Strategies for neural control of prosthetic limbs: From electrode interfacing to 3D printing

Limb amputation is a major cause of disability in our community, for which motorised prosthetic devices offer a return to function and independence. With the commercialisation and increasing availability of advanced motorised prosthetic technologies, there is a consumer need and clinical drive for intuitive user control. In this context, rapid additive fabrication/prototyping capacities and biofabrication protocols embrace a highly-personalised medicine doctrine that marries specific patient biology and anatomy to high-end prosthetic design, manufacture and functionality. Commercially-available prosthetic models utilise surface electrodes that are limited by their disconnect between mind and device. As such, alternative strategies of mind-prosthetic interfacing have been explored to purposefully drive the prosthetic limb. This review investigates mind to machine interfacing strategies, with a focus on the biological challenges of long-term harnessing of the user\u27s cerebral commands to drive actuation/movement in electronic prostheses. It covers the limitations of skin, peripheral nerve and brain interfacing electrodes, and in particular the challenges of minimising the foreign-body response, as well as a new strategy of grafting muscle onto residual peripheral nerves. In conjunction, this review also investigates the applicability of additive tissue engineering at the nerve-electrode boundary, which has led to pioneering work in neural regeneration and bioelectrode development for applications at the neuroprosthetic interface

Electrical stimulation using conductive polymer polypyrrole promotes differentiation of human neural stem cells: a biocompatible platform for translational neural tissue engineering

Conductive polymers (CPs) are organic materials that hold great promise for biomedicine. Potential applications include in vitro or implantable electrodes for excitable cell recording and stimulation, and conductive scaffolds for cell support and tissue engineering. Here we demonstrate the utility of electroactive CP Polypyrrole (PPy) containing the anionic dopant dodecylbenzenesulfonate (DBS) to differentiate novel clinically relevant human neural stem cells (hNSCs). Electrical stimulation of PPy(DBS) induced hNSCs to predominantly β-III Tubulin (Tuj1) expressing neurons, with lower induction of glial fibrillary acidic protein (GFAP) expressing glial cells. In addition, stimulated cultures comprised nodes or clusters of neurons with longer neurites and greater branching than unstimulated cultures. Cell clusters showed a similar spatial distribution to regions of higher conductivity on the film surface. Our findings support the use of electrical stimulation to promote neuronal induction and the biocompatibility of PPy(DBS) with hNSCs, and opens up the possibility of identifying novel mechanisms of fate determination of differentiating human stem cells for advanced in vitro modelling, translational drug discovery and regenerative medicine

Wet-Spun Trojan Horse Cell Constructs for Engineering Muscle

Engineering of 3D regenerative skeletal muscle tissue constructs (skMTCs) using hydrogels containing muscle precursor cells (MPCs) is of potential benefit for repairing Volumetric Muscle Loss (VML) arising from trauma (e.g., road/industrial accident, war injury) or for restoration of functional muscle mass in disease (e.g., Muscular Dystrophy, muscle atrophy). Additive Biofabrication (AdBiofab) technologies make possible fabrication of 3D regenerative skMTCs that can be tailored to specific delivery requirements of VML or functional muscle restoration. Whilst 3D printing is useful for printing constructs of many tissue types, the necessity of a balanced compromise between cell type, required construct size and material/fabrication process cyto-compatibility can make the choice of 3D printing a secondary alternative to other biofabrication methods such as wet-spinning. Alternatively, wet-spinning is more amenable to formation of fibers rather than (small) layered 3D-Printed constructs. This study describes the fabrication of biosynthetic alginate fibers containing MPCs and their use for delivery of dystrophin-expressing cells to dystrophic muscle in the mdx mouse model of Duchenne Muscular Dystrophy (DMD) compared to poly(DL-lactic-co-glycolic acid) copolymer (PLA:PLGA) topically-seeded with myoblasts. In addition, this study introduces a novel method by which to create 3D layered wet-spun alginate skMTCs for bulk mass delivery of MPCs to VML lesions. As such, this work introduces the concept of “Trojan Horse” Fiber MTCs (TH-fMTCs) and 3d Mesh-MTCs (TH-mMTCs) for delivery of regenerative MPCs to diseased and damaged muscle, respectively

