Material Selection Techniques for Polymer Hubs of Novel Spinal Stem Cell Introducers using Finite Element and Weighted Property Method

Stem cell studies have progressed significantly in the last decades. Similarly, stem cell therapies have also been applied in clinical settings, includingthe spine. However, stem cells’ delivery to the spinal region requires delicate procedures, demanding reliable introducers in addition to the surgeons' psychomotor skills. The introducer is a tubular sleeve to guide a syringe needle to deliver and draw