Background: Benign stenoses, digestive tract ruptures and fistulas are conditions that endanger l... more Background: Benign stenoses, digestive tract ruptures and fistulas are conditions that endanger life and are often treated surgically. Recently, the placement of partially or fully covered metal stents has emerged as a minimally invasive treatment option. This article looks at a new design for stents to determine its clinical effectiveness. The new stent is a completely covered nitinol stent for treatment of gastrointestinal perforations and anastomotic leaks. This article places special emphasis on evaluating reactive hyperplasia. Methods: Fifteen had the new completely covered self-expanding nitinol stent placed for treatment of benign esophageal perforations, anastomotic leaks, and stenoses following upper or lower gastrointestinal surgery during 2012 and 2013. The stents are 20 mm in diameter in the middle and 28 mm in diameter at the proximal end. Information about patient demographics, type of lesion, lesion locations, stent removal, clinical success and complications was coll...
Energy dissipating materials are used in a variety of impact events to protect more important par... more Energy dissipating materials are used in a variety of impact events to protect more important parts of a system; one example of this is a football player’s helmet protecting a brain. A major drawback to classic energy dissipating materials however is that they dissipate energy through plastic deformation, meaning that after a single use, permanent deformations will prevent the material from being reusable to the same capacity as initially. We have designed a 1D cellular material in which geometric phase transformations in cylindrical shell elements are the primary energy dissipating mechanism, allowing for recoverability after use while keeping high energy dissipation functionality. Finite element analysis was used to evaluate cylindrical shell ligaments of differing lengths and angle of inclination, after which key parameter combinations were identified. Physical samples were manufactured and tested in loading and unloading cycles. It was found through simulations that ligaments wi...
Journal of the Mechanical Behavior of Biomedical Materials, 2020
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Phase Transforming Cellular Materials (PXCMs) are periodic cellular materials whose unit cells ex... more Phase Transforming Cellular Materials (PXCMs) are periodic cellular materials whose unit cells exhibit multiple stable or meta-stable configurations. Transitions between the various (meta-) stable configurations at the unit cell level enable these materials to exhibit reusable solid state energy dissipation. This energy dissipation arises from the storage and non-equilibrium release of strain energy accompanying the limit point traversals underlying these transitions. The material deformation is fully recoverable, and thus the material can be reused to absorb and dissipate energy multiple times. In this work, we present two designs for functionally two-dimensional PXCMs: the S-type with four axes of reflectional symmetry based on a square motif and, the T-type with six axes of symmetry based on a triangular motif. We employ experiments and simulations to understand the various mechanisms that are triggered under multiaxial loading conditions. Our numerical and experimental results i...
Introducción: se han descrito diferentes técnicas endoscópicas para la ablación del epitelio meta... more Introducción: se han descrito diferentes técnicas endoscópicas para la ablación del epitelio metaplásico esofágico manteniendo la integridad de la submucosa, bajo una agresiva terapia supresora de ácido con los bloqueadores de bomba. En lo evaluado hasta la fecha existen muy pocos estudios que comparen las diferentes técnicas endoscópicas que reviertan el esófago de Barrett (EB) con displasia y los factores que pueden influir en el pronóstico con estas terapias no se han evaluado consistentemente. Objetivo: este es un estudio piloto que compara la eficacia en la erradicación completa del EB con displasia, entre la resección endoscópica mucosa (EMR) con bandas y la terapia con argón plasma (APC) con bajo voltaje. Los puntos a evaluar son: a) la completa ablación del EB; b) el número de sesiones requeridas; c) las complicaciones. Métodos: la APC se realizó con un catéter flexible 10 Fr. El voltaje utilizado fue de 50 W y el flujo varió entre 1 y 2 l/min. En el grupo de EMR con bandas,...
