The NeuroMedicator An Intra-Cerebral Drug Delivery System
for Neural Research
Dissertation
zur Erlangung des Doktorgrades der Technischen Fakultt
der Albert-Ludwigs-Universitt Freiburg im Breisgau
vorgelegt von
Sven Spieth
Freiburg im Breisgau, 2013
ii
Dekan
Prof. Dr. Yiannos Manoli
Referenten
Prof. Dr. Roland ZengerleProf. Dr. Oliver Paul
Tag der Abgabe: 9. Juli 2013Tag der Prfung: 27. November 2013
Lehrstuhl fr AnwendungsentwicklungInstitut fr Mikrosystemtechnik (IMTEK)Technische FakulttAlbert-Ludwigs-Universitt Freiburg im Breisgau
Die Dissertation wurde angefertigt am
Institut fr Mikro- und Informationstechnik der Hahn-Schickard-Gesellschaftfr angewandte Forschung e.V. (HSG-IMIT)Villingen-Schwenningen
Abstract
In neuroscience, microinfusions of drugs directly into the central nervous systemof awake animals are often used to unravel brain functions related to behavior.In this respect, implantable multifunctional microprobes enabling drug deliveryalongside electrophysiological recording of neural activity are desirable. How-ever, whereas small, skull-mounted transmitters (headstages) for wireless electro-physiological recordings are already commercially available, drug delivery reliesstill mostly on tethered liquid infusion systems with a stationary syringe pump,possibly interfering with behavior. Ideally, one headstage can be used for bothelectrophysiology as well as control of drug delivery.
To address the aspect of drug delivery, this thesis presents the NeuroMedicator,a micropump system with an application-specific, comb-like silicon microprobe.With outer dimensions of 2017.55 mm3, the device is small and lightweightenough to be placed directly on the skull of a rat. The operational concept of theNeuroMedicator is based on 2 8 discrete liquid reservoirs interconnected in apearl-chain-like manner whose contents can be infused on demand. Thereby, thedrug liquid is loaded to the reservoirs prior to implantation. After implantation andelectrical connection, individual 0.25 L infusions can be sequentially releasedthrough the two shafts of the silicon microprobe.
Delivery of the reservoir contents is realized by thermally expandable Expancel
microspheres embedded into an elastic polydimethylsiloxane (PDMS) matrix. Byapplying short heating pulses to the composite underneath a reservoir, the result-ing irreversible expansion displaces the liquid and releases it through the micro-probe directly to the neural tissue. To enable localized energy-efficient heating,two different microheater array designs are fabricated by thin-film processing onmultilayer printed circuit boards (PCBs) and subsequently evaluated. Addition-ally, in order to get a better understanding for the different expansion phases of themicrospheres, the associated idealized thermodynamic processes are described.
Microprobe designs featuring different microfluidic channel layouts and elec-trodes to be used with the NeuroMedicator as well as other assemblies are im-plemented. The microprobe comb used with the NeuroMedicator features two
iv Abstract
8-mm-long shafts each having a cross-sectional area of 250250 m2 and an in-tegrated drug delivery channel of 5050 m2 with an outlet of 25 m in diameter.The microprobes are fabricated in a two-wafer silicon direct bonding process byapplying deep reactive ion etching (DRIE), wafer grinding, and thin film process-ing.
Following fabrication, in particular the fluidic properties of the microprobes arecharacterized. Furthermore, a miniaturized system assembly for the microprobesis developed which allows to demonstrate the combination of drug delivery andelectrophysiological recording during in vivo functional inactivations.
After implantation of the microprobe, leakage of drug by diffusion occurs due tothe open outlets. In order to evaluate this, the amount of leakage as well as its ef-fect on the NeuroMedicator is simulated for a worst case scenario. The maximumleakage within the first three days after implantation is calculated to be equivalentto less than 0.07 L of drug solution.
The development of the NeuroMedicator is implemented in two stages with twoprototypes NeuroMedicator I and II which are both characterized and evaluated.The general proof-of-concept and device functionality is first demonstrated withthe NeuroMedicator I being mainly based on PDMS technologies. On this basis,critical aspects such as increased storage stability of the drug liquid and prac-tical handling issues are addressed by the NeuroMedicator II. This includes anevaluation of the barrier properties of different polymer foils. After connectionto an external electronic control unit, the NeuroMedicator II provides individualmetered infusions of 0.250.01L each requiring 3.375 W s of electric energy.Finally, the NeuroMedicator II is successfully applied in the 5-choice serial reac-tion time task (5-CSRTT), a behavioral test of visual attention and impulsivity.
Zusammenfassung
In den Neurowissenschaften werden Mikroinfusionen von pharmazeutischenWirkstoffen ins zentrale Nervensystem von sich frei bewegenden Versuchstierenhufig zur Untersuchung von verhaltensbezogenen Gehirnfunktionen herangezo-gen. Hierfr sind implantierbare, multifunktionale Mikrosonden wnschenswert,die sowohl Wirkstoffe abgeben als auch die Aktivitt der Neuronen elektrophy-siologisch aufnehmen knnen. Whrend zur Aufnahme und bermittlung vonelektrophysiologischen Daten bereits kleine, drahtlose Sendeinheiten, die direktauf dem Kopf montiert werden knnen, kommerziell verfgbar sind, werden zurWirkstoffabgabe immer noch hauptschlich schlauchgebundene Infusionssyste-me mit stationren Spritzenpumpen eingesetzt. Unter Umstnden kann dies dasnormale Tierverhalten beeinflussen. Idealerweise sollte eine einzige drahtloseSende-/Empfangseinheit sowohl elektrophysiologische Untersuchungen als auchdie Wirkstoffabgabe ermglichen.
Fr den Aspekt der Wirkstoffabgabe wird in dieser Arbeit der NeuroMedica-tor vorgestellt, ein Mikropumpensystem mit einer anwendungsspezifischen Si-liziummikrosonde in Form einer Kammstruktur. Mit Auenabmessungen von2017,55 mm3 ist das System klein und leicht genug um direkt auf dem Kopfeiner Ratte befestigt zu werden. Das Funktionsprinzip des NeuroMedicators be-ruht auf 28 diskreten Flssigkeitsreservoiren, die perlenschnurartig verbundensind und deren Inhalte auf Anforderung infundiert werden knnen. Die Wirkstoff-lsung wird dabei vor der Implantation in die Reservoire gefllt. Nach erfolgterImplantation und elektrischem Anschluss knnen dann sequentiell individuelle0,25 l Infusionen durch die zwei Schfte der Siliziummikrosonde abgegebenwerden.
Die Abgabe der Reservoirinhalte erfolgt durch thermisch expandierbareExpancel Mikrosphren, die in eine elastische Matrix aus Polydimethylsiloxan(PDMS) eingebettet sind. Durch kurzzeitiges Heizen des Komposits unterhalb ei-nes Reservoirs expandiert dieses irreversibel und verdrngt so die Flssigkeit, diedann durch die Mikrosonde direkt ans Gehirngewebe abgegeben wird. Um rtlichbegrenztes, energieeffizientes Heizen zu ermglichen, werden zwei verschiedeneMikroheizerarrays durch Dnnschichtprozessierung auf mehrlagigen Leiterplat-
vi Zusammenfassung
ten hergestellt und bewertet. Zustzlich werden zum besseren Verstndnis derverschiedenen Expansionsphasen der Mikrosphren die zugrundeliegenden idea-lisierten thermodynamischen Prozesse beschrieben.
Mikrosondendesigns mit unterschiedlichen mikrofluidischen Kanallayouts undElektroden werden realisiert, die sowohl fr den NeuroMedicator als auch an-dere Systemaufbauten verwendet werden knnen. Die mit dem NeuroMedicatorverwendete Mikrosonde besteht dabei aus einer Kammstruktur mit zwei 8 mmlangen Schften, die jeweils eine Querschnittsflche von 250250 m2 undeinen 5050 m2 Flssigkeitskanal mit einem runden Auslass von 25 m imDurchmesser besitzen. Die Mikrosonden werden durch adhsives Bonden zweierSiliziumwafer in Verbindung mit Tiefentzen, Waferabdnnen und Dnnschicht-prozessierung hergestellt.
Die hergestellten Mikrosonden werden insbesondere im Hinblick auf ihre fluidi-schen Eigenschaften hin charakterisiert. Darber hinaus wird ein miniaturisierterSystemaufbau fr die Sonden entwickelt, der es ermglicht die Kombination ausWirkstoffabgabe und Elektrophysiologie in vivo whrend einer funktionalen In-aktivierung zu zeigen.
Nach der Implantation der Mikrosonde diffundiert Wirkstoff aus den offenen Aus-lssen. Um diesen Effekt einschtzen zu knnen, werden die Ausdiffusion sowiedie Auswirkungen auf den NeuroMedicator fr ein Worst-Case-Szenario simu-liert. Die maximale Ausdiffusion innerhalb der ersten drei Tage nach Implantationentspricht dabei weniger als 0,07 l Wirkstofflsung.
Die Entwicklung des NeuroMedicators erfolgt in zwei Stufen mit zwei Prototy-pen NeuroMedicator I und II, die jeweils charakterisiert und bewertet werden. Diegenerelle Machbarkeit sowie Funktionalitt werden zunchst anhand des Neuro-Medicators I gezeigt, der hauptschlich auf PDMS-Technologien basiert. Auf die-ser Basis werden dann mit dem NeuroMedicator II kritische Aspekte wie erhhteLagerstabilitt der Wirkstofflsung und praktische Handhabung adressiert. Diesschliet eine Evaluation der Barriereeigenschaften von verschiedenen Kunststoff-folien ein. Nach Anstecken an eine externe elektronische Kontrolleinheit kannder NeuroMedicator II einzelne, definierte Infusionen von 0,250,01l abgeben.Hierfr wird jeweils eine elektrische Energiemenge von 3,375 W s bentigt.
