Advanced Wound Care: Working towards standardised test platforms for novel therapies

Hill, Catherine Ann

Abstract

There used to be limited knowledge on how wounds heal in response to clinical treatments, therefore therapies were often ineffective, causing a need to heal the wounds that failed to respond to treatment (Gregor et al., 2008; Huang et al., 2014). Developments in advanced wound care to meet this clinical need have led to novel therapies, such as negative pressure wound therapy. Over £110 million is spent by the UK’s NHS on advanced wound dressing. Limited clinical evidence supporting their use and significantly increased cost compared to other wound care options means that advanced wound care options are utilised less often (NICE, 2016; Tickle, 2016). This is due to current test platforms approved by the FDA offering only basic insight into the performance of these devices (FDA, 2018a). These test platforms are limited due to focusing on user safety and basic functionality, with little regard for the influence biomechanical stimulation has on wound healing. Performance evaluation can be achieved through animal and clinical testing, but these have ethical and cost limitations. This research addresses the gap in current test methodologies by designing, developing and manufacturing a novel test platform and protocol to accurately provide data on key performance indicators related to healing wounds. Testing environments that simulate real-world scenarios aid in the optimisation and comparison of the performance of advance wound care therapies. The novel test platform consists of a flexible multi-axis sensor made from materials mechanically representative of human skin incorporated onto a human knee model with full range of motion. Knee model control systems allow for detailed in-situ analysis of lower limb dressing performance under physiologically relevant environments and conditions such as natural gait.
To find a biomechanically similar skin substitute for the sensor, porcine skin, human skin, and a range of elastomers (MB Fibreglass, UK) were tested in accordance with ISO527-2. Tension samples of type 5A were tested to destruction with a pre-load and load of 0.5 mm/sec on a 1kN load cell (Lloyds LS1 Tensile Tester). Bulk elastic modulus was measured under compression using DMA (TA Q800, Veryst Engineering) with a pre-load and load of 0.5 N/min to a maximum of 10N. Localised elastic modulus was measured using a Bio-indenter (UNHT³ Bio, Anton Paar Bioindenter) with a loading rate of 0.70 mN/min until a maximum of 0.35 mN. Sample numbers of n=8 were used for tension tests and n=5 for compression tests. An elastomer-based sensor was manufactured based on high and low resistance silicone-carbon mixtures adapted from (Matsuda et al., 2020). A 6x6 nodal array was created in a silicone sheet (MB Fibreglass, UK) and a stiffer elastomer (MB Fibreglass, UK) used to minimise deformations when the sensor was under tension. Holes 5mm in diameter were filled with a porous silicone created by a simple porogen leaching process (caster sugar) and combined with a high resistance composite of polyvinylidene difluoride (PVDF), N-Methyl-2-pyrrolidone (NMP) and carbon black. A low resistance conductive paste was added to the sensor to connect the nodes, creating strain sensors in the X and Y directions. The sensor was read using an Arduino. Changes in resistances were assumed to be proportional to change in pressure and strain. To have a physiologically relevant environment a representative knee model was designed and manufactured using 3D printing. Representation included the geometry of natural anatomy and fulfilling full range of motion in six degrees of freedom. The design was based on a constrained ball and socket joint, with springs to provide passive forces of ligaments and use of a universal tester (Lloyds EZ50 Tensile Tester) for active forces. The movement and angles were tracked and analysed using the software Kinovea.
The elastomer testing method used to test the skin yielded Young's moduli values consistent with those in literature (Kalra & Lowe, 2016). A one-way ANOVA analysis was performed using a Tukey test to provide the mean difference between elastomers and human skin. This showed that in tension T20 with a mean difference of 3.00 MPa ranked top and in both types of compression tests PT flex with mean differences of -0.20 MPa and -0.39 MPa ranked top. To choose a material which encompassed all properties of skin, the mean differences were weighted 50% tension and 25% to each compression technique resulting in Zhermack ZA13 being the most representative elastomer. Preliminary testing of the sensor showed the low resistance elastomer had a voltage change of 5V to a minimum of 2V when undergoing cycles of 50% strain. The high resistance elastomer went from 5V to 4V when undergoing 10N of pressure. When a PICO negative pressure wound therapy dressing (Smith+Nephew PLC, UK) is applied to the sensor and activated at a pressure of -80mmHg, voltage changes can be observed above 0V. This suggests it operates within the ranges to detect useful information and perform both statically and dynamically. This highly elastic sensing array exploited structural and resistive control to obtain independent detection of pressure and strain, making it ideal for differentiating readings of negative pressure and larger pressures applied to the wound through movement. Testing showed the knee model to have the full range of motion to be representative of the knee joint. The programmed walking gait resulted in accurate flexion-extension and internal-external movements, which were actively controlled and matched anterior-posterior movements.
Static and dynamic tests were compared to the data obtained from current test platforms. The results demonstrated that the novel platform provides insights into pressure distribution and changes which current test platforms cannot capture. The findings suggest that the pressure applied by NPWT devices increases under increased strain and dynamic movement, highlighting the need for advanced testing methodologies. The novel test platform offers a valuable complementary tool to current platforms, with the potential for further refinement and calibration to enhance its accuracy. This research also reduces the reliance on animal testing and facilitates innovation in research and development without the immediate need for clinical trials to obtain performance data, accelerating the advancement of NPWT technology and improving wound care outcomes, ultimately reducing patient suffering.

Citation

Hill, C. A. (2024). Advanced Wound Care: Working towards standardised test platforms for novel therapies. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4927712

• Thesis Type: Thesis
• Deposit Date: Nov 26, 2024
• Publicly Available Date: Nov 19, 2026
• Keywords: Medical engineering
• Public URL: https://hull-repository.worktribe.com/output/4927712
• Additional Information: Medical Engineering; School of Engineering; University of Hull
• Award Date: Nov 18, 2024