Bradley J. Nelson

ORCID

Publication Activity (10 Years)

Years Active: 1984-2024
Publications (10 Years): 98

Top Venues

Sci. Robotics

ICRA

IEEE Robotics Autom. Lett.

CoRR

Publications

Jasan Zughaibi, Bradley J. Nelson, Michael Muehlebach
Balancing a 3D Inverted Pendulum using Remote Magnetic Manipulation. CoRR (2024)
Roland Dreyfus, Quentin Boehler, Sean Lyttle, Philipp Gruber, Jonas Lussi, Christophe Chautems, Simone Gervasoni, Jatta Berberat, Dominic Emanuel Seibold, Nicole Ochsenbein-Kölble, Michael Reinehr, M. Weisskopf, Luca Remonda, Bradley J. Nelson
Dexterous helical magnetic robot for improved endovascular access. Sci. Robotics 9 (87) (2024)
Denis von Arx, Cedric Fischer, Harun Torlakcik, Salvador Pané, Bradley J. Nelson, Quentin Boehler
Simultaneous Localization and Actuation Using Electromagnetic Navigation Systems. IEEE Trans. Robotics 40 (2024)
Bradley J. Nelson
Technology matters, especially when deploying robots in the real world. Sci. Robotics 8 (79) (2023)
Quentin Boehler, Simone Gervasoni, Samuel L. Charreyron, Christophe Chautems, Bradley J. Nelson
On the Workspace of Electromagnetic Navigation Systems. IEEE Trans. Robotics 39 (1) (2023)
Amos Matsiko, Bradley J. Nelson
Robotics and AI in the Global South. Sci. Robotics 8 (85) (2023)
Florian Heemeyer, Christophe Chautems, Quentin Boehler, José Luis Merino, Bradley J. Nelson
An Evaluation Platform for Catheter Ablation Navigation. ISMR (2023)
Hao Jiang, Hongri Gu, Bradley J. Nelson, Teng Zhang
Numerical Study of Metachronal Wave-Modulated Locomotion in Magnetic Cilia Carpets. Adv. Intell. Syst. 5 (10) (2023)
Jiaen Wu, Barna Becsek, Alessandro Schaer, Henrik Maurenbrecher, George Chatzipirpiridis, Olgaç Ergeneman, Salvador Pané, Hamdi Torun, Bradley J. Nelson
Real-Time Gait Phase Detection on Wearable Devices for Real-World Free-Living Gait. IEEE J. Biomed. Health Informatics 27 (3) (2023)
Min Ye, Yan Zhou, Hongyu Zhao, Ziya Wang, Bradley J. Nelson, Xiaopu Wang
A Review of Soft Microrobots: Material, Fabrication, and Actuation. Adv. Intell. Syst. 5 (11) (2023)
Hongri Gu, Emre Hanedan, Quentin Boehler, Tian-Yun Huang, Arnold J. T. M. Mathijssen, Bradley J. Nelson
Artificial microtubules for rapid and collective transport of magnetic microcargoes. Nat. Mach. Intell. 4 (8) (2022)
Bradley J. Nelson
: Helping build better robots for a better future. Sci. Robotics 7 (68) (2022)
Cedric Fischer, Quentin Boehler, Bradley J. Nelson
Using Magnetic Fields to Navigate and Simultaneously Localize Catheters in Endoluminal Environments. IEEE Robotics Autom. Lett. 7 (3) (2022)
Lidong Yang, Jiangfan Yu, Shihao Yang, Ben Wang, Bradley J. Nelson, Li Zhang
A Survey on Swarm Microrobotics. IEEE Trans. Robotics 38 (3) (2022)
Bradley J. Nelson, Simone Gervasoni, Philip W. Y. Chiu, Li Zhang, Ajmal Zemmar
Magnetically Actuated Medical Robots: An in vivo Perspective. Proc. IEEE 110 (7) (2022)
Bradley J. Nelson, Yuan Luo
Topology-Preserving Dimensionality Reduction via Interleaving Optimization. CoRR (2022)
Jonas Lussi, Simone Gervasoni, Michelle Mattille, Roland Dreyfus, Quentin Boehler, Michael Reinehr, Nicole Ochsenbein, Bradley J. Nelson, Ueli Moehrlen
Magnetically Guided Laser Surgery for the Treatment of Twin-to-Twin Transfusion Syndrome. Adv. Intell. Syst. 4 (11) (2022)