Abschlieend wird der NeuroMedicator II erfolgreich im 5-Choice Serial Reac-tion Time Task (5-CSRTT) eingesetzt, einem Verhaltenstest zur visuellen Auf-merksamkeit und Impulsivitt.
Preface
The work presented in this thesis was performed at the Institut fr Mikro-und Informationstechnik der Hahn-Schickard-Gesellschaft fr angewandte For-schung e.V. (HSG-IMIT) in Villingen-Schwenningen, Germany, in the frame ofthe Information Society Technologies (IST) Integrated Project NeuroProbes ofthe 6th Framework Program (FP6) of the European Commission (project numberIST-027017).
NeuroProbes Development of multifunctional probe arrays for cerebral appli-cations targeted the development of a common system platform for multifunc-tional one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D)neural microprobes. The central aspect was the realization of a two-way interac-tion with the brain, both electrically and chemically, by combining
electrodes for recording and stimulation drug delivery for chemical stimulation and inactivation biosensors for chemical monitoring
In addition, aspects of biocompatibility and the integration of complementarymetal oxide semiconductor (CMOS) electronics directly on the probes were ad-dressed. In order to realize the transition from 2D combs to complete 3D arrays,a novel modular concept enabling the insertion of individual probe combs of dif-ferent functionality into a common slim-base platform was pursued. Correspond-ingly, all technological efforts were adapted to comply with this concept.
In order to realize this ambitious approach, fourteen institutions from ten Euro-pean countries having technological, neuroscientific, and industrial backgroundcollaborated from 2006 to 2010. Thereby, HSG-IMIT coordinated all aspects re-lated to microfluidic drug delivery involving partners at the
Interuniversity Microelectronics Center (IMEC), Leuven, Belgium Department of Microsystems Engineering (IMTEK) at the University of
Freiburg, Germany Institute of Microengineering at the cole Polytechnique Fdrale de Lau-
sanne (EPFL), Switzerland University of Cambridge (UCam), United Kingdom
viii Preface
Katholieke Universiteit Leuven (KUL), Belgium Micronit Microfluidics, Enschede, The Netherlands
The complete NeuroProbes project was coordinated by IMEC, whereas the tech-nological and scientific coordination was taken over by IMTEK and KUL, respec-tively.
The scope of this thesis is the development of the intra-cerebral drug deliverysystem NeuroMedicator with a silicon microprobe for liquid infusion. The de-scribed work required close collaboration with co-workers at HSG-IMIT and Neu-roProbes partners whose contributions are credited throughout the thesis:
Silicon microprobes combining drug delivery with recording electrodeswere jointly developed with IMTEK addressing neurophysiological de-mands of UCam and KUL.
Requirements to the NeuroMedicator and aspects of behavioral neuro-science as well as in vivo experiments were addressed with UCam.
The presented silicon micromachining and thin film processes were implementedby the cleanroom team at HSG-IMIT in collaboration with partners at IMTEK andthe associated cleanroom service center (RSC). Required mechanical componentsand electronic circuits were realized by the machine shop and the electronic sys-tems group at HSG-IMIT, respectively. The shown scanning electron micrographswere taken with support of the micrograph service of HSG-IMIT.
All animal related procedures and experiments were conducted by UCam andcomplied with the legal and ethical requirements of the UK Animals (Scien-tific Procedures) Act 1986 in accordance with the local institutional guidelinesat UCam.
Publications
Parts of this work have been published in the following journals and conferenceproceedings:
Journals
1. K. Seidl, S. Spieth, S. Herwik, J. Steigert, R. Zengerle, O. Paul, and P.Ruther, In-plane silicon probes for simultaneous neural recording anddrug delivery, Journal of Micromechanics and Microengineering, vol. 20,no. 10, 105006 (11pp), 2010.
2. O. Frey, P. D. van der Wal, S. Spieth, O. Brett, K. Seidl, O. Paul, P. Ruther,R. Zengerle, and N. F. de Rooij, Biosensor microprobes with integratedmicrofluidic channels for bi-directional neurochemical interaction, Jour-nal of Neural Engineering, vol. 8, no. 6, 066001 (9pp), 2011.
3. S. Spieth, O. Brett, K. Seidl, A. A. A. Aarts, M. A. Erismis, S. Herwik, F.Trenkle, S. Ttzner, J. Auber, M. Daub, H. P. Neves, R. Puers, O. Paul, P.Ruther, and R. Zengerle, A floating 3D silicon microprobe array for neuraldrug delivery compatible with electrical recording, Journal of Microme-chanics and Microengineering, vol. 21, no. 12, 125001 (16pp), 2011.
4. S. Spieth, A. Schumacher, C. Kallenbach, S. Messner, and R. Zengerle,The NeuroMedicator - a micropump integrated with silicon microprobesfor drug delivery in neural research, Journal of Micromechanics and Mi-croengineering, vol. 22, no. 6, 065020 (11pp), 2012.
5. S. Spieth, A. Schumacher, T. Holtzman, P. D. Rich, D. E. Theobald, J. W.Dalley, R. Nouna, S. Messner, and R. Zengerle, An intra-cerebral drugdelivery system for freely moving animals, Biomedical Microdevices,vol. 14, no. 5, pp. 799809, 2012.
x Publications
6. S. Spieth, A. Schumacher, F. Trenkle, O. Brett, K. Seidl, S. Herwik, S.Kisban, P. Ruther, O. Paul, A. A. A. Aarts, H. P. Neves, P. D. Rich, D.E. Theobald, T. Holtzman, J. W. Dalley, B.-E. Verhoef, P. Janssen, andR. Zengerle, Approaches for drug delivery with intracortical probes,Biomedizinische Technik / Biomedical Engineering, 2013, advance onlinepublication, doi:10.1515/bmt-2012-0096.
Conference Proceedings
1. K. Seidl, S. Spieth, J. Steigert, O. Paul, and P. Ruther, Fabrication processof silicon in-plane probes for simultaneous neural recording and drug de-livery, in Proceedings 7th International Workshop on High-Aspect-RatioMicro-Structure Technology (HARMST), Besanon, France, June 79 2007,pp. 247248.
2. K. Seidl, S. Spieth, S. Herwik, O. Paul, and P. Ruther, Cerebral micro-probes for neural recording combined with fluidic functionality, in Pro-ceedings 41. Jahrestagung der DGBMT im VDE (BMT), Aachen, Germany,Sep. 2629 2007.
3. P. Ruther, S. Herwik, S. Kisban, K. Seidl, S. Spieth, B. Rubehn, N. Haj-Hosseini, J. Steigert, M. Daub, O. Paul, T. Stieglitz, R. Zengerle, and H.Neves, NeuroProbes development of modular multifunctional probearrays for neuroscience, in Proceedings MikroSystemTechnik Kongress2007, Berlin, Germany, Oct. 1517 2007, pp. 739741.
4. P. Ruther, A. Aarts, O. Frey, S. Herwik, S. Kisban, K. Seidl, S. Spieth, A.Schumacher, M. Koudelka-Hep, O. Paul, T. Stieglitz, R. Zengerle, and H.Neves, The NeuroProbes project multifunctional probe arrays for neuralrecording and stimulation, in Proceedings 13th Annual Conference of theIFESS, Sep. 2125 2008, Freiburg, Germany, ser. Biomedizinische Tech-nik / Biomedical Engineering, 2008, vol. 53, suppl. 1, pp. 238240.
5. S. Spieth, A. Schumacher, K. Seidl, K. Hiltmann, S. Haeberle, R. McNa-mara, J. W. Dalley, T. Holtzman, S. A. Edgley, P. Ruther, and R. Zengerle,Microprobe systems for neural recording and drug delivery, programno. 863.14, 2008 Neuroscience Meeting Planner, Society for NeuroscienceAnnual Meeting, Washington DC, USA, Nov. 1519 2008. [Online].
Publications xi
6. S. Spieth, A. Schumacher, K. Seidl, K. Hiltmann, S. Haeberle, R. Mc-Namara, J. W. Dalley, S. A. Edgley, P. Ruther, and R. Zengerle, Robustmicroprobe systems for simultaneous neural recording and drug delivery,in Proceedings 4th European Conference of the International Federationfor Medical and Biological Engineering (ECIFMBE), Nov. 2327 2008,Antwerp, Belgium, ser. IFMBE Proceedings, R. Magjarevic, J. Sloten, P.Verdonck, M. Nyssen, and J. Haueisen, Eds. Berlin, Germany: Springer,2009, vol. 22, pp. 24262430.
7. S. Spieth, A. Schumacher, S. van de Moosdijk, S. Haeberle, and R.Zengerle, Silicon microprobe systems for neural drug delivery: Exper-imental characterization of liquid distribution, in Proceedings WorldCongress on Medical Physics and Biomedical Engineering, Sep 7122009, Munich, Germany, ser. IFMBE Proceedings, R. Magjarevic, O. Ds-sel, and W. C. Schlegel, Eds. Berlin, Germany: Springer, 2009, vol. 25/9,pp. 158161.
8. S. Spieth, A. Schumacher, S. Haeberle, S. Messner, T. Holtzman, P. D.Rich, J. W. Dalley, and R. Zengerle, NeuroMedicator a novel drugdelivery system with microprobes for chronic neural research, programno. 664.9, 2009 Neuroscience Meeting Planner, Society for NeuroscienceAnnual Meeting, Chicago, IL, USA, Oct. 1721 2009. [Online].
9. F. Trenkle, S. Spieth, S. Kisban, K. Seidl, S. Taetzner, K. Hiltmann, S. Hae-berle, B. Verhoef, P. Janssen, P. Ruther, and R. Zengerle, Robust and MRIcompatible electro-fluidic microprobe systems used for behavioral neuro-science, program no. 664.11, 2009 Neuroscience Meeting Planner, Soci-ety for Neuroscience Annual Meeting, Chicago, IL, USA, Oct. 1721 2009.[Online].