Introducción: el esófago de Barrett (EB) con displasia tiene un reconocido potencial maligno. Ni ... more Introducción: el esófago de Barrett (EB) con displasia tiene un reconocido potencial maligno. Ni la supresión ácida, ni la cirugía antirreflujo producen una regresión consistente o completa del epitelio metaplásico o displásico. La termoablación endoscópica a través de la coagulación con argón plasma (APC) ofrece la posibilidad de su erradicación (ablación), pero los posibles factores que pueden influir en el desenlace de la terapia aún no han sido evaluados consistentemente. Objetivo: evaluar la eficacia de la APC en la erradicación del EB con displasia y los factores que influencian su desenlace inmediato y tras por lo menos un año de seguimiento. Métodos: A 33 pacientes con EB y displasia (19 hombres-58%), con una media de edad de 56,9±6,35 años (rango 45 a 69 años) con una longitud media del EB de 4,1 cm (rango de 2-8 cm), se les practicó terapia con APC a intervalos de cada 4-6 semanas, hasta lograr la erradicación del EB, mientras estaban recibiendo terapia con doble dosis de ...
The mechanics of phase transforming cellular materials (PXCMs) with three different chiral honeyc... more The mechanics of phase transforming cellular materials (PXCMs) with three different chiral honeycomb architectures, viz., hexachiral, tetra-anti-chiral, and tetra-chiral, are investigated under quasi-static loading/unloading. Each PXCM comprises interconnected unit cells consisting of tape springs rigidly affixed to circular nodes that can rotate and/or translate. The phase change is associated with snap-through instability due to bending of the tape springs and corresponds to sudden changes in the geometry of the unit cells from one stable configuration to another stable (or metastable) configuration during loading/unloading. When compared with similar chiral materials with flat ligaments, the chiral PXCMs exhibit a significantly higher energy dissipation in quasi-static experiments. The hexachiral PXCM was selected for detailed parametric analysis with finite element simulations including 21 models constructed to investigate the effects of PXCM geometry on phase change and energy ...
International Journal of Solids and Structures, 2016
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service... more This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Highlights We present a new type of programmable materials (PCMs) based on cellular solids. A shape memory polymer is used as the base material. Programming allows modifying the properties of a programmable material. Programming is reversible and repeatable. Significant changes in the response of PCMs is achieved by programming.
Revista Colombiana De Gastroenterologia, Dec 1, 2014
Introduction: Barrett's esophagus (BE) is the result of chronic damage to the esophageal epitheli... more Introduction: Barrett's esophagus (BE) is the result of chronic damage to the esophageal epithelium caused by the acid of gastrointestinal reflux diseases (GERD). It is defined as replacement of the by stratified squamous epithelium that normally lines the esophagus with metaplastic columnar epithelium. BE represents a risk factor for esophageal adenocarcinoma. The aim of radiofrequency ablation (RFA) is to destroy the metaplastic epithelium with the electric current of a radiofrequency to stimulate the reappearance of stratified squamous epithelium in the distal esophagus. Objective: The objective of this study was to evaluate the efficiency and safety of RFA which has recently been introduced in the city of Medellin, Colombia for management of BE with in situ dysplasia or carcinoma. Materials and Methods: Ten patients were treated with RFA. BE patients with in situ dysplasia or carcinoma and histological diagnoses were chosen for treatment. RFA procedures were done with BARRX equipment, the circular HALO360 system or the HALO90 system. Macroscopic and microscopic effects of RFA, patient tolerance to treatment and complications were evaluated. Results: A group of 10 patients received treatment with RFA. All ten finished treatment and follow-up. Seven patients had low-grade dysplasia, two had high-grade dysplasia, and one patient had in situ carcinoma. Two patients had undergone endoscopic resections of nodules and treatments of mucosa with bands prior to therapy with RFA. A total of 13 procedures were performed: 10 RFAs with the HALO360, 2 RFAs with the HALO90 and one argon plasma treatment for residual BE in one patient. Eradication of metaplastic and dysplastic esophageal epithelium was achieved in all patients and confirmed by endoscopic and histologic evaluation. No significant complications related to RFA were found, but in two patients esophageal stenoses were found. One required endoscopic dilatation three weeks after therapy. Conclusions: Based on these preliminary results we believe that this is a promising method that is welltolerated by patients and which has no major complications. Metaplastic and dysplastic epithelium of the distal esophagus were successfully eradicated in most patients.