Bradley J. Nelson, Pierre E. Dupont, Dario Floreano, Ken Goldberg, Hongri Gu, Neil Jacobstein, Cecilia Laschi, Hod Lipson
. Sci. Robotics 7 (71) (2022)
Xiaopu Wang, Chengzhi Hu, Salvador Pané, Bradley J. Nelson
Dynamic Modeling of Magnetic Helical Microrobots. IEEE Robotics Autom. Lett. 7 (2) (2022)
Deqing Fu, Bradley J. Nelson
Topological Regularization for Dense Prediction. ICMLA (2022)
Quentin Peyron, Quentin Boehler, Patrick Rougeot, Pierre Roux, Bradley J. Nelson, Nicolas Andreff, Kanty Rabenorosoa, Pierre Renaud
Magnetic concentric tube robots: Introduction and analysis. Int. J. Robotics Res. 41 (4) (2022)
Michael Mattmann, Quentin Boehler, Xiangzhong Chen, Salvador Pané, Bradley J. Nelson
Shape memory polymer variable stiffness magnetic catheters with hybrid stiffness control. IROS (2022)
Bradley J. Nelson
Parameterized Vietoris-Rips Filtrations via Covers. CoRR (2022)
Tian-Yun Huang, Hongri Gu, Bradley J. Nelson
Increasingly Intelligent Micromachines. Annu. Rev. Control. Robotics Auton. Syst. 5 (2022)
Roland Dreyfus, Quentin Boehler, Bradley J. Nelson
A Simulation Framework for Magnetic Continuum Robots. IEEE Robotics Autom. Lett. 7 (3) (2022)
Lek-Heng Lim, Bradley J. Nelson
What is an equivariant neural network? CoRR (2022)
Samuel L. Charreyron, Quentin Boehler, Byungsoo Kim, Cameron Weibel, Christophe Chautems, Bradley J. Nelson
Modeling Electromagnetic Navigation Systems. IEEE Trans. Robotics 37 (4) (2021)
Yuan Luo, Bradley J. Nelson
Accelerating Iterated Persistent Homology Computations with Warm Starts. CoRR (2021)
Pierre E. Dupont, Bradley J. Nelson, Michael Goldfarb, Blake Hannaford, Arianna Menciassi, Marcia K. O'Malley, Nabil Simaan, Pietro Valdastri, Guang-Zhong Yang
A decade retrospective of medical robotics research from 2010 to 2020. Sci. Robotics 6 (60) (2021)
Peer Fischer, Bradley J. Nelson
Tiny robots make big advances. Sci. Robotics 6 (52) (2021)
Harun Torlakcik, Can Sarica, Patrick Bayer, Kazuaki Yamamoto, Christian Iorio-Morin, Mojgan Hodaie, Suneil K. Kalia, Joseph S. Neimat, Juha Hernesniemi, Anuj Bhatia, Bradley J. Nelson, Salvador Pané, Andres M. Lozano, Ajmal Zemmar
Magnetically Guided Catheters, Micro- and Nanorobots for Spinal Cord Stimulation. Frontiers Neurorobotics 15 (2021)
Raul Vicente, Youssef Mohamed, Víctor M. Eguíluz, Emal Zemmar, Patrick Bayer, Joseph S. Neimat, Juha Hernesniemi, Bradley J. Nelson, Ajmal Zemmar
Modelling the Impact of Robotics on Infectious Spread Among Healthcare Workers. Frontiers Robotics AI 8 (2021)
Ayoung Hong, Andrew J. Petruska, Ajmal Zemmar, Bradley J. Nelson
Magnetic Control of a Flexible Needle in Neurosurgery. IEEE Trans. Biomed. Eng. 68 (2) (2021)
Jiaen Wu, Kiran Kuruvithadam, Alessandro Schaer, Richie Stoneham, George Chatzipirpiridis, Christopher Awai Easthope, Gill Barry, James Martin, Salvador Pané, Bradley J. Nelson, Olgaç Ergeneman, Hamdi Torun
An Intelligent In-Shoe System for Gait Monitoring and Analysis with Optimized Sampling and Real-Time Visualization Capabilities. Sensors 21 (8) (2021)
Alessandro Schaer, Oskar Helander, Francesco Buffa, Alexis Müller, Kevin Schneider, Henrik Maurenbrecher, Barna Becsek, George Chatzipirpiridis, Olgaç Ergeneman, Salvador Pané, Bradley J. Nelson, Nina Schaffert