10. S. Spieth, A. Schumacher, C. Kallenbach, S. Messner, and R. Zengerle,NeuroMedicator a disposable drug delivery system with silicon micro-probes for neural research, in Proceedings 23rd IEEE International Con-ference on Micro Electro Mechanical Systems (MEMS), Hong Kong, Jan.2428 2010, pp. 983986.
11. S. Spieth, S. Herrlich, A. Schumacher, S. Messner, and R. Zengerle, Drugdelivery microsystems for the treatment of neural diseases, in Proceedings7th International Conference on Wearable Micro and Nano Technologiesfor Personalized Health (pHealth), Berlin, Germany, May 2628 2010.
xii Publications
12. S. Spieth, A. Schumacher, S. Messner, T. Holtzman, P. D. Rich, J. W.Dalley, and R. Zengerle, A miniaturized on-demand drug delivery sys-tem for neural research, in Proceedings 6th International Conference onMicrotechnologies in Medicine and Biology (MMB), Luzern, Switzerland,May 46 2011, pp. 6263.
Table of Contents
Abstract iii
Zusammenfassung v
Preface vii
Publications ixJournals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ixConference Proceedings . . . . . . . . . . . . . . . . . . . . . . . . . . x
List of Symbols xvii
List of Abbreviations xxi
1 Introduction 11.1 Neuroscience and Neural Engineering . . . . . . . . . . . . . . . 11.2 Biology of the Nervous System . . . . . . . . . . . . . . . . . . . 21.3 Monitoring and Manipulation of Neurons . . . . . . . . . . . . . 41.4 Applications in Behavioral Neuroscience . . . . . . . . . . . . . . 6
1.4.1 The 5-Choice Serial Reaction Time Task . . . . . . . . . 61.4.2 Pharmacological Modification . . . . . . . . . . . . . . . 7
1.5 Multifunctional Neural Probes . . . . . . . . . . . . . . . . . . . 91.6 Aim and Structure of the Thesis . . . . . . . . . . . . . . . . . . 11
2 Design of the NeuroMedicator 132.1 Requirements to the Drug Delivery System . . . . . . . . . . . . 132.2 Fluidic Microprobes . . . . . . . . . . . . . . . . . . . . . . . . . 142.3 Drug Delivery and Microdosage Systems . . . . . . . . . . . . . 19
2.3.1 Neural Drug Delivery Systems State of the Art . . . . . 202.3.2 General Miniaturized Drug Delivery and Microdosage
Systems State of the Art . . . . . . . . . . . . . . . . . 252.4 Concept of the NeuroMedicator . . . . . . . . . . . . . . . . . . 29
xiv Table of Contents
2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3 Fundamentals 333.1 Microfluidic Flows . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.1.1 Continuity, Navier-Stokes, and Energy ConservationEquations . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.1.2 Hagen-Poiseuille Flow . . . . . . . . . . . . . . . . . . . 343.1.3 Reynolds Number . . . . . . . . . . . . . . . . . . . . . 353.1.4 Bernoulli Equation . . . . . . . . . . . . . . . . . . . . . 363.1.5 Microfluidic Networks . . . . . . . . . . . . . . . . . . . 36
3.2 Capillary Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.3 Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4 Fluidic Silicon Microprobes for Drug Delivery 454.1 Design and Layout of the Microprobes . . . . . . . . . . . . . . . 46
4.1.1 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . 484.1.2 Hydrodynamics . . . . . . . . . . . . . . . . . . . . . . . 514.1.3 Diffusion-Based Drug Leakage . . . . . . . . . . . . . . 52
4.2 Microprobe Fabrication . . . . . . . . . . . . . . . . . . . . . . . 544.2.1 Fabrication Process . . . . . . . . . . . . . . . . . . . . . 544.2.2 Fabricated Microprobes . . . . . . . . . . . . . . . . . . 584.2.3 Process Characteristics . . . . . . . . . . . . . . . . . . . 60
4.3 Microprobe Characterization . . . . . . . . . . . . . . . . . . . . 644.3.1 Mechanical Stability and Insertion Behavior . . . . . . . . 644.3.2 Microfluidic Properties . . . . . . . . . . . . . . . . . . . 644.3.3 Liquid Distribution . . . . . . . . . . . . . . . . . . . . . 674.3.4 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.4 Assembly for In Vivo Experiments . . . . . . . . . . . . . . . . . 704.4.1 Assembly and Packaging . . . . . . . . . . . . . . . . . . 714.4.2 Acute and Semi-Chronic Experiments . . . . . . . . . . . 74
4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
5 Actuator Unit 795.1 Expandable Microspheres as Actuators for Microfluidic Devices . 79
5.1.1 Integration Aspects of Expancel for Microfluidic Actu-ation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.1.2 Actuation Modes . . . . . . . . . . . . . . . . . . . . . . 90
Table of Contents xv
5.2 Microheater Array with Expancel-PDMS Composite . . . . . . 935.2.1 Substrate Material . . . . . . . . . . . . . . . . . . . . . 935.2.2 Fabrication Process . . . . . . . . . . . . . . . . . . . . . 995.2.3 Characterization . . . . . . . . . . . . . . . . . . . . . . 102
5.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
6 NeuroMedicator I 1076.1 Technical Implementation . . . . . . . . . . . . . . . . . . . . . 108
6.1.1 Diffusion-Based Drug Leakage . . . . . . . . . . . . . . 1096.1.2 Low-Leakage Optimized Microfluidic Design . . . . . . . 111
6.2 Fabrication and Assembly . . . . . . . . . . . . . . . . . . . . . 1146.2.1 Cover with Interconnected Reservoirs . . . . . . . . . . . 1146.2.2 Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . 116
6.3 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 1186.3.1 Filling with Liquid . . . . . . . . . . . . . . . . . . . . . 1186.3.2 Micropump . . . . . . . . . . . . . . . . . . . . . . . . . 1186.3.3 Storage Stability . . . . . . . . . . . . . . . . . . . . . . 1206.3.4 In Vivo Experiments . . . . . . . . . . . . . . . . . . . . 124
6.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
7 NeuroMedicator II 1277.1 Liquid Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
7.1.1 Thin Film Material . . . . . . . . . . . . . . . . . . . . . 1297.1.2 Reservoir Plate . . . . . . . . . . . . . . . . . . . . . . . 136
7.2 Technical Implementation . . . . . . . . . . . . . . . . . . . . . 1377.3 Fabrication and Assembly . . . . . . . . . . . . . . . . . . . . . 139
7.3.1 Insert with Liquid Reservoirs and Microprobe . . . . . . . 1397.3.2 Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . 1407.3.3 Electronic Control Unit . . . . . . . . . . . . . . . . . . . 141
7.4 In Vitro Characterization . . . . . . . . . . . . . . . . . . . . . . 1427.5 In Vivo Experiments . . . . . . . . . . . . . . . . . . . . . . . . 146
7.5.1 Device Preparation and Implantation . . . . . . . . . . . . 1467.5.2 Assessment of Storage Stability, Passive Leakage, and
Fluid Spread . . . . . . . . . . . . . . . . . . . . . . . . 1477.5.3 (R)-CPP Infusion in the 5-CSRTT . . . . . . . . . . . . . 148
7.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
8 General Conclusions and Outlook 153
xvi Table of Contents
Appendix 157A.1 Viscosity and Hardness of Selected Silicones . . . . . . . . . . . 157A.2 Thermophysical Properties of Selected Materials . . . . . . . . . 157A.3 Barrier, Absorption, and Mechanical Properties of Selected Poly-
mer Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
References 194
Acknowledgements 195
List of Symbols
Symbol Description Unit
a Thermal diffusivity m2/s
A Area m2
b Thermal effusivity W s0.5/(m2 K)
c, c Concentration, average concentration mol/m3
cp Specific heat capacity J/(kg K)
Cel Electric capacitance F
Chyd Hydraulic capacitance m3/Pa
d Diameter m
dhyd Hydraulic diameter m
d0.5 Volume median diameter m
D Diffusion coefficient m2/s
Dp Plate flexural rigidity Pa m3
E Youngs modulus Pa
f Force per unit volume N/m3
g Gravitational acceleration m/s2
h Height, depth, thickness m
I Electric current A
Ihyd Hydraulic inertance Pa s2/m3
j Diffusion flux mol/(m2 s)
J Molar flow rate mol/s
K Permeability coefficient (osmosis) m/(Pa s)
l Length m
L Characteristic length m
m Mass kg
xviii List of Symbols
m Mass flow rate m3/s
M Molar mass g/mol
N Number, particles
p Pressure Pa
P Perimeter m
Pel Electric power W
Po Poiseuille number
q, q Volumetric flow rate,average volumetric flow rate
m3/s
qh Heat flux W/m2
Q Electric charge C
Qh Heat J
Qh Heat transfer rate W
r Radius m
r, , x Cylindrical coordinates m, , mR Universal gas constant J/(mol K)
R2 Coefficient of determination
Rel Electric resistance Rhyd Hydraulic resistance Pa s/m3
Rm Tensile strength Pa
Re Reynolds number
t Time s
T Temperature CTg Glass transition temperature CTm Melting temperature CU Voltage V
v, v Velocity, average velocity m/s
V Volume m3
w Width m
wd Deflection m
wi Weight percent of component i wt%
x, y, z Cartesian coordinates m, m, m
List of Symbols xix
Some ratio Some ratio Surface tension N/m2
max Elongation at break % Dynamic viscosity Pa s[ ] Intrinsic viscosity Contact angle Thermal conductivity W/(m K) Poissons ratio Osmotic pressure Pa Density kg/m3
Mechanical stress Pao Osmotic reflection coefficient w, w Wall shear stress,
average wall shear stressPa
Volumetric packing factor %
List of Abbreviations
Abbreviation Description
Al Aluminum
Al2O3, AlxOy Aluminum oxide / sapphire(stoichiometric, non-stoichiometric)
Ar Argon
ASTM American Society for Testing and Materials, WestConshohocken, PA, USA
Au Gold
BBB Blood-brain barrier
bECF Brain extracellular fluid
BeO Beryllium oxide
BSA Bovine serum albumin
CaCl2 Calcium chloride
CAS Chemical Abstracts Service, Columbus, OH, USA
CED Convection-enhanced delivery
CFD Computational fluid dynamics
Cg3 Prelimbic cortex
CH Cellulose hydrate (Cellophane)
ChoOx Choline oxidase
CMOS Complementary metal oxide semiconductor
CNS Central nervous system
COC Cycloolefin copolymer
COP Cycloolefin polymer
Cr Chrome
CSF Cerebrospinal fluid
Cu Copper