We extend the notion of phase transformations to periodic cellular materials by introducing mater... more We extend the notion of phase transformations to periodic cellular materials by introducing materials whose unit cells have multiple stable configurations. Each stable configuration of the unit cell corresponds to a stable phase, and transitions between these phases are regarded as phase transformations of the cellular material. We present a cellular material that exhibits phase transformation, identify key elements of the unit cell geometry that yield its mechanical behavior and introduce an analytical model that describes its constitutive behavior. Cellular materials that exhibit phase transformations show hysteresis and their response is characterized by a long serrated loading and unloading plateaus, making these materials attractive for energy absorption applications. While these phase transforming cellular material offer an energy absorption performance that is comparable to metal and polymeric honeycombs, the key difference is that the deformation on these materials is fully recoverable.
Conference Proceedings of the Society for Experimental Mechanics Series, 2011
We propose the concept of periodic cellular materials with programmable effective properties and ... more We propose the concept of periodic cellular materials with programmable effective properties and present initial results from a computational study of a prototypical material that exhibits this behavior. Nonlinear FEA shows that programmed geometric imperfections at the cell level can be used to modify the effective compressive storage modulus of shape memory polymer (SMP) based periodic cellular materials after they have been manufactured. The ability of SMPs to freeze a temporary deformation for an extended ...
This article presents a fast and suitable method to derive the spatial distribution of seabed gra... more This article presents a fast and suitable method to derive the spatial distribution of seabed grain sizes on tidal flats. A successful validation was carried out in the Dithmarschen Bight, a meso-tidal area of the German North Sea coast. In particular, the study is built on statistical correlations between observations of sediment sizes from 197 seabed samples and the numerical model output of flow-wave variables under specific marine and atmospheric forcings. The correlation between orbital velocities and mean grain sizes (d 50) exhibited the best results, but it can be affected under low relative wave height ratios. The methodology was applied for the construction of a high-resolution map of grain sizes in the Dithmarschen Bight, which resulted in absolute and relative errors of 16 μm and 13%, respectively. The proposed relationship was also adopted to examine scenarios of field data scarcity.
Background: Benign stenoses, digestive tract ruptures and fistulas are conditions that endanger l... more Background: Benign stenoses, digestive tract ruptures and fistulas are conditions that endanger life and are often treated surgically. Recently, the placement of partially or fully covered metal stents has emerged as a minimally invasive treatment option. This article looks at a new design for stents to determine its clinical effectiveness. The new stent is a completely covered nitinol stent for treatment of gastrointestinal perforations and anastomotic leaks. This article places special emphasis on evaluating reactive hyperplasia. Methods: Fifteen had the new completely covered self-expanding nitinol stent placed for treatment of benign esophageal perforations, anastomotic leaks, and stenoses following upper or lower gastrointestinal surgery during 2012 and 2013. The stents are 20 mm in diameter in the middle and 28 mm in diameter at the proximal end. Information about patient demographics, type of lesion, lesion locations, stent removal, clinical success and complications was coll...
Energy dissipating materials are used in a variety of impact events to protect more important par... more Energy dissipating materials are used in a variety of impact events to protect more important parts of a system; one example of this is a football player’s helmet protecting a brain. A major drawback to classic energy dissipating materials however is that they dissipate energy through plastic deformation, meaning that after a single use, permanent deformations will prevent the material from being reusable to the same capacity as initially. We have designed a 1D cellular material in which geometric phase transformations in cylindrical shell elements are the primary energy dissipating mechanism, allowing for recoverability after use while keeping high energy dissipation functionality. Finite element analysis was used to evaluate cylindrical shell ligaments of differing lengths and angle of inclination, after which key parameter combinations were identified. Physical samples were manufactured and tested in loading and unloading cycles. It was found through simulations that ligaments wi...