Handheld Co-Axial Bioprinting: Application to in situ surgical cartilage repair

Three-dimensional (3D) bioprinting is driving major innovations in the area of cartilage tissue engineering. Extrusion-based 3D bioprinting necessitates a phase change from a liquid bioink to a semi-solid crosslinked network achieved by a photo-initiated free radical polymerization reaction that is known to be cytotoxic. Therefore, the choice of the photocuring conditions has to be carefully addressed to generate a structure stiff enough to withstand the forces phisiologically applied on articular cartilage, while ensuring adequate cell survival for functional chondral repair. We recently developed a handheld 3D printer called Biopen . To progress towards translating this freeform biofabrication tool into clinical practice, we aimed to define the ideal bioprinting conditions that would deliver a scaffold with high cell viability and structural stiffness relevant for chondral repair. To fulfill those criteria, free radical cytotoxicity was confined by a co-axial Core/Shell separation. This system allowed the generation of Core/Shell GelMa/HAMa bioscaffolds with stiffness of 200KPa, achieved after only 10seconds of exposure to 700mW/cm2 of 365nm UV-A, containing \u3e90% viable stem cells that retained proliferative capacity. Overall, the Core/Shell handheld 3D bioprinting strategy enabled rapid generation of high modulus bioscaffolds with high cell viability, with potential for in situ surgical cartilage engineering

Efficacy testing as a primary purpose of phase 1 clinical trials: is it applicable to first-in-human bionics and optogenetics trials?

In her article, Pascale Hess raises the issue of whether her proposed modelmay be extrapolated and applied to clinical research fields other than stem cell-based interventions in the brain (SCBI-B) (Hess 2012). Broadly summarized, Hess\u27s model suggests prioritizing efficacy over safety in phase 1 trials involving irreversible interventions in the brain, when clinical criteria meet the appropriate population suffering from degenerative brain diseases (Hess 2012). Although there is a need to reconsider the traditional phase 1 model, especially with respect to first-in-human clinical trials involving novel technologies, the question arises as to whether it is appropriate to advocate for a new model that prioritizes efficacy over safety across all phase 1 clini- cal research trials involving irreversible interventions in the brain

Precision wet-spinning of cell-impregnated alginate fibres for tissue engineering

The selective assembly of functionalised fibres produced by wet-spinning into implantable three dimensional contructs presents attractive prospects for the field of medical bionics[1]. In particular, the incorporation of biological factors and large numbers of cells within biocompatible and macroporous fibres is expected to deliver improvements to drug delivery platforms as well as to tissue engineering biotechnology[2, 3]

Optical and electrochemical methods for determining the effective area and charge density of conducting polymer modified electrodes for neural stimulation

Neural stimulation is used in the cochlear implant, bionic eye, and deep brain stimulation, which involves implantation of an array of electrodes into a patient\u27s brain. The current passed through the electrodes is used to provide sensory queues or reduce symptoms associated with movement disorders and increasingly for psychological and pain therapies. Poor control of electrode properties can lead to suboptimal performance; however, there are currently no standard methods to assess them, including the electrode area and charge density. Here we demonstrate optical and electrochemical methods for measuring these electrode properties and show the charge density is dependent on electrode geometry. This technique highlights that materials can have widely different charge densities but also large variation in performance. Measurement of charge density from an electroactive area may result in new materials and electrode geometries that improve patient outcomes and reduce side effects

Influence of biopolymer loading on the physiochemical and electrochemical properties of inherently conducting polymer biomaterials

The physicochemical and electrochemical properties of polypyrrole (PPy) doped with the biological dopant dextran sulphate (DS) were shown to be significantly altered as a function of varying the salt concentration (0.2, 2 or 20 mg/ml) in the polymerisation electrolyte. Films grown in the presence of 0.2 mg/ml DS generated the highest potential during galvanostatic growth, with the potential decreasing with each subsequent increase in DS concentration. The electroactivity of the polymers was similar for all three DS concentrations, with the 20 mg/ml film drawing slightly more current upon reduction in PBS. Increasing the DS concentration reduced film interfacial roughness and increased polymer hydrophilicity. Polymer mass and thickness was larger for DS films grown from 0.2 mg/ml and 2 mg/ml DS electrolyte solutions, compared to the 20 mg/ml films. The latter also demonstrated a much higher shear modulus than the 2 mg/ml and 0.2 mg/ml films, respectively. The changes in the polymer physicochemical properties were associated with an increase in polymer densification with increasing DS loading, correlating with a likely higher conjugation generated during polymerisation at a potential closer to the ideal oxidation potential of pyrrole. Herein we describe a facile approach through which polymer properties may be varied significantly by varying the dopant concentration in the electrolyte, providing the ability to tune polymer properties for enhanced functionality while preserving fundamental polymer chemistry

Organic Bionics

The first reference on this emerging interdisciplinary research area at the interface between materials science and biomedicine is written by pioneers in the field, who address the requirements, current status and future challenges. Focusing on inherently conducting polymers, carbon nanotubes and graphene, they adopt a systematic approach, covering all relevant aspects and concepts: synthesis and fabrication, properties, introduction of biological function, components of bionic devices and materials requirements. Established bionic devices, such as the bionic ear are examined, as are emerging areas of application, including use of organic bionic materials as conduits for bone re-growth, spinal cord injury repair and muscle regeneration. The whole is rounded off with a look at future prospects in sustainable energy generation and storage. Invaluable reading for materials scientists, polymer chemists, electrotechnicians, chemists, biologists, and bioengineers