xxii List of Abbreviations
CVD Chemical vapor deposition
DDS Drug delivery system
DIN Deutsche Institut fr Normung e.V., Berlin, Germany
DRIE Deep reactive ion etching
EAP Electroactive polymer
ECS Extracellular space
ECTFE Ethylene-chlorotrifluoroethylene copolymer
EDP Ethylenediamine pyrocatechol
EN European standard
EPFL cole Polytechnique Fdrale de Lausanne, Switzerland
ETFE Ethylene-tetrafluoroethylene copolymer
EVA Ethylene vinylacetate
EVOH Ethylene-vinyl alcohol copolymer
FDA U.S. Food and Drug Administration, Rockville, MD, USA
FEM Finite element method
FP6 6th Framework Program of the European Commission
FPCB Flexible printed circuit board
FR-4 Flame retardant glass reinforced epoxy
GABA gamma-aminobutyric acid
GUI Graphical user interface
Hg Mercury
HSG-IMIT Institut fr Mikro- und Informationstechnik der Hahn-Schickard-Gesellschaft fr angewandte Forschung e.V.,Villingen-Schwenningen, Germany
H2O Water
H2O2 Hydrogen peroxide
IC Integrated circuit
ICP Inductively coupled plasma
ICP Intracranial pressure
ID Inner diameter
IL Infralimbic cortex
IMEC Interuniversity Microelectronics Center, Leuven, Belgium
List of Abbreviations xxiii
IMTEK Department of Microsystems Engineering, University ofFreiburg, Germany
IPA Isopropanol
IR Infrared
ISO International Organization for Standardization, Geneva,Switzerland
IST Information Society Technologies
KOH Potassium hydroxide
KUL Katholieke Universiteit Leuven, Belgium
LCP Liquid-crystal polymer
LED Light-emitting diode
LFP Local field potentials
LPCVD Low pressure chemical vapor deposition
LTO Low temperature oxide
MEMS Microelectromechanical systems
mPFC Medial prefrontal cortex
MRI Magnetic resonance imaging
MRI Micro-magnetic resonance imaging
NaCl Sodium chloride
Ni Nickel
NiAu Nickel gold
NiFe Invar
NIH National Institutes of Health, Bethesda, MD, USA
NMDA N-methyl-D-aspartate
O2 Molecular oxygen
OD Outer diameter
oPFC Orbital prefrontal cortex
OTR Oxygen transmission rate
Pa Palladium
PaCo Palladium cobalt
PA6 Polyamide 6
PAN Polyacrylonitrile
xxiv List of Abbreviations
PC Polycarbonate
PCB Printed circuit board
PCTFE Polychlorotrifluoroethylene
PDMS Polydimethylsiloxane
PE Polyethylene
PE-HD High-density polyethylene
PE-LD Low-density polyethylene
PECVD Plasma enhanced chemical vapor deposition
PEEK Polyetheretherketone
PEN Poly(ethylene naphtalate)
PET Poly(ethylene terephthalate)
PET Positron emission tomography
PFC Prefrontal cortex
PI Polyimide
PLA Polylactic acid
PMMA Poly(methyl methacrylate)
PNS Peripheral nervous system
Poly-Si Polysilicon, polycrystalline silicon
POM Polyoxymethylene
PP Polypropylene
PS Polystyrene
PSB Pontamine sky blue
PSG Phosphosilicate glass
PSU Polysulfone
Pt Platinum
PTFE Polytetrafluoroethylene
PVAL Poly(vinyl alcohol)
PVC-P Poly(vinyl chloride), plasticized
PVC-U Poly(vinyl chloride), unplasticized (hard)
PVD Physical vapor deposition
PVDC Poly(vinylidene chloride)
List of Abbreviations xxv
PVDF Poly(vinylidene fluoride)
PVF Poly(vinyl fluoride)
Py, PyC Parylene, parylene-C
RCA Radio Corporation of America
(R)-CCP 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid
RIE Reactive ion etching
RSC Cleanroom service center at IMTEK
RTV Room temperature vulcanizing
SEM Scanning electron microscope
SF6 Sulfur hexaflouride
SfN Society for Neuroscience, Washington, DC, USA
Si Silicon
SiC Silicon carbide
Si3N4, SixNy Silicon nitride (stoichiometric, non-stoichiometric)
SiO2, SiOx Silicon oxide (stoichiometric, non-stoichiometric)
SMA Shape memory alloy
SMP Shape memory polymer
SOI Silicon on insulator
STP Standard temperature and pressure
Ti Titanium
TiO2 Titanium dioxide
TiW Titanium tungsten
TMAH Tetramethylammonium hydroxide
TMS Transcranial magnetic stimulation
TPA Polyamide thermoplastic elastomer
TPC Copolyester thermoplastic elastomer
TPE Thermoplastic elastomer
TPO Olefinic thermoplastic elastomer
TPS Styrenic thermoplastic elastomer
TPS-SBS Styrene-butadiene-styrene block copolymer
TPS-SEBS Styrene-ethylene-butylene-styrene block copolymer
xxvi List of Abbreviations
TPU Urethane thermoplastic elastomer
TPV Dynamically vulcanized thermoplastic elastomer
UCam University of Cambridge, United Kingdom
UK United Kingdom
USB Universal serial bus
WA Water absorption
WVTR Water vapor transmission rate
XeF2 Xenon diflouride
1D, 2D, 3D One-, two-, three-dimensional
5-CSRTT 5-choice serial reaction time task
1 Introduction
1.1 Neuroscience and Neural Engineering
Understanding the human mind in biological terms is often considered to be thecentral challenge for science in the 21st century [1]. The term neuroscience, in-troduced in the mid 1960s to signal a new era of interdisciplinary cooperation [2],reflects all research activities related to the brain, or more general the nervous sys-tem. Today, neuroscience is one of the fastest growing fields of research with theSociety for Neuroscience (SfN) having more than 42000 members in 2012 [3].
Beside the ultimate goal to understand the biological basis of consciousness, thetreatment of diseases related to the nervous system has become increasingly im-portant. Today, more than 1000 disorders of the nervous system are known. Thisincludes childhood, psychiatric, and degenerative disorders such as Alzheimersand Parkinsons disease as well as addiction and neural injuries [4,5]. From eco-nomic perspective, brain related diseases are considered to be the largest unmetmedical market affecting approximately 2 billion people worldwide and incur-ring costs of US$ 2 trillion in 2010 [6]. With respect to Europe, studies estimate179 million people to be affected by prevalent neurological diseases resulting ineconomic costs of C 798 billion in 2010 [7].
In addition, prosthetic approaches to restore parts of the nervous system havegained increasing significance. Today, more than 200000 cochlear implantsworldwide allow profoundly deaf people to hear again by electrical stimulationof the auditory nerve directly in the cochlea, the auditory portion of the innerear [8]. Beyond that, there are first successes in applying microelectromechanicalsystems (MEMS) to interface with the human neural system. For instance, retinaimplants inside the eye demonstrated partial regain of vision by conversion ofvisual impulses into electrical stimulation of the retinal nerve cells [9]. Further-more, micromachined electrode arrays implanted directly into the brain enabledthe acquisition of neural signals thereby proving the feasibility of bypassingdamaged parts of the nervous system in order to operate motor prostheses [10].
The increasing demand for technological neural devices addressing investigative
2 1 Introduction
as well as therapeutic and prosthetic needs resulted in the formation of an engi-neering discipline within neuroscience. Neural engineering is a relatively newdiscipline which coalesces engineering techniques with neuroscience to under-stand, repair, replace, enhance, or treat diseases of neural systems [11,12]. Neu-rotechnology is one of the fastest emerging sectors growing from 2005 to 2010by annual rates of 2530 % and being expected to increase further by 1020 %during the coming years [6].
1.2 Biology of the Nervous System
The nervous system is a network of specialized cells which monitors and controlsalmost every organ in the body. It consists of two main parts, the central nervoussystem (CNS) and the peripheral nervous system (PNS) [13]. The CNS representsthe majority of the nervous system and includes the brain and the spinal cord. Itis connected to organs and limbs by the PNS.
The brain can be subdivided into cell tissue and extra cellular space (ECS) sur-rounding the cells, both representing a certain volume fraction. Thereby, the ECSis filled with brain extracellular fluid (bECF) containing ions and other substancesaffecting the cell functions [14]. Consequently, from the engineering point ofview, the brain can be considered as a liquid filled poroelastic medium with aporosity of about 20 % [15,16].
Neurons (nerve cells) are the basic working units of the nervous system and aredesigned to transmit information by bioelectrical signals. Together with the sup-porting glial cells, these two cell types are the building blocks of the nervoussystem. The human brain consists of around 100 billion neurons and ten timesas many glial cells [13,14]. An illustration of two connected neurons is shown inFig. 1.1 and explained in the following according to [2,13,14].
Central part of a neuron is the cell body, called soma, which contains the nucleus.The soma hast two different types of cellular extensions which receive and trans-mit signals from and to other neurons: dendrites and the axon. Beside the somaitself, the dendrites receive signals from other neurons and conduct them to thesoma. The other way around, signals are transmitted by the electrically excitableaxon. The axon leaves the soma at the axon hillock and often undergoes extensivebranching before ending at the axon terminals. At these terminals, the axon isconnected to other neurons or cells for information transfer. An individual con-tact point is called a synapse. In a synapse, information is transferred from the
1.2 Biology of the Nervous System 3
Presynaptic cell
Dendrites
Soma Nucleus
Axon hillock
Axon
Myelin sheath
Axon terminals
Presynaptic axon terminal
Postsynaptic cell
Synaptic cleft
Neurotransmitter
Synapse
Postsynaptic dendrite
Receptors
Signal
Signal
Node of Ranvier
Schwanncell
Figure 1.1: Illustration of two connected neurons according to [14] with a close-up viewof the synapse, drawings adapted from [17].