Journal of the Mechanical Behavior of Biomedical Materials, 2020
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Phase Transforming Cellular Materials (PXCMs) are periodic cellular materials whose unit cells ex... more Phase Transforming Cellular Materials (PXCMs) are periodic cellular materials whose unit cells exhibit multiple stable or meta-stable configurations. Transitions between the various (meta-) stable configurations at the unit cell level enable these materials to exhibit reusable solid state energy dissipation. This energy dissipation arises from the storage and non-equilibrium release of strain energy accompanying the limit point traversals underlying these transitions. The material deformation is fully recoverable, and thus the material can be reused to absorb and dissipate energy multiple times. In this work, we present two designs for functionally two-dimensional PXCMs: the S-type with four axes of reflectional symmetry based on a square motif and, the T-type with six axes of symmetry based on a triangular motif. We employ experiments and simulations to understand the various mechanisms that are triggered under multiaxial loading conditions. Our numerical and experimental results i...
Introducción: se han descrito diferentes técnicas endoscópicas para la ablación del epitelio meta... more Introducción: se han descrito diferentes técnicas endoscópicas para la ablación del epitelio metaplásico esofágico manteniendo la integridad de la submucosa, bajo una agresiva terapia supresora de ácido con los bloqueadores de bomba. En lo evaluado hasta la fecha existen muy pocos estudios que comparen las diferentes técnicas endoscópicas que reviertan el esófago de Barrett (EB) con displasia y los factores que pueden influir en el pronóstico con estas terapias no se han evaluado consistentemente. Objetivo: este es un estudio piloto que compara la eficacia en la erradicación completa del EB con displasia, entre la resección endoscópica mucosa (EMR) con bandas y la terapia con argón plasma (APC) con bajo voltaje. Los puntos a evaluar son: a) la completa ablación del EB; b) el número de sesiones requeridas; c) las complicaciones. Métodos: la APC se realizó con un catéter flexible 10 Fr. El voltaje utilizado fue de 50 W y el flujo varió entre 1 y 2 l/min. En el grupo de EMR con bandas,...
Introducción: el esófago de Barrett (EB) con displasia tiene un reconocido potencial maligno. Ni ... more Introducción: el esófago de Barrett (EB) con displasia tiene un reconocido potencial maligno. Ni la supresión ácida, ni la cirugía antirreflujo producen una regresión consistente o completa del epitelio metaplásico o displásico. La termoablación endoscópica a través de la coagulación con argón plasma (APC) ofrece la posibilidad de su erradicación (ablación), pero los posibles factores que pueden influir en el desenlace de la terapia aún no han sido evaluados consistentemente. Objetivo: evaluar la eficacia de la APC en la erradicación del EB con displasia y los factores que influencian su desenlace inmediato y tras por lo menos un año de seguimiento. Métodos: A 33 pacientes con EB y displasia (19 hombres-58%), con una media de edad de 56,9±6,35 años (rango 45 a 69 años) con una longitud media del EB de 4,1 cm (rango de 2-8 cm), se les practicó terapia con APC a intervalos de cada 4-6 semanas, hasta lograr la erradicación del EB, mientras estaban recibiendo terapia con doble dosis de ...
The mechanics of phase transforming cellular materials (PXCMs) with three different chiral honeyc... more The mechanics of phase transforming cellular materials (PXCMs) with three different chiral honeycomb architectures, viz., hexachiral, tetra-anti-chiral, and tetra-chiral, are investigated under quasi-static loading/unloading. Each PXCM comprises interconnected unit cells consisting of tape springs rigidly affixed to circular nodes that can rotate and/or translate. The phase change is associated with snap-through instability due to bending of the tape springs and corresponds to sudden changes in the geometry of the unit cells from one stable configuration to another stable (or metastable) configuration during loading/unloading. When compared with similar chiral materials with flat ligaments, the chiral PXCMs exhibit a significantly higher energy dissipation in quasi-static experiments. The hexachiral PXCM was selected for detailed parametric analysis with finite element simulations including 21 models constructed to investigate the effects of PXCM geometry on phase change and energy ...
International Journal of Solids and Structures, 2016
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service... more This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Highlights We present a new type of programmable materials (PCMs) based on cellular solids. A shape memory polymer is used as the base material. Programming allows modifying the properties of a programmable material. Programming is reversible and repeatable. Significant changes in the response of PCMs is achieved by programming.
