Abstract
Intradermal bioanalysis via microneedle array (MNA) based electrodes are attracting significant research interests because their active components can be modified to selectively detect and monitor analytes of clinical interest in real-time; ranging from biofluids to solid tissues in a minimally invasive manner. The electrochemical detection strategies employed by these sensors are distinctly advantageous as they are generally low-cost, non complex and can be effectively utilised by a minimally trained individual. However, clinical acceptability of MNA-based electrochemical sensors is dependent on developing highly selective and sensitive electrodes through a scalable fabrication strategy. The aim of this research was to therefore evaluate the potential of using carbon nanotubes (CNT’s) based polymeric nanocomposites to develop electrochemical MNA-based electrodes.A simple fabrication strategy of dispersing CNT’s into epoxy resin via add/solution mixing and casting as MNA’s was initially investigated. However, a poor interfacial interaction between the epoxy-resin and CNT’s was found. Thus, a biodegradable polymer (poly (lactic) acid – PLA) was chosen as an alternative for the production of nanocomposites. A combinatorial approach involving carboxyl-functionalised MWCNT’s and sonication produced nanocomposites with excellent interfacial interaction and minimal aggregation. Using this approach, nanocomposite MNA’s were fabricated effectively at a maximum CNT loading of 6 wt%, at which the MNA’s displayed improved mechanical strength and the optimal electrochemical properties. The nanocomposite MNA’s were able to detect varying concentrations of ascorbic acid (AA), and demonstrated a linear oxidative response with a limit of detection (LOD) at 164.38µM and 16.79µM using differential pulse voltammetric (DPV) and amperometric, respectively. In situ electrochemical performance was further assessed in porcine skin. MNA’s could detect active changes in the skin, characterised by the appearance of two oxidative peaks. Upon inducing an artificial burn wound, the oxidative response was significantly attenuated, and importantly the impact of the burn could be measured at progressive distances from the burn site.
Further optimisation was however required to reduce the MNA’s resistive behaviour. Firstly, the method of sonication was investigated. Replacing the bath sonicator with a probe-type sonicator improved dispersions. The resultant 6 wt% CNT-loaded MNA’s had a better LOD of 0.51μM for AA, when measured amperometrically. Secondly, the influence of different dimensionalities (i.e. shorter lengths and smaller diameters) and functional groups (-NH2, - N2 and -COOH) were evaluated. Results highlighted the need to lower the concentration at which the CNT’s (particularly -NH2 and -N2 functionalised MWCNT’s) were dispersed. The maximum loading that could be incorporated for the shorter MWCNT’s with various functional groups was found to be 1 wt%. At this low CNT loading, the performance of N2 functionalised MWCNT-based MNA’s was comparable to the sonication-optimised MNA’s.
The biocompatibility of -N2 and -COOH MNA’s was then assessed in vitro. Exposure to MNA device-based extracts on the human keratinocyte cell line (HaCaT) and human malignant melanoma cell line (A375) produced contradictory results, possibly due the influence of released nanoparticulates from the MNA’s. Toxicity was however only evident after a prolonged exposure to the extracts (i.e. ≥24 hrs). Further assessment using DPV and amperometric simulations showed no evidence of voltage mediated cell toxicity.
In conclusion, this research highlights the potential of using CNT’s based nanocomposites to produce electroactive MNA’s, with limited toxicity, for bioanalytical studies. The fabrication steps are simple and easy to scale-up, and result in the development of MNA based electrodes in a single step.
| Date of Award | 2018 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Keng Wooi Ng (Supervisor), Bhavik Patel (Supervisor) & Melanie Flint (Supervisor) |