Integrated Pedal System for Data Driven Rehabilitation. Sensors 21 (23) (2021)
Daniel Ahmed, Alexander Sukhov, David Hauri, Dubon Rodrigue, Gian Maranta, Jens Harting, Bradley J. Nelson
Bioinspired acousto-magnetic microswarm robots with upstream motility. Nat. Mach. Intell. 3 (2) (2021)
Samuel L. Charreyron, Quentin Boehler, Aschraf N. Danun, Alexandre Mesot, Matthias Becker, Bradley J. Nelson
A Magnetically Navigated Microcannula for Subretinal Injections. IEEE Trans. Biomed. Eng. 68 (1) (2021)
Deqing Fu, Bradley J. Nelson
Topological Regularization for Dense Prediction. CoRR (2021)
Anzhu Gao, Robin R. Murphy, Weidong Chen, Giulio Dagnino, Peer Fischer, Maximiliano G. Gutierrez, Dennis Kundrat, Bradley J. Nelson, Naveen Shamsudhin, Hao Su, Jingen Xia, Ajmal Zemmar, Dandan Zhang, Chen Wang, Guang-Zhong Yang
Progress in robotics for combating infectious diseases. Sci. Robotics 6 (52) (2021)
François Drielsma, Qing Lin, Pierre Côte de Soux, Laura Dominé, Ran Itay, Dae Heun Koh, Bradley J. Nelson, Kazuhiro Terao, Ka Vang Tsang, Tracy L. Usher
Clustering of Electromagnetic Showers and Particle Interactions with Graph Neural Networks in Liquid Argon Time Projection Chambers Data. CoRR (2020)
Ruoxi Yu, Samuel L. Charreyron, Quentin Boehler, Cameron Weibel, Christophe Chautems, Carmen C. Y. Poon, Bradley J. Nelson
Modeling Electromagnetic Navigation Systems for Medical Applications using Random Forests and Artificial Neural Networks. ICRA (2020)
Qianqian Wang, Ben Wang, Jiangfan Yu, Kathrin Schweizer, Bradley J. Nelson, Li Zhang
Reconfigurable Magnetic Microswarm for Thrombolysis under Ultrasound Imaging. ICRA (2020)
Christophe Chautems, Alice Tonazzini, Quentin Boehler, Seung Hee Jeong, Dario Floreano, Bradley J. Nelson
Magnetic Continuum Device with Variable Stiffness for Minimally Invasive Surgery. Adv. Intell. Syst. 2 (6) (2020)
Ajmal Zemmar, Andres M. Lozano, Bradley J. Nelson
The rise of robots in surgical environments during COVID-19. Nat. Mach. Intell. 2 (10) (2020)
Rickard Brüel Gabrielsson, Bradley J. Nelson, Anjan Dwaraknath, Primoz Skraba
A Topology Layer for Machine Learning. AISTATS (2020)
Guang-Zhong Yang, Bradley J. Nelson, Robin R. Murphy, Howie Choset, Henrik I. Christensen, Steven H. Collins, Paolo Dario, Ken Goldberg, Koji Ikuta, Neil Jacobstein, Danica Kragic, Russell H. Taylor, Marcia McNutt
Combating COVID-19 - The role of robotics in managing public health and infectious diseases. Sci. Robotics 5 (40) (2020)
Gunnar E. Carlsson, Anjan Dwaraknath, Bradley J. Nelson
Persistent and Zigzag Homology: A Matrix Factorization Viewpoint. CoRR (2019)
Ruoxi Yu, Samuel L. Charreyron, Quentin Boehler, Cameron Weibel, Carmen C. Y. Poon, Bradley J. Nelson
Modeling Electromagnetic Navigation Systems for Medical Applications using Random Forests and Artificial Neural Networks. CoRR (2019)
Samuel L. Charreyron, Edoardo Gabbi, Quentin Boehler, Matthias Becker, Bradley J. Nelson
A Magnetically Steered Endolaser Probe for Automated Panretinal Photocoagulation. IEEE Robotics Autom. Lett. 4 (2) (2019)
Jan T. Burri, Hannes Vogler, Gautam Munglani, Nino Laubli, Ueli Grossniklaus, Bradley J. Nelson
