarımlardan farklı olarak, damar kaplama alanının azalması sayesinde strat çakışması önlenmiştir. Arter yüzeyine temas eden strat oranının azalması, çökme basıncını damarın doğal basıncına yakın hale getirmiştir. Buna göre, erken dönemde restenozun oluşmasına neden olan ve arter katmanlarında genişleme esnasında meydana gelen aşırı çevresel gerilmelerin önüne geçilmiştir. Biyouyumlu stent tasarımlarında en büyük dezavantajın, mekanik özelliklerin yeterli olmamasından kaynaklandığı önceki çalışmalarda özellikle vurgulanmıştır. Buna rağmen, stent tasarımlarında geometrik faktörlerle birlikte mekanik özelliklerin de geliştirilmesiyle istenilen radyal dayanım sağlanabilmektedir. Ayrıca, polimer esaslı stentlerin, üretim proseslerine bağlı olarak, yapı iskelesi içerisinde kristalinite oranı biyobozunurluğa doğrudan etki etmektedir. PLA/PHA karışımında termogravimetrik verilere göre yüzdesi daha fazla olan PLA'nın bu karışımla birlikte kırılganlığı azaltılırken, PHA malzemenin hidrofobik davranışıyla uygun bir biyouyumlu malzeme meydana gelmektedir. Buna ilave olarak, PLA/PHA karışımınıyla yapılan çalışmalarda 37°C'de 2 ve 7 günlük inkübasyon süresince 3B baskı boşluklarında canlı hücre büyümesinin sağlanması ve biyomedikal uygulamalar için gelecek araştırmalarda kullanılabilir özelliğe sahip olduğunu göstermiştir. Stentlerin lezyonlu bölgeye ulaştırılması esnasında, damar kıvrımlarından rahat ilerleyebilmesi, eğilme rijitliği parametresi olarak adlandırılmaktadır. Bu çalışma için, eklemeli imalat yöntemlerinden biri olan eriyik yığma modellemeyle iki katmanlı olarak PLA/PHA malzemeden üretilen plaka modeller, ısıtıcı bir tambur etrafında sarılarak stent formuna getirilmiştir. Üretilen stent modeli için, farklı sıcaklıklarda eğilme testleri yapılmış ve sonuçlar yorumlanmıştır. Literatürde yer alan çalışmalarda, polimer ve diğer stentler için standart olarak vücut sıcaklığında eğilme testleri gerçekleştirilmektedir. Yeni bir bakış açısıyla, ısıtıcılı özel balon kateter yapısına göre vücut sıcaklığı ile camsı geçiş sıcaklığı arasındaki bir değerde lezyonlu bölgeye ilerleyen stent için eğilme testleri yapılmıştır. Hibrit stent geometrisinde kapalı birim hücreye sahip olan tasarımın eğilme rijitliği, vücut sıcaklığı ve camsı geçiş sıcaklığı arasında uygun sonuç vermiştir ve yeterli eğilme rijitliğine ulaşılmıştır.
Numerical and experimental analysis studies carried out to date have been based on the assumption that biodegradable polymer-based cardiovascular stent designs are in the form of a curvilinear strut and in thick strut thickness. In the last five years, as an alternative to metallic stents and in order to meet the needs of clinical applications, concept of a new generation stent has been introduced along with novel geometric model ideas. In parallel, although the approval for commercial use of polymer-based stents has been obtained, there are some disadvantages and shortcomings as a result of the feedback received from preliminary clinical studies. Therefore, it is important that biodegradable polymer-based stents must be able to support artery and maintain the efficiency of performance parameters until complete degradation. In this study, a cardiovascular stent design with a new unit cell geometry was carried out by considering the disadvantageous situations presented in clinical feedback within the scope of geometric factors. The stent performance parameters were compared with the results from clinical applications and the design improvement was interpreted by numerical and analytical methods by evaluating the geometric effects. The inverse or parallel effects of stent performance parameters were determined and the results were interpreted according to the priority of importance. Crimping and expansion of polymer-based stents in real conditions are performed under temperature. On this basis, a numerical analysis model was created and solutions were made in a special balloon catheter system with a heater, which are compatible with the principle of coaxial expansion between the glass transition and the body temperature. Unlike the biodegradable polymer-based stent designs in the literature, in the designs made according to the closed unit cell geometry and the importance of performance parameters; since the shortening amount is close to zero, malposition and slippage of the stent over time are prevented. In this study, It was ensured that both the strut thickness was reduced by 50% and amount of acute recoil with increasing the plastic deformation in the strut connection regions. The number of unit cells and vessel coverage ratio in the circumferential direction have been reduced and a result close to the coverage ratio of the existing metallic stents has been obtained by the new unit cell geometry. Unlike the peak-to-walley strut form designs on which various studies have been carried out to date, strut overlap are prevented through to the reduction of the artery coverage area. The ratio of strat contacting the arterial surface has been reduced, thus making the collapse pressure close to the natural pressure of the vessel. Accordingly, excessive circumferencial stresses are prevented that cause in-stent restenosis and occur during expansion in the arterial layers in the early period. It has been especially emphasized in previous studies that the biggest disadvantage in biocompatible stent designs is the insufficient mechanical properties. However, the desired radial strength can be achieved by improving mechanical properties along with geometric factors in stent designs. In addition, the rate of crystallization of polymer-based stents in the scaffold, depending on the production processes, directly affects biodegradability. PLA is a brittle material and its mechanical properties are improved with a mixture of PHA. However, through to the hydrophobic properties of PHA, the biodegradation time of the mixture is prolonged. In addition, studies with PLA/PHA mixture have shown that it can be used in future researches for biomedical applications and ensuring live cell growth in 3D printing blanks during 2 and 7 days of incubation at 37°C. The bending stiffness parameter is determined by the fact that the stents can easily move through the vascular folds during delivery to the lesioned area. For this study, plate models produced from two-layered PLA/PHA material as three-dimensional printing in the fused deposition manufacturing process, which is one of the additive manufacturing methods, were wrapped around a heating tube and turned into stent form. For the stent model produced, the results were interpreted by performing a bending tests at different temperatures. In the studies in literature, bending tests performed at body temperature as standard for stents were performed at a value between body temperature and glass transition temperature as a new perspective. The sufficient bending stiffness was achieved of the new design, which has a closed unit cell in the stent geometry, gave a suitable results between body temperature and glass transition temperature.