Table 1.1: Dimensions and numbers of neuronal cellular components [14]
Soma 50 m and more in diameterAxon 0.220 m in diameter, 0.1 mm to more than 3 m in lengthSynaptic cleft Distance of 2040 nm for chemical synapsesNumber of contacts Large dendrite trees receive about 150000 contacts
sending presynaptic axon terminal to the receiving postsynaptic dendrite across anarrow space, the synaptic cleft.
One possibility to classify neurons is to group them according to their functioninto sensory, motor, and interneuronal neurons [13]. Sensory neurons transmitsensory information from the body to the nervous system, whereas motor neuronstransmit signals from the nervous system to muscles and glands. Interneuronsform the majority of neurons and transmit either signals over long distances orwithin local circuits. Typical dimensions and numbers of the neuronal cellularcomponents are listed in Table 1.1.
At rest, neurons maintain an electrical potential difference across their externalmembrane [14]. By convention, this membrane potential is measured with the
4 1 Introduction
extracellular space as reference. The resting membrane potential of most neuronsranges from 40 to 80 mV, for muscle cells sometimes to 90 mV. An actionpotential is a depolarizing pulse of 1 to 10 ms duration and an amplitude of 70to 110 mV. Hence, the membrane potential switches from negative to positiveduring an action potential. An electrochemical information transfer occurs in thefollowing steps:
1. The internal charge of the presynaptic neuron switches from negative topositive.
2. The generated action potential travels along the axon toward the presynapticaxon terminal.
3. Upon achieving the terminal, the release of neurotransmitters into thesynaptic cleft is triggered.
4. The neurotransmitters diffuse across the synaptic cleft and bind to the re-ceptors of the postsynaptic dendrite.
5. Once the neurotransmitters are in place, the membrane potential of the cellis changed and an action is initiated. This can be the generation of a furtheraction potential, a muscle contraction, an inhibition of neurotransmitter re-lease, etc. In case of a further action potential, the sequence starts again atthe first step.
Due to their distinct colors, the outer region of the CNS containing the cell bodiesis named gray matter, whereas the inner region composed of glial cells and axonsis called white matter. The CNS does not come in direct contact with the skull butis protected by three membranes commonly referred to as the meninges. Thereby,the soft and thin pia mater adheres closest to the brain followed by the web-likearachnoid membrane and the leather-like dura mater. Around the CNS, as wellas in the subarachnoid space between the pia mater and the arachnoid membrane,cerebrospinal fluid (CSF) is circulating. This fluid layer around the brain and thespinal cord serves as a shock absorber to protect the CNS from injury. In addition,it removes metabolic waste products. The CSF and consequently the whole CNSis separated from the circulating blood in the body by the blood-brain barrier(BBB). The BBB is a network of thin capillaries with special membranes whichprevents that undesirable substances from the blood enter into the CSF.
1.3 Monitoring and Manipulation of Neurons
As described in the previous section, electrical impulses and chemical substancesare used to transmit signals in the nervous system. To get a better understand-
1.3 Monitoring and Manipulation of Neurons 5
ing for the mechanisms and underlying processes in the brain, it is necessary tomonitor and possibly also to modify these transmission pathways on the cellu-lar level. The main experimental approaches for reversible, i.e. non-permanent,modification are briefly summarized in the following.
Electrical Approaches
Since the resting as well as the action potential of a neuron is a measurable volt-age, it is possible to record signals from individual neurons [2,13,14]. However,taking into account the dimensions of the structures listed in Table 1.1, micro-probes with tiniest recording electrodes are required which have to be placed nextto the cells of interest [18,19]. This is complicated by the fact that the relevantneural structures are often not located directly on the brain surface. Apart fromrecording, electrodes can also be used to introduce electrical signals into the neuralsystem, i.e. to stimulate the neural tissue. However, electrical stimulation cannotbe limited to individual neurons and covers a fairly large area.
Chemical Approaches
Compared to electrical signals, the localized monitoring and modification ofchemical concentrations in the bECF is more complicated. Whereas electricalsignals can be directly recorded and processed, chemical properties have to bemonitored by applying voltammetry [20] or have to be first converted into electri-cal signals by microfabricated biosensors [21,22]. Alternatively, chemical com-pounds such as neurotransmitters can be extracted, e.g. by microdialysis [23,24],followed by external analysis or marked with radiotracers for positron emissiontomography (PET) [25]. Selective modification of the brain chemistry requireslocal pharmacological intervention as the administration over the bloodstreamdoes not allow to localize drugs in specific areas and is restricted to drugs beingable to pass the BBB. Depending on the administered drugs, chemical stimulationas well as inactivation can be achieved. However, it has to be taken into accountthat the administered drug starts immediately to diffuse in the bECF.
Other Approaches
Alternative research methods to temporarily alter neural activity include transcra-nial magnetic stimulation (TMS) inducing weak electric currents in the brain orcryogenic techniques using a probe to locally deactivate neural tissue by cool-ing [26]. The most recent approach becoming increasingly popular applies op-togenetics, i.e. the combination of optical and genetic methods [27,28]. Opto-genetics includes (a) the genetic transfection of selected neural cells with a virus
6 1 Introduction
making them light-sensitive and (b) the control of the modified cells by light en-abling to switch them on and off.
1.4 Applications in Behavioral Neuroscience 1
A major goal of neuroscience is to understand the fundamental cognitive pro-cesses in the brain such as learning, memory, and attention. Elucidation of theneurobiological basis of behavior is a mandatory first step to reach this goal. Thereare two general research methods applied in behavioral neuroscience with ani-mals: (i) recording of selected electrophysiological or neurochemical parametersduring behavior and (ii) modification of the nervous system to alter behavior dur-ing an experiment. Amongst the methods for altering behavior during an exper-iment, electrical stimulation is non-specific and only increases neuronal activity.On the other hand, application of optogenetics enables the control of specific cells.However, optogenetics in behaving animals is still an evolving approach. There-fore, intra-cerebral pharmacological intervention to manipulate cells representsthe present state of the art in behavioral neuroscience. Furthermore, pharmacol-ogy forms the basis for future therapeutic applications.
1.4.1 The 5-Choice Serial Reaction Time Task
The majority of behavioral studies are performed in freely-moving rodents such asrats and mice. One example is the 5-choice serial reaction time task (5-CSRTT),a behavioral test of visual attention and impulsivity with rats [31]. A detaileddescription of the 5-CSRTT apparatus illustrated in Fig. 1.2 is provided in [32].The test is performed in a box of 252525 cm3 into which the rat is placed.One side of the box is designed as a curved wall with five equally spaced apertures
1Abridged versions of this section have been published as part of the original journal articles
[29] S. Spieth, A. Schumacher, C. Kallenbach, S. Messner, and R. Zengerle, The NeuroMedica-tor a micropump integrated with silicon microprobes for drug delivery in neural research,Journal of Micromechanics and Microengineering, vol. 22, no. 6, 065020 (11pp), 2012.
[30] S. Spieth, A. Schumacher, T. Holtzman, P. D. Rich, D. E. Theobald, J. W. Dalley, R. Nouna,S. Messner, and R. Zengerle, An intra-cerebral drug delivery system for freely movinganimals, Biomedical Microdevices, vol. 14, no. 5, pp. 799809, 2012.
Related author contributions included in this section:
S. Spieth (HSG-IMIT): literature search, manuscript preparation T. Holtzman, J. W. Dalley (UCam): scientific advice on behavioral neuroscience and pharma-
cological modification
1.4 Applications in Behavioral Neuroscience 7
5 apertures with
LEDs and
IR light barriers
Light stimulus
Food magazine
with IR light barrier
Trained rat
Curved wallHouse light
Box
Figure 1.2: Illustration of the 5-CSRTT apparatus according to [32].
of 1.51.5 cm2, whereas the opposite side incorporates a food magazine. Eachaperture as well as the food magazine is equipped with a light-emitting diode(LED) and an infrared (IR) light barrier to detect nose pokes of the rat. The testis started after the rat enters the food magazine for the first time. During the test,brief light stimuli are generated on a pseudorandom basis inside the apertures.The rats are trained to answer such a light stimulus with a nose poke into thecorresponding aperture. If the response is correct, a reward in the form of a foodpellet is provided by the magazine and the next light stimulus is initiated. In thecase of incorrect or omitted responses, no pellet is provided and the house lightis extinguished for a 5 s time-out period. During the test, impulsive responsesare defined as responses made before the onset of the light stimulus resultingimmediately in a time-out period.
1.4.2 Pharmacological Modification
Typically, invasive pharmacological manipulation in the research area employs ei-ther microiontophoresis (microelectrophoresis) or pressure-driven liquid infusion(pressure ejection) [33,34]. Comparing the two approaches, microiontophoresisallows to inject charged compounds from a solution by passing currents. Conse-quently, this method is limited to drugs that can be electrically polarized and is notsuitable for drugs with low charge-to-mass ratios. Additionally, the effective drugflow cannot be calculated from the applied current and is influenced by severaladditional factors, e.g. geometry and material of the micropipette, solvent, andthe medium into which it is iontophoresed [34]. This makes it impossible to de-termine accurately the amount of drug injected. A state-of-the-art hot-pulled glasspipette used for microiontophoresis is shown in Fig. 1.3a. Although the probe isvery narrow at the tip, the probe diameters widens rapidly along the shaft.