A Microrobotic System for Simultaneous Measurement of Turgor Pressure and Cell-Wall Elasticity of Individual Growing Plant Cells. IEEE Robotics Autom. Lett. 4 (2) (2019)
Rickard Brüel Gabrielsson, Bradley J. Nelson, Anjan Dwaraknath, Primoz Skraba, Leonidas J. Guibas, Gunnar E. Carlsson
A Topology Layer for Machine Learning. CoRR (2019)
Robert J. Full, Neil Jacobstein, Peer Fischer, James G. Bellingham, Howie Choset, Henrik I. Christensen, Paolo Dario, Bradley J. Nelson, Russell H. Taylor
Ten robotics technologies of the year. Sci. Robotics 4 (26) (2019)
Sungwoong Jeon, Sangwon Kim, Shinwon Ha, Seungmin Lee, Eunhee Kim, So Yeun Kim, Sun Hwa Park, Jung Ho Jeon, Sung Won Kim, Cheil Moon, Bradley J. Nelson, Jin-young Kim, Seong-Woon Yu, Hongsoo Choi
Magnetically actuated microrobots as a platform for stem cell transplantation. Sci. Robotics 4 (30) (2019)
Hongri Gu, Quentin Boehler, Daniel Ahmed, Bradley J. Nelson
Magnetic quadrupole assemblies with arbitrary shapes and magnetizations. Sci. Robotics 4 (35) (2019)
George Chatzipirpiridis, Simone Gervasoni, Cedric Fischer, Olgaç Ergeneman, Eva Pellicer, Bradley J. Nelson, Salvador Pané
3D Printing of Thermoplastic-Bonded Soft- and Hard-Magnetic Composites: Magnetically Tuneable Architectures and Functional Devices. Adv. Intell. Syst. 1 (6) (2019)
Janis Edelmann, Andrew J. Petruska, Bradley J. Nelson
Estimation-Based Control of a Magnetic Endoscope without Device Localization. J. Medical Robotics Res. 3 (1) (2018)
Quentin Peyron, Quentin Boehler, Kanty Rabenorosoa, Bradley J. Nelson, Pierre Renaud, Nicolas Andreff
Kinematic Analysis of Magnetic Continuum Robots Using Continuation Method and Bifurcation Analysis. IEEE Robotics Autom. Lett. 3 (4) (2018)
Hen-Wei Huang, Sean Lyttle, Bradley J. Nelson
Bioinspired navigation in shape morphing micromachines for autonomous targeted drug delivery. RoboSoft (2018)
James G. Bellingham, Pierre E. Dupont, Peer Fischer, Luciano Floridi, Robert J. Full, Neil Jacobstein, Vijay Kumar, Marcia McNutt, Robert D. Merrifield, Bradley J. Nelson, Brian Scassellati, Mariarosaria Taddeo, Russell H. Taylor, Manuela M. Veloso, Zhong Lin Wang, Robert J. Wood
. Sci. Robotics 3 (14) (2018)
Salvador Pané, Bradley J. Nelson
4D printing and robotics. Sci. Robotics 3 (18) (2018)
Bumjin Jang, Amanda Aho, Bradley J. Nelson, Salvador Pané
Fabrication and Locomotion of Flexible Nanoswimmers. IROS (2018)
Franziska Ullrich, Jonas Lussi, Vasileios Chatzopoulos, Stephan Michels, Andrew J. Petruska, Bradley J. Nelson
A Robotic Diathermy System for Automated Capsulotomy. J. Medical Robotics Res. 3 (1) (2018)
Chengzhi Hu, Salvador Pané, Bradley J. Nelson
Soft Micro- and Nanorobotics. Annu. Rev. Control. Robotics Auton. Syst. 1 (2018)
Christophe Chautems, Sean Lyttle, Quentin Boehler, Bradley J. Nelson
Design and Evaluation of a Steerable Magnetic Sheath for Cardiac Ablations. IEEE Robotics Autom. Lett. 3 (3) (2018)
Sumit Mohanty, Ayoung Hong, Carlos Alcantara, Andrew J. Petruska, Bradley J. Nelson
Stereo Holographic Diffraction Based Tracking of Microrobots. IEEE Robotics Autom. Lett. 3 (1) (2018)
Bradley J. Nelson, Guang-Zhong Yang
New materials for next-generation robots. Sci. Robotics 3 (18) (2018)
