Printing Carbon Black based multi-material silicone with Additive Manufacturing for embedding sensing in soft structures

MSc assignment

Soft robots are a promising paradigm for a range of applications which could benefit from their inherent safety and adaptability. However, connections between rigid electronics both in
hardware and software remain the mainstay [1]. Although recent efforts have created soft analogues of individual rigid components, integrating sensing and control systems is challenging to achieve without compromising the complete softness, form factor, or capabilities [1]. Soft sensors that can discriminate shear and normal force could help provide machines with the fine control desirable for safe and effective physical interactions with the environment [2].

One area where soft robotics could be utilised is in the research of Pelvic Organ Prolapse (POP). POP is a disorder where one or more pelvic organs prolapse from their original position within the abdomen. The pelvic organs consist of the vagina, uterus, bladder, urethra, and rectum and are kept in place by the pelvic floor which consists of muscles, ligaments and tissues. POP is caused by the pelvic floor being damaged and weakened. One of the main causes of this trauma is pregnancy and childbirth, in particular vaginal childbirth. Others include ageing and obesity. There are different treatments for POP. The less invasive option is the use of a pessary. A pessary is an orthotic device which is inserted into the vagina to act as a physical support for the pelvic floor organs. There are different types of pessary, characterised by the variety of prolapsed organs that need support and differing in severity in which support is needed.

Studies conducted in the US showed a 56 % success rate 12 months after insertion and up to 89 % after three months [3]. If a pessary is used within the context of urinary incontinence the success rate was up to 59 % after approximately one year of usage [3]. In the study conducted by Manchana et al., 52 % of the participants continued the use of the pessary at a 13th-month consultancy. The majority of women who stopped the usage reported pessary expulsion as the main reason, which is caused by a poor fit [4]. Since age is a contributing factor to the development of POP, it is expected that with the increased life expectancy, the presence of POP will also become more prevalent [4]. To reduce the rate of rejection, it is hypothesised that a sensorised pessary could help identify the problematic areas and provide clinicians with insight on how to tailor the pessary.

This thesis will aim to contribute to this goal by acquiring more knowledge on the forces' pessary experiences inside the abdomen and at what positions these forces are on the pessary. The use of 3D printing is opted for since it can realize arbitrary shapes due to its inherent design freedom and could therefore realise the complex shapes required to elevate the symptoms.
It can therefore provide the possibility for personalized pessary shapes, if that proves to be beneficial. There are several types of 3D printing, which can be divided into two methods. In the first method, the printer deposits a full layer of the material of the intended product [5]. From this deposit layer, areas that are part of the printed product are indicated and solidified. All the access areas of the layer are removed once the entire print has been completed. The benefit of this method is that there is no or little need for support structures since the unprocessed material functions as support and therefore complex shapes can be realised. Meanwhile, this method is confined to its building volume and the use of multi-material is not trivial. To this group belongs binder jetting, sheet lamination, VAT polymerization and powder bed fusion [5]. In the second method, the building material is added from one central point and is therefore only
deposited where it is needed to form the product. Examples include material jetting, direct energy deposit, and material extrusion [5]. In this work, the latter will be applied. This is due to its ability to print with silicone, which is the material the sensorized pessary will consist of within the context of this study.

To produce a sensorized pessary, the strain of a deforming pessary needs to be measured. To achieve this, this study will make use of the piezoresistive effect, which is widely used in
sensorized devices. Here, a change in electrical resistance is observed when a conductor is subjected to an applied strain. To utilise this effect, the silicone basis needs to be augmented to exhibit conductive behaviour. Four methods are applicable to accomplish the conductivity: liquid conductors, conductive inks, and conductive elastomer composites, which can be compatible with 3D printing. The conductive elastomer composites consist of a conductive filler and an elastomeric matrix, which in this study is silicone. The electrical and mechanical properties of the elastomer are dictated by the choice of elastomeric matrix and conductive filler. Of the available conductive fillers carbon black (CB) is selected for this study as it has advantageous rheological properties, good processability and it is easily accessible.1

Research Question
From the aforementioned literature review, the need for integrating sensing into soft structures could prove to be beneficial in providing a solution to the discomfort women with POP still experience. However, embedded sensing in soft structures still faces several challenges and is an active research field. The goal of this thesis is to further develop the integration of soft structures with flexible strain sensors by incorporating silicone with CB-loaded silicone. This study aims to answer the following research question: How can multi-material carbon-loaded silicone printing be used to realise soft sensing?

During this research, the following research sub-questions are investigated:
• How do the characteristics of the combined silicone change to its individual components
• What composition of silicone is beneficial for printing?
• What is the optimal carbon loading for multi-material sensing?
• Which carbon loading provides the most optimal signal without sacrificing the flexibility of the sensor?
• What are the best strategies and recipes for multi-material printing of carbon-loaded and unloaded silicones?

Intended Methodology
This research will complete three different stages, each developing a deeper understanding of the material characteristics that could ultimately lead to sensing within soft structures. The first stage is concerned with developing an understanding of the structural properties of the different silicones and how these can be used to their advantage. Using a decision tree the best-suited silicone will be selected to advance to the next stage. Since this study is related to the development of an instrumented pessary, medical (bio-compatible) types of silicone can also be explored.

The second stage aims to research the conductive (carbon-doped)silicone materials and how to perform characterization of the response signal.

The third stage focuses on the integration of the above-mentioned stages into the fabrication of a functional prototype sensor. The crux here is to marry the two different silicon types into one product and to identify the resulting adaptations for processing, investigating the effects of the multi-material on the curing and carbon black loading characteristics. The fabricated sensor prototype will be tested for compatibility between the neutral silicone and the carbon black-loaded silicone. This includes characterising the resulting adhesion between the two materials and the stiffness of the combination of the two. Here different configurations will be further inspected and evaluated on the previously mentioned material characteristics. These include a study of the different material consistencies, the influence of the order of stacking the different silicone layers in the z-direction and how these choices influence the curing process.

Finally, the prototype sensor's signal response will be characterised. This will be executed by deforming the sensor under force and/or position control, and further, documenting its material characteristics according to the test configuration established in Phase 2. During the different stages, literature will be consulted.2

[1] J. K. Choe, J. Kim, H. Song, J. Bae, and J. Kim, “A soft, self-sensing tensile valve for perceptive soft robots,” Nature Communications, vol. 14, no. 1, p. 3942, 7 2023.
[2] M. S. Sarwar, R. Ishizaki, K. Morton, C. Preston, T. Nguyen, X. Fan, B. Dupont, L. Hogarth, T. Yoshiike, R. Qiu, Y. Wu, S. Mirabbasi, and J. D. W. Madden, “Touch, press and stroke: a soft capacitive sensor skin,” Scientific Reports, vol. 13, no. 1, p. 17390, 10 2023.
[3] R. Oliver, R. Thakar, and A. H. Sultan, “The history and usage of the vaginal pessary: A review,” pp. 125–130, 2011.
[4] T. Manchana, “Ring pessary for all pelvic organ prolapse,” Archives of Gynecology and Obstetrics, vol. 284, no. 2, pp. 391–395, 8 2011.
[5] P. Schmitt, S. Zorn, and K. Gericke, “ADDITIVE MANUFACTURING RESEARCH LANDSCAPE: A LITERATURE REVIEW,” Proceedings of the Design Society, vol. 1, pp. 333–344, 8  2021