Pressure-driven liquid infusion systems are more commonly used and are inde-
8 1 Introduction
10 mm(a) (b)
Implantabledouble guide
Protectioncap
Capillaries
5 mm
10 m
Figure 1.3: Commercially available probes for pharmacological modification: (a) hot-pulled multi-barrel glass pipette having a central carbon fiber electrode (Carbostar-3, im-ages courtesy of Denes Budai, Ph.D., Kation Scientific, LLC, Minneapolis, MN, USA);(b) stainless steel cannula system for small rodents (C232G, Plastics One Inc., Roanoke,VA, USA).
pendent of the charge states of the drug molecules. However, diffusion-basedleakage of drug from the probe, channel blockages, and physical displacementof the surrounding tissue during infusion can be disadvantageous [33]. Depend-ing on the infusion rate, low-flow (L/h) and high-flow (L/min) infusions canbe further distinguished [35]. Whereas diffusion-driven drug transport domi-nates in low-flow regime and limits the speed and area of the drug distribution,in high-flow regime convection of the drug in the tissue is promoted [3537]. Theconvection-enhanced delivery (CED) method was pioneered by researchers fromthe National Institutes of Health (NIH) in the United States and is especially bene-ficial for larger drug molecules for which diffusion-driven distribution is too slow.Depending on the application and area of interest, catheters of rigid or flexibletubing or microprobes are used to administer the liquid drug. In the non-humanresearch area, microprobes fabricated from stainless steel or glass capillaries arecommonly used. A conventional stainless steel cannula system for small rodentsis exemplarily shown in Fig. 1.3b. The system consists of a double guide which isimplanted onto the skull, capillaries which are inserted for liquid infusion, and aprotection cap.
Typically, implanted microprobes require external connections, e.g. electrical orfluidic connections, and it is important that these interfaces do not interfere withthe behavior of the animal during the experiment. In the case of electrophysiol-ogy, electrodes can be easily connected to pluggable electrical micro connectors.Hence, this issue has been optimally solved using miniature wireless transmitters(headstages) mounted on the animal just during the experiment [3840].
1.5 Multifunctional Neural Probes 9
Pharmacological modification in awake, freely-moving animals is technicallymore demanding and often constrained by the need for tethered infusion systemswhich restrict behavioral output. The reason is the increased complexity of liquidhandling compared to the electrical access to neural signals. There is no pluggableand self-sealing microfluidic connector compatible with volume deliveries on theorder of submicroliters and the wireless transmission of liquid is not possible.In addition, a pluggable connection can introduce contaminations into the fluidicsystem, possibly causing infections. Therefore, a permanent fluidic connectionof the microprobes either to a stationary macroscopic pump or a miniaturizeddrug delivery system (DDS) which can be placed directly next to the probes isrequired. Comparing both approaches, a miniature DDS not constraining theanimal is preferable. However, particular constraints are placed by the size andthe weight that can be carried by a small animal.
1.5 Multifunctional Neural Probes 2
Wire electrodes and electrolyte-filled micropipettes for recording and stimulationof electrical signals [19,42] as well as glass or steel capillaries used to deliverpharmaceutical substances directly into the brain [43] are established tools in neu-roscience research. However, the rapid development of MEMS made from siliconwithin the last decades has opened completely new technological perspectiveswith respect to system integration and multifunctionality [42]. Following the pio-neering work of Wise et al. in 1970 [44], a wide variety of microfabricated probeshave been developed in the past.
Depending on the number and geometrical arrangement of the probe shafts, one-dimensional (1D) single-shaft probes, two-dimensional (2D) probe combs withmultiple shafts, and fully three-dimensional (3D) microprobe arrays can be dis-tinguished. Additionally, acute and chronic probes have to be differentiated.Whereas acute probes are only operated during a neurological surgery and are typ-
2A version of this section has been published as part of the original journal article
[41] S. Spieth, O. Brett, K. Seidl, A. A. A. Aarts, M. A. Erismis, S. Herwik, F. Trenkle,S. Ttzner, J. Auber, M. Daub, H. P. Neves, R. Puers, O. Paul, P. Ruther, and R. Zengerle,A floating 3D silicon microprobe array for neural drug delivery compatible with electri-cal recording, Journal of Micromechanics and Microengineering, vol. 21, no. 12, 125001(16pp), 2011.
Compared to the published version, additional information is provided. Related author contribu-tions included in this section:
S. Spieth (HSG-IMIT): literature search, manuscript preparation
10 1 Introduction
5 mm
(a)
2 mm
(b)
Figure 1.4: Out-of-plane and in-plane fabrication: (a) scanning electron microscope(SEM) micrograph of the out-of-plane fabricated 3D Utah microprobe array [46],reprinted with permission from Elsevier; (b) multiple in-plane 2D microprobe combs fab-ricated on a wafer (NeuroProbes project).
ically fixed to laboratory equipment, chronic probes are implanted over a longerperiod of time. In the latter, the probes are often required to be mechanicallydecoupled from the skull resulting in floating assemblies.
Restricted by the general concept of micromachining thin silicon wafers and thesubtractive nature of the available shaping processes, two fabrication technologiescan be distinguished: out-of-plane and in-plane fabrication sequences.
In the case of out-of-plane fabrication, microprobe shafts oriented perpendicularlyto the wafer plane are fabricated by removing excess bulk material. Consequently,the length of the probe shafts cannot exceed the wafer thickness. Among the out-of-plane probe arrays, the so-called Utah array [45] shown in Fig. 1.4a is certainlythe most popular structure meanwhile commercialized by Blackrock Microsys-tems, Salt Lake City, UT, USA. Whereas the shafts of the commercially availablearray variant are currently restricted to a length of 1.5 mm [47], shaft lengths ofup to 9 mm have been demonstrated by applying alternative machining technolo-gies [48]. In general, the array shafts comprise one tip electrode per shaft onlyand have no further functionalities such as drug delivery.
Alternatively, 1D probes and 2D probe combs can be fabricated in the wafer planeusing in-plane fabrication technologies, as exemplarily shown in Fig. 1.4b. Thisenables shafts with lengths of several millimeters combined with multiple elec-trodes for electrophysiological recording [49,50] or stimulation [51] as well asmonolithic integration of electronic circuits [52], fluidic microchannels [53], andbiosensors [54]. Particularly well known are the so-called Michigan probes whichare commercially available through NeuroNexus Technologies Inc., Ann Arbor,MI, USA, since 2012 part of Greatbach, Inc., Clarence, NY, USA.
1.6 Aim and Structure of the Thesis 11
1.6 Aim and Structure of the Thesis
The scope of this thesis is the development of the miniaturized drug delivery sys-tem NeuroMedicator for behavioral experiments with small animals which offers(i) an implantable multifunctional microprobe, (ii) multiple independently trig-gered drug infusions into both brain hemispheres, and (iii) a power consumptionlow enough to be covered by state-of-the-art wireless headstages.
In detail, the thesis is structured as follows:
Chapter 2 derives the concept and design of the NeuroMedicator. On the ba-sis of the current state of the art of fluidic microprobes compatible with siliconprocessing as well as drug delivery systems for liquids, a system concept combin-ing a micropump with an application-specific silicon microprobe for infusion isdeveloped.
Chapter 3 provides the necessary theoretical background of the physical effectsrelevant to the development of the NeuroMedicator. Thereby, the focus is set onthe description of microfluidic flows, capillary effects, and diffusion-based speciestransport.
Chapter 4 describes the design, layout, fabrication, and characterization of thefluidic silicon microprobes. With respect to design and layout, especially the hy-draulic resistance and the diffusion-based drug leakage after implantation are con-sidered. After description of the fabrication process, the general properties of themicroprobes are first characterized followed by the assembly of complete micro-probe systems for in vivo experiments.
Chapter 5 develops the actuator unit of the NeuroMedicator used to infuse thedrug liquid. After a general thermodynamic description of the thermally expand-able microspheres used for actuation, integration aspects are discussed and a suit-able approach is selected. To realize thermal expansion, a suitable microheaterarray is developed and evaluated. This is followed by fabrication and characteri-zation of the complete actuator unit.
Chapter 6 presents the NeuroMedicator I, the first prototype of the NeuroMedi-cator. After detailing the fabrication and assembly sequences, the general charac-teristics of the device are evaluated and first in vivo tests are conducted.
Chapter 7 presents the NeuroMedicator II, the second prototype of the Neu-roMedicator addressing critical aspects derived from the first implementation.Following the evaluation of appropriate materials for the device, the fabricationand assembly steps are described. Finally, the NeuroMedicator II is assessed in
12 1 Introduction
vitro as well as in vivo and evaluated as a tool for pharmacological manipulationof behavior.
Chapter 8 summarizes the thesis and provides a general outlook.
2 Design of the NeuroMedicator
In this chapter, the concept of the miniaturized intra-cerebral drug delivery sys-tem (DDS) NeuroMedicator for freely moving animals in neural research is de-rived. The NeuroMedicator combines a metering micropump with an application-specific fluidic microprobe for infusions into the neural tissue. The basic require-ments of the microprobe and the drug delivery mechanism are first determinedfrom the application point of view. Restricting to fluidic microprobes compatiblewith silicon processing, the state of the art of microprobe in-plane fabrication tech-nologies is reviewed and a suitable fabrication approach is motivated. Similarly,an overview of the state of the art of neural DDS as well as general miniaturizeddrug delivery and microdosage systems is provided. On this basis, the special mi-cropumping and metering concept of the NeuroMedicator is developed. Finally,the overall system concept of the NeuroMedicator is disclosed.
2.1 Requirements to the Drug Delivery System
In general, a drug delivery system (DDS) is defined as a formulation or a devicethat enables the introduction of a therapeutic substance in the body and improvesits efficacy and safety by controlling the rate, time, and place of release of drugsin the body [55]. As general as this definition of a DDS is, as versatile are itsapplications, administration routes, and technical implementations. Furthermore,the term drug delivery system limits not only to the drug release mechanism butincludes also the required administration interfaces.