Burak Zeydan, Andrew J. Petruska, Luca Somm, Roel Pieters, Yang Fang, David F. Sargent, Bradley J. Nelson
Automated Particle Collection for Protein Crystal Harvesting. IEEE Robotics Autom. Lett. 2 (3) (2017)
Hen-Wei Huang, Qianwen Chao, Mahmut Selman Sakar, Bradley J. Nelson
Optimization of Tail Geometry for the Propulsion of Soft Microrobots. IEEE Robotics Autom. Lett. 2 (2) (2017)
Peer Fischer, Bradley J. Nelson
New materials for next-generation robots. Sci. Robotics 2 (10) (2017)
Christophe Chautems, Alice Tonazzini, Dario Floreano, Bradley J. Nelson
A variable stiffness catheter controlled with an external magnetic field. IROS (2017)
Christophe Chautems, Bradley J. Nelson
The tethered magnet: Force and 5-DOF pose control for cardiac ablation. ICRA (2017)
Ayoung Hong, Burak Zeydan, Samuel Charreyron, Olgaç Ergeneman, Salvador Pané, Muhammed Fatih Toy, Andrew J. Petruska, Bradley J. Nelson
Real-Time Holographic Tracking and Control of Microrobots. IEEE Robotics Autom. Lett. 2 (1) (2017)
Janis Edelmann, Andrew J. Petruska, Bradley J. Nelson
Magnetic control of continuum devices. Int. J. Robotics Res. 36 (1) (2017)
Samuel L. Charreyron, Burak Zeydan, Bradley J. Nelson
Shared control of a magnetic microcatheter for vitreoretinal targeted drug delivery. ICRA (2017)
Chengzhi Hu, Qi Zhang, Tobias Meyer, Hannes Vogler, Jan T. Burri, Naveen Shamsudhin, Ueli Grossniklaus, Bradley J. Nelson
In vivo tracking and measurement of pollen tube vesicle motion. ICRA (2017)
Ima Avalos Vizcarra, Jens Möller, Mahmut Selman Sakar, Berna Özkale, André Machado Lindo, Fajer Mushtaq, Ingmar Schoen, Salvador Pané, Viola Vogel, Bradley J. Nelson
Robotically controlled microprey to resolve initial attack modes preceding phagocytosis. Sci. Robotics 2 (2) (2017)
Yun Ding, Famin Qiu, Xavier Casadevall i Solvas, Flora Wing Yin Chiu, Bradley J. Nelson, Andrew deMello
Microfluidic-Based Droplet and Cell Manipulations Using Artificial Bacterial Flagella. Micromachines 7 (2) (2016)
Hen-Wei Huang, Mahmut Selman Sakar, Katharina Riederer, Naveen Shamsudhin, Andrew J. Petruska, Salvador Pané, Bradley J. Nelson
Magnetic microrobots with addressable shape control. ICRA (2016)
Jan T. Burri, Chengzhi Hu, Naveen Shamsudhin, Xiaopu Wang, Hannes Vogler, Ueli Grossniklaus, Bradley J. Nelson
Dual-axis Cellular Force Microscope for mechanical characterization of living plant cells. CASE (2016)
Chengzhi Hu, Katharina Riederer, Michael Klemmer, Salvador Pané, Bradley J. Nelson
Electrosynthesis of magnetoresponsive microrobot for targeted drug delivery using calcium alginate. EMBC (2016)
Franziska Ullrich, Famin Qiu, Juho Pokki, Tianyun Huang, Salvador Pané, Bradley J. Nelson
Swimming characteristics of helical microrobots in fibrous environments. BioRob (2016)
Hen-Wei Huang, Andrew J. Petruska, Mahmut Selman Sakar, Maria Skoura, Franziska Ullrich, Qi Zhang, Salvador Pané, Bradley J. Nelson
Self-folding hydrogel bilayer for enhanced drug loading, encapsulation, and transport. EMBC (2016)
Roel Pieters, Sievi Lombriser, Alejandro Alvarez-Aguirre, Bradley J. Nelson
Model Predictive Control of a Magnetically Guided Rolling Microrobot. IEEE Robotics Autom. Lett. 1 (1) (2016)
James G. Bellingham, Howie Choset, Paolo Dario, Peer Fischer, Toshio Fukuda, Neil Jacobstein, Bradley J. Nelson, Manuela M. Veloso, Jeremy M. Berg