Numerical and experimental analysis studies carried out to date have been based on the assumption that biodegradable polymer-based cardiovascular stent designs are in the form of a curvilinear strut and in thick strut thickness. In the last five years, as an alternative to metallic stents and in order to meet the needs of clinical applications, concept of a new generation stent has been introduced along with novel geometric model ideas. In parallel, although the approval for commercial use of polymer-based stents has been obtained, there are some disadvantages and shortcomings as a result of the feedback received from preliminary clinical studies. Therefore, it is important that biodegradable polymer-based stents must be able to support artery and maintain the efficiency of performance parameters until complete degradation. In this study, a cardiovascular stent design with a new unit cell geometry was carried out by considering the disadvantageous situations presented in clinical feedback within the scope of geometric factors. The stent performance parameters were compared with the results from clinical applications and the design improvement was interpreted by numerical and analytical methods by evaluating the geometric effects. The inverse or parallel effects of stent performance parameters were determined and the results were interpreted according to the priority of importance. Crimping and expansion of polymer-based stents in real conditions are performed under temperature. On this basis, a numerical analysis model was created and solutions were made in a special balloon catheter system with a heater, which are compatible with the principle of coaxial expansion between the glass transition and the body temperature. Unlike the biodegradable polymer-based stent designs in the literature, in the designs made according to the closed unit cell geometry and the importance of performance parameters; since the shortening amount is close to zero, malposition and slippage of the stent over time are prevented. In this study, It was ensured that both the strut thickness was reduced by 50% and amount of acute recoil with increasing the plastic deformation in the strut connection regions. The number of unit cells and vessel coverage ratio in the circumferential direction have been reduced and a result close to the coverage ratio of the existing metallic stents has been obtained by the new unit cell geometry. Unlike the peak-to-walley strut form designs on which various studies have been carried out to date, strut overlap are prevented through to the reduction of the artery coverage area. The ratio of strat contacting the arterial surface has been reduced, thus making the collapse pressure close to the natural pressure of the vessel. Accordingly, excessive circumferencial stresses are prevented that cause in-stent restenosis and occur during expansion in the arterial layers in the early period. It has been especially emphasized in previous studies that the biggest disadvantage in biocompatible stent designs is the insufficient mechanical properties. However, the desired radial strength can be achieved by improving mechanical properties along with geometric factors in stent designs. In addition, the rate of crystallization of polymer-based stents in the scaffold, depending on the production processes, directly affects biodegradability. PLA is a brittle material and its mechanical properties are improved with a mixture of PHA. However, through to the hydrophobic properties of PHA, the biodegradation time of the mixture is prolonged. In addition, studies with PLA/PHA mixture have shown that it can be used in future researches for biomedical applications and ensuring live cell growth in 3D printing blanks during 2 and 7 days of incubation at 37°C. The bending stiffness parameter is determined by the fact that the stents can easily move through the vascular folds during delivery to the lesioned area. For this study, plate models produced from two-layered PLA/PHA material as three-dimensional printing in the fused deposition manufacturing process, which is one of the additive manufacturing methods, were wrapped around a heating tube and turned into stent form. For the stent model produced, the results were interpreted by performing a bending tests at different temperatures. In the studies in literature, bending tests performed at body temperature as standard for stents were performed at a value between body temperature and glass transition temperature as a new perspective. The sufficient bending stiffness was achieved of the new design, which has a closed unit cell in the stent geometry, gave a suitable results between body temperature and glass transition temperature.
Açıklama
Anahtar Kelimeler
Biyouyumlu Stent Tasarımı, Kardiyovasküler Stent, Stent Performans Parametreleri, Eklemeli İmalat, Kapalı Birim Hücre Geometrisi, Biocompatible Polymer Stents, Cardiovascular Stent, Stent Performance Parameters, Additive Manufacturing, Closed Unit Cell Geometries