Revista Colombiana De Gastroenterologia, Dec 1, 2014
Introduction: Barrett's esophagus (BE) is the result of chronic damage to the esophageal epitheli... more Introduction: Barrett's esophagus (BE) is the result of chronic damage to the esophageal epithelium caused by the acid of gastrointestinal reflux diseases (GERD). It is defined as replacement of the by stratified squamous epithelium that normally lines the esophagus with metaplastic columnar epithelium. BE represents a risk factor for esophageal adenocarcinoma. The aim of radiofrequency ablation (RFA) is to destroy the metaplastic epithelium with the electric current of a radiofrequency to stimulate the reappearance of stratified squamous epithelium in the distal esophagus. Objective: The objective of this study was to evaluate the efficiency and safety of RFA which has recently been introduced in the city of Medellin, Colombia for management of BE with in situ dysplasia or carcinoma. Materials and Methods: Ten patients were treated with RFA. BE patients with in situ dysplasia or carcinoma and histological diagnoses were chosen for treatment. RFA procedures were done with BARRX equipment, the circular HALO360 system or the HALO90 system. Macroscopic and microscopic effects of RFA, patient tolerance to treatment and complications were evaluated. Results: A group of 10 patients received treatment with RFA. All ten finished treatment and follow-up. Seven patients had low-grade dysplasia, two had high-grade dysplasia, and one patient had in situ carcinoma. Two patients had undergone endoscopic resections of nodules and treatments of mucosa with bands prior to therapy with RFA. A total of 13 procedures were performed: 10 RFAs with the HALO360, 2 RFAs with the HALO90 and one argon plasma treatment for residual BE in one patient. Eradication of metaplastic and dysplastic esophageal epithelium was achieved in all patients and confirmed by endoscopic and histologic evaluation. No significant complications related to RFA were found, but in two patients esophageal stenoses were found. One required endoscopic dilatation three weeks after therapy. Conclusions: Based on these preliminary results we believe that this is a promising method that is welltolerated by patients and which has no major complications. Metaplastic and dysplastic epithelium of the distal esophagus were successfully eradicated in most patients.
We extend the notion of phase transformations to periodic cellular materials by introducing mater... more We extend the notion of phase transformations to periodic cellular materials by introducing materials whose unit cells have multiple stable configurations. Each stable configuration of the unit cell corresponds to a stable phase, and transitions between these phases are regarded as phase transformations of the cellular material. We present a cellular material that exhibits phase transformation, identify key elements of the unit cell geometry that yield its mechanical behavior and introduce an analytical model that describes its constitutive behavior. Cellular materials that exhibit phase transformations show hysteresis and their response is characterized by a long serrated loading and unloading plateaus, making these materials attractive for energy absorption applications. While these phase transforming cellular material offer an energy absorption performance that is comparable to metal and polymeric honeycombs, the key difference is that the deformation on these materials is fully recoverable.
Conference Proceedings of the Society for Experimental Mechanics Series, 2011
We propose the concept of periodic cellular materials with programmable effective properties and ... more We propose the concept of periodic cellular materials with programmable effective properties and present initial results from a computational study of a prototypical material that exhibits this behavior. Nonlinear FEA shows that programmed geometric imperfections at the cell level can be used to modify the effective compressive storage modulus of shape memory polymer (SMP) based periodic cellular materials after they have been manufactured. The ability of SMPs to freeze a temporary deformation for an extended ...
This article presents a fast and suitable method to derive the spatial distribution of seabed gra... more This article presents a fast and suitable method to derive the spatial distribution of seabed grain sizes on tidal flats. A successful validation was carried out in the Dithmarschen Bight, a meso-tidal area of the German North Sea coast. In particular, the study is built on statistical correlations between observations of sediment sizes from 197 seabed samples and the numerical model output of flow-wave variables under specific marine and atmospheric forcings. The correlation between orbital velocities and mean grain sizes (d 50) exhibited the best results, but it can be affected under low relative wave height ratios. The methodology was applied for the construction of a high-resolution map of grain sizes in the Dithmarschen Bight, which resulted in absolute and relative errors of 16 μm and 13%, respectively. The proposed relationship was also adopted to examine scenarios of field data scarcity.
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Papers by David Restrepo