Considering the boundary conditions described in Sect. 1.4, the ideal DDS for be-havioral studies with rats should be small in size to allow skull-mounting, enableon-demand drug delivery from multifunctional microprobes alongside electro-physiological recording, and be controllable by a wireless headstage connectedonly during the trials. Consequently, the two basic components of such a DDSare (i) a fluidic microprobe with electrodes and (ii) a miniaturized on-demandmicrodosage system for liquids. The microdosage system must be able to storeand release a specified number of defined liquid volumes and should not alter
14 2 Design of the NeuroMedicator
Table 2.1: Basic specifications of the neural DDS
(a) 2D fluidic microprobe comb dimensionsNumber of shafts 2Shaft length 6.5 mmShaft distance 1.5 mm
(b) Typical infusion parameters (per hemisphere)Drugs 0.15 mM muscimol, 1.5 mM baclofen+Liquid 0.01 M phosphate buffer in 0.9 % salineInfusion rate 0.2 L/minInfusion volume 0.5 LFrequency max. 8 infusions over 21 days
the drug during operation. In addition, a disposable DDS is desirable to preventcross-contamination between experiments.
In order to enable parallel infusions into both hemispheres of the rat brain, a comb-like microprobe having two shafts is required. In the present case, the medialand orbital prefrontal cortices (mPFC, oPFC) located at depths of 4.55.5 mmare primarily targeted and the distance between the two hemispheres is 1.5 mm.During implantation, the probe shafts are inserted through holes which are drilledinto the skull. Taking into account the thickness of the pia mater, the subarachnoidspace, the arachnoid membrane, the dura mater, and the skull (cf. Sect. 1.2), thisresults in the basic 2D microprobe comb dimensions listed in Table 2.1a.
One possible application of the NeuroMedicator is the temporary, localized inac-tivation of brain regions. For this kind of experiments, the receptors for the chiefinhibitory neurotransmitter gamma-aminobutyric acid (GABA) can be activatedwith the GABAA and GABAB receptor agonists muscimol (CAS 2763-96-4) andbaclofen+ (CAS 1134-47-0), respectively. Typical infusion parameters presentlyapplied with stationary syringe pumps are summarized in Table 2.1b.
2.2 Fluidic Microprobes
In order to prevent damage to the brain, the microprobe shafts have to be ex-tremely thin but also long and mechanically stiff enough to penetrate into the brainregion of interest enabling a targeted and localized drug deposition. In addition,multifunctional microprobes allowing in parallel the recording from electrodes orbiosensors are desirable. Comparing in- and out-of-plane fabrication technologiesas introduced in Sect. 1.5, out-of-plane technologies result in rather short probes
2.2 Fluidic Microprobes 15
(a) (b) (c)
Sacrificial core
Hollow structure
Etched cavity
Closed trenches
Trenches Patterned channel
Bonded cover
Figure 2.1: Microfluidic channel fabrication: (a) surface micromachined channel, (b) bur-ied channel, and (c) bonded channel.
of only a few hundred micrometers in shaft length with few exceptions [45]. Suchlengths are especially useful for transdermal drug delivery. Therefore, out-of-plane probes have been extensively evaluated for this purpose [56,57]. In order toenable probe lengths of up to 10 mm, the NeuroProbes system approach requiresin-plane fabrication technologies. In addition, to ensure material compatibilityamong the different functionalities to be implemented in the project (electrodes,biosensors, complementary metal oxide semiconductors (CMOS), and microflu-idics) as well as compatibility with the 3D platform integration approach [5860],silicon (Si) was selected as the standard probe material. Restricting to in-planefabrication based on silicon, three main concepts to realize microfluidic channelstructures exist as illustrated in Fig. 2.1 and described in the following: surfacemicromachined channels, buried channels, and bonded channels.
The individual channel properties are further determined by the applied fabri-cation technologies. Table 2.2 provides an overview of the state of the art ofneedle-like in-plane probes with microfluidic channels. Beyond the applied mate-rials and technologies, the shape of the channel cross section, the incorporation ofelectrodes, and the targeted application are compared. Depending on the combina-tion of microchannel concept and fabrication technology, the microfluidic designis more or less restricted. Targeting for high flexibility in the microfluidic design,the design flexibility with respect to the channel layout and geometry is evaluatedfor each approach.
Surface Micromachined Channels
Surface micromachined channels (Fig. 2.1a) are based on the deposition and etch-ing of thin films (surface micromachining). To achieve channel structures, sac-
16 2 Design of the NeuroMedicator
Table2.2:Fabrication
approachesofin-plane
fluidicprobes
andevaluation
ofthem
icrofluidicdesign
flexibility
SubstrateC
hannelcrosssection
Channel
Channel
Electrodes
Application
Ref.
Flexibilityofchannel
determined
byshape
sealinglayout/geom
etrya
(a)Surfacem
icromachined
channels:Si
PatternedPSG
,SiO2
Rectangular
Six Ny
PreparedH
ypodermic/N
eural[61]
+/
SiPatterned
SiOx
TrapezoidalSix N
y ,SiOx
No
Neural
[62]+
/Si
Patternedphotoresist
Rectangular
PyCN
oN
eural[63]
+/
SOI
PatternedSiO
2Trapezoidal
Poly-SiN
oM
icrodialysis[64]
+/
Si bPatterned
photoresistR
ectangularPa/PaC
o/Ni
No
Hypoderm
ic[65,66]
+/
Si bO
2R
IEofPyC
,photoresistR
ectangularPyC
Yes
Neural
[67,68]+
/Si b
Patternedphotoresist
Rectangular
PyCY
esN
eural[69]
+/
Si bPatterned
photoresistR
ectangularPyC
No
Neural
[70]+
/(b)B
uriedchannels:
SiE
DP
wetetching
TrapezoidalSiO
2 ,Si3 N4
Yes
Neural
[53,71,72]+
/Si
SF6
RIE
Circular
Poly-SiN
o/Yes
Hypoderm
ic/Neural
[73]/[74]+
/Si
XeF
2dry
etchingSem
icircularPyC
Yes
Neural
[75]+
/SO
IK
OH
wetetching
TriangularPoly-Si
No
Hypoderm
ic[76]
/
SOI
KO
Hw
etetchingTrapezoidal
Six Ny ,SiO
2Y
esN
eural[77]
/
SOI
TM
AH
wetetching
TriangularSix N
y ,PyCY
esN
eural[78]
/
(c)Bonded
channels:Si
DR
IER
ectangularSi
Yes
Neural
[7981]+
/+Si
DR
IER
ectangularPD
MS
Yes
Neural
[82]+
/+Si
KO
Hw
etetchingTriangular
PDM
SN
oH
ypodermic
[83]
/SO
ID
RIE
Rectangular
SiN
oH
ypodermic
[84]+
/+Si b
PIlithographyR
ectangularPI
Yes
Neural
[85]+
/+Si b
Pyin
DR
IEm
oldR
ectangularPy
Yes
Neural
[86]+
/+Si b
SU-8
lithographyR
ectangularSU
-8Y
esN
eural[87]
+/+
Si bSU
-8lithography
Rectangular
PIY
esN
eural[88]
+/+
aM
icrofluidicdesign
flexibility:+high,
fair,low
; bProbe
iscom
pletelyreleased
fromthe
substrate;A
pproachdeveloped
inthis
thesis
2.2 Fluidic Microprobes 17
rificial layers made of photoresist or oxide have to be used. The sacrificial layeris patterned and forms the core of the fluidic channel during fabrication. Afterdeposition of the channel material by applying technologies such as low pressurechemical vapor deposition (LPCVD) or plasma enhanced chemical vapor depo-sition (PECVD), the sacrificial core is etched away leaving a hollow structure.Afterwards, bulk micromachining by dry or wet etching is most often required topattern the overall probe shape. Alternatively, in case the channel has been fabri-cated as a self-contained structure, the entire surface micromachined structure canbe completely released from the substrate.
By applying surface micromachining, various probes have been fabricated on Sias well as on silicon-on-insulator (SOI) substrates (Table 2.2a). The most popu-lar materials used for the channel walls are CVD materials such as silicon nitride(Si3N4, non-stoichiometric SixNy), silicon oxide (SiO2, non-stoichiometric SiOx),polysilicon (poly-Si), and the polymer parylene-C (PyC). But also the electro-chemical deposition of metals such as palladium (Pa), palladium cobalt (PaCo),and nickel (Ni) was reported.
Surface micromachining allows the channel layout to be freely determined, butthe sacrificial layer limits the achievable channel cross sections. The height of thelayer must be low enough to ensure sufficient edge coverage during the subsequentdeposition steps. Additionally, a diffusion-limited release of the sacrificial layerneeds to be accounted for possibly demanding special design measures. There-fore, the technology results typically in rather low channel profiles not higher thanten micrometers having widths of a few ten micrometers.
Buried Channels
Buried channels (Fig. 2.1b) combine etching techniques of bulk micromachin-ing with surface micromachining. Typically, narrow trenches are first created byisotropic deep reactive ion etching (DRIE). This is followed by a second etch-ing step creating a cavity at the bottom of the trenches which defines the channelcross section. Thereby, the cavity can be implemented by anisotropic wet etching,e.g. using ethylenediamine pyrocatechol (EDP), potassium hydroxide (KOH), ortetramethylammonium hydroxide (TMAH), as well as isotropic dry etching usingxenon diflouride (XeF2) gas or reactive ion etching (RIE) with sulfur hexafluoride(SF6), as summarized in Table 2.2b. The original trenches are then closed takingadvantage of the volume increase during thermal oxidation of Si to SiO2 and/ordeposition of additional CVD layers, e.g. SiO2, SixNy, or PyC. Finally, etchingtechniques are applied to pattern the overall probe shape.
18 2 Design of the NeuroMedicator
The most sensitive aspect of this approach is the adjustment of the trench width.The trench width needs to be narrow enough to be closed again by the afore men-tioned methods but at the same time wide enough to prevent significant reductionof the diffusion-limited etch rate in the cavity. In the case of anisotropic etch-ing of the cavity, the microfluidic layout is constrained by the orientation of thecrystallographic planes of Si.