Science for robotics and robotics for science. Sci. Robotics 1 (1) (2016)
Andrew J. Petruska, Fabio Ruetz, Ayoung Hong, Luca Regli, Oguzkan Surucu, Ajmal Zemmar, Bradley J. Nelson
Magnetic needle guidance for neurosurgery: Initial design and proof of concept. ICRA (2016)
Naveen Shamsudhin, Huseyin Baris Atakan, Nino Laubli, Hannes Vogler, Chengzhi Hu, Abu Sebastian, Ueli Grossniklaus, Bradley J. Nelson
Probing the micromechanics of the fastest growing plant cell - The pollen tube. EMBC (2016)
John J. Andersen, Bradley J. Nelson, Jeremy M. Brown
EmpPrior: using outside empirical data to inform branch-length priors for Bayesian phylogenetics. BMC Bioinform. 17 (2016)
Franziska Ullrich, Stephan Michels, Matthias Becker, Bradley J. Nelson
Automated Device for Intravitreal Injections. CURAC (2016)
Ayoung Hong, Andrew J. Petruska, Bradley J. Nelson
Tracking a magnetically guided catheter with a single rotating C-Arm. ICRA (2015)
Andrew J. Petruska, Bradley J. Nelson
Minimum Bounds on the Number of Electromagnets Required for Remote Magnetic Manipulation. IEEE Trans. Robotics 31 (3) (2015)
Seungmin Lee, Sangwon Kim, Bradley J. Nelson, Hongsoo Choi
Fabrication and targeted particle delivery using microrobots. URAI (2015)
George Chatzipirpiridis, Pascal Erne, Olgaç Ergeneman, Salvador Pané, Bradley J. Nelson
A magnetic force sensor on a catheter tip for minimally invasive surgery. EMBC (2015)
Dimitrios Felekis, Hannes Vogler, Geraldo Mecja, Simon Muntwyler, Anna Nestorova, Tianyun Huang, Mahmut Selman Sakar, Ueli Grossniklaus, Bradley J. Nelson
Real-time automated characterization of 3D morphology and mechanics of developing plant cells. Int. J. Robotics Res. 34 (8) (2015)
Franziska Ullrich, Kanika S. Dheman, Simone Schürle, Bradley J. Nelson
Magnetically actuated and guided milli-gripper for medical applications. ICRA (2015)
Alex H. Barnett, Bradley J. Nelson, J. Matthew Mahoney
High-order boundary integral equation solution of high frequency wave scattering from obstacles in an unbounded linearly stratified medium. J. Comput. Phys. 297 (2015)
Samuel Charreyron, Roel S. Pieters, Hsi-Wen Tung, Maurice Gonzenbach, Bradley J. Nelson
Navigation of a rolling microrobot in cluttered environments for automated crystal harvesting. IROS (2015)
Sangwon Kim, Shinwon Ha, Seungmin Lee, Famin Qiu, Hongsoo Choi, Seungwoon Yu, Bradley J. Nelson
Scaffold-type microrobots for targeted cell delivery. URAI (2015)
Roel Pieters, Hsi-Wen Tung, Samuel Charreyron, David F. Sargent, Bradley J. Nelson
RodBot: A rolling microrobot for micromanipulation. ICRA (2015)
Dimitrios Felekis, Hannes Vogler, Geraldo Mecja, Simon Muntwyler, Mahmut Selman Sakar, Ueli Grossniklaus, Bradley J. Nelson
High-throughput analysis of the morphology and mechanics of tip growing cells using a microrobotic platform. IROS (2014)
Hsi-Wen Tung, Massimo Maffioli, Dominic R. Frutiger, Kartik M. Sivaraman, Salvador Pané, Bradley J. Nelson
Polymer-Based Wireless Resonant Magnetic Microrobots. IEEE Trans. Robotics 30 (1) (2014)

Bradley J. Nelson.css-eqlu8i{font-size:small;padding-left:4px;}

Publication Activity (10 Years)

Top Topics

Top Venues

Bradley J. Nelson