Bonded Channels
Bonded channels (Fig. 2.1c) are fabricated by first implementing an open channelstructure in/on the substrate. This is followed by bonding of a second structurewhich seals the open channel, i.e. a structured or unstructured cover. After bond-ing, further etching techniques need to be applied to pattern the overall probegeometry.
The bonding technology can be applied to Si, glass, or polymers such as polyimide(PI), parylene (Py), polydimethylsiloxane (PDMS), and the photoresist SU-8 (Mi-croChem Corp., Newton, MA, USA), as listed in Table 2.2c. Thereby, polymericprobes are commonly realized as self-contained structures which are completelyreleased from the substrate.
The bonding approach allows to realize the fluidic structure on a separate waferindependent of the cover. Consequently, the channel cross-sectional area as wellas the fluidic layout is not restricted by preceding or subsequent processing steps.In addition, DRIE allows to independently adjust the channel depth and width. Onthe other hand, two wafers as well as a non-trivial bonding step are required.
Other Approaches
In addition to the main approaches described above, there are less common in-plane technologies previously used for the fabrication of hypodermic probes notlisted in Table 2.2. For instance, the deposition of poly-Si into a reusable Simold [89] as well as the thermal oxidation of a negative Si lost mold resultingin a SiO2 probe [90] was reported. Hybrid assemblies affixing a silicon micro-electrode array to a fused silica capillary were also demonstrated [91]. Furtherapproaches based on microstructuring and bonding of glass [92] and titanium(Ti) [93] differ from the Si technology but are mentioned here for completeness.This includes also attempts to fabricate probes by microinjection molding usingcycloolefin copolymers (COC) [94].
The design flexibility with respect to the channel layout as well as the channelgeometry of the different approaches is evaluated in the last column of Table 2.2.
2.3 Drug Delivery and Microdosage Systems 19
Thereby, the bonding approach offers the highest flexibility in the design of themicrofluidic layout and allows the layout to be optimally adapted to the applica-tion. Furthermore, various in- and outlet types can be implemented and an entirelyplanar probe surface is maintained which is advantageous for the intended Neuro-Probes platform assembly concept. Therefore, a bonding process was consideredto be most suitable and has been further pursued. The developed two-wafer Sibonding process combined with DRIE [79,95,96] is highlighted in Table 2.2. Asimilar process has been meanwhile implemented by other groups [80,81].
2.3 Drug Delivery and Microdosage Systems
The current state of the art of MEMS-based DDS is summarized in several gen-eral reviews [56,9799] as well as reviews addressing only selected aspects suchas implantable [100], osmotic [101], or reservoir-based [102] devices. Restrictingto the pressure-driven administration of liquid drugs, a DDS is basically repre-sented by an application-specific microdosage system which combines infusionequipment with liquid storage, micropumping, and metering. In general, micro-dosage systems are found in diverse applications where defined liquid volumeshave to be provided.
In principle, connecting a micropump to a reservoir or using a microvalve to mod-ulate the flow from a pressurized source results already in a microdosage system.Micropumps and -valves are considered to be the building blocks of microflu-idic systems. Correspondingly, numerous technical implementations have beeninvestigated and reviewing the complete state of the art goes beyond the scope ofthis thesis. However, there are several good reviews available addressing micro-pumps [103105] as well as microvalves [106] in general or with special focus onbiomedical and drug delivery applications [107110].
Depending on the principle of flow generation, micropumps can be classified intodisplacement pumps exerting pressure forces on the working fluid and dynamicpumps continuously adding energy directly to the working fluid [103]. In the lat-ter, the pump rate is often highly dependent on specific properties of the fluid. Dis-placement pumps are based on periodic or aperiodic principles. Periodic pumpsrely on a repetitive or rotary motion of the actuator, whereas the actuator of an ape-riodic pump performs an unidirectional motion. In contrast to aperiodic pumps,periodic pumps can be operated in continuous flow mode. The very limited vol-ume displacement of MEMS-based actuators often requires the implementationof periodic pump principles in order to deliver reasonably large liquid volumes.
20 2 Design of the NeuroMedicator
The pumping mechanism has significant influence on the technical implementa-tion and the resulting release profile of a DDS. Drug delivery devices for liquidscan be classified according to their release profiles into four idealized classes IIVas shown and described in Table 2.3.
In the simplest case, a DDS provides a fixed and continuous liquid flow upon ac-tivation (class I). Additional technical measures are required if the device has notonly to provide fixed flow rates but defined amounts of liquid upon request result-ing in a drug-on-demand system (classes IIIV). Such systems require meteringmechanisms either in an open-loop or even more sophisticated in a closed-loopconfiguration involving sensoric components. In the order of increasing technicalcomplexity, this includes release of discrete amounts (class II), pulse-width mod-ulated release (class III), and fully variable release rates and profiles (class IV).Targeting a drug-on-demand system, only devices of class IIIV are of interest.However, given the operational conditions specified in Table 2.1, the fully variableadjustment offered by class IV is not necessarily required.
2.3.1 Neural Drug Delivery Systems State of the Art 3
In human medicine, drug delivery directly to the brain is of general interest fortherapeutic approaches where the bodys BBB needs to be circumvented in orderto sustain a certain intracranial drug concentration [111], e.g. for the chemother-apy of brain tumors. In this case, drug can be directly provided to the neuraltissue by using brain catheters connected to implanted multipurpose drug deliverypumps. Examples include the implantable Ommaya reservoir operated by hand,aperiodic constant rate pumps requiring no electric power, and electrically con-trollable periodic solenoid as well as peristaltic pumps [111,112]. However, beingoften designed for implantation times of several years and storing fairly large liq-uid volumes, such devices are far too large and heavy for use with small animals.For instance, the implantable SynchroMed II Drug Infusion System (Medtronic
3Abridged versions of this section have been published as part of the original journal articles
[29] S. Spieth, A. Schumacher, C. Kallenbach, S. Messner, and R. Zengerle, The NeuroMedica-tor a micropump integrated with silicon microprobes for drug delivery in neural research,Journal of Micromechanics and Microengineering, vol. 22, no. 6, 065020 (11pp), 2012.
[30] S. Spieth, A. Schumacher, T. Holtzman, P. D. Rich, D. E. Theobald, J. W. Dalley, R. Nouna,S. Messner, and R. Zengerle, An intra-cerebral drug delivery system for freely movinganimals, Biomedical Microdevices, vol. 14, no. 5, pp. 799809, 2012.
Related author contributions included in this section:
S. Spieth (HSG-IMIT): literature search, manuscript preparation
2.3 Drug Delivery and Microdosage Systems 21
Table 2.3: Idealized classes of DDS according to their characteristic release profiles andtechnical implementations
Time
RateConst. pressure
Hydraulic resistance
Uncontrolled aperiodic micropumps
Collapsiblereservoir
Class I: Devices of class I provide continuous liquid flows over time at constantrates. Thereby, it is not possible to modify the release rate during operation andthe release stops not before the driving mechanism is exhausted. Technically, thesedevices correspond to uncontrolled aperiodic micropumps, e.g. realized by constantpressure acting on a collapsible reservoir, connected to a hydraulic resistance.
Time
Rate
Openable membraneDiscrete reservoirs
Class II: Devices of class II provide predefined discrete liquid amounts on demand.Thereby, the amounts can be of identical or varying quantity and multiple amountscan be released at the same time. Typically, this is implemented by discrete reser-voirs which can be opened on demand.
Time
RateConst. pressure
On/off microvalve
Pulse-width modulated aperiodic micropumps
Collapsiblereservoir
Class III: Devices of class III provide constant liquid flows over freely adjustabletime intervals on demand. This can be realized by applying an on/off microvalvefor pulse-width modulation of a constant pressure aperiodic micropump allowing tovirtually decrease the average release rate over time.
Time
Rate
Collap./ventedreservoir
Const. pressure
Adjustable microvalve
Controlledperiodic micropumps
Modulated / controlledaperiodic micropumps
Periodic micropump
Collapsiblereservoir
Class IV: Devices of class IV release liquids at variable rates and over adjustabletime intervals enabling complete freedom in the adjustment of the release profile.This can be realized by modulated and controlled aperiodic micropumps or con-trolled periodic micropumps connected to collapsible or vented reservoirs.
22 2 Design of the NeuroMedicator
45 mm
16 mm
Skull
(b)(a)
Step motorScrew
Electrolysisin drugInjectioncannula
Guide cannula
Constantcurrentsource
Figure 2.2: Examples of DDS for small animals with microprobes: (a) electrolysis-basedDDS [115], adapted with permission from Elsevier; (b) skull-mounted intracranial infu-sion system based on a step motor and steel capillaries [134], adapted with permissionfrom Elsevier.
Neuromodulation, Minneapolis, MN, USA) is primarily designed for intrathecalpain therapy and features a peristaltic pump enclosed in a titanium housing. Thesmallest model 8637-20 has dimensions of 87.5mm 19.5mm, weighs 165 gwhen empty, and can store up to 20 mL of drug liquid [113].
Special miniature DDS for use with small animals together with their propertiesare listed in Table 2.4. Only few systems with attached microprobes for liquidinfusions directly into the neural tissue have been proposed (Table 2.4a).
Early approaches applied electrolysis directly in the drug liquid to generate thepressure required for infusion through stainless steel capillaries as illustratedin Fig. 2.2a while accepting possible degradation of the drug itself [114,115].Thereby, the amount of infused liquid needs to be derived from the applied cur-rent. In addition, a miniaturized mechanical pump based on a step motor andcoupled to steel capillaries was suggested for intracranial infusions as shown inFig. 2.2b [134].
With respect to micromachined probes, bubble-powered pumps [61,117,118] andthermopneumatic [71,116] as well as piezo-operated [82] peristaltic micropumpswere integrated into the backbones of Si probes (Table 2.4b). However, theseapproaches included neither reservoirs for liquid storage nor was the operation ofthe devices verified with the exception of [117,118].
On-demand openable discrete microreservoirs (Table 2.4c) can be implanted close
2.3 Drug Delivery and Microdosage Systems 23
Table2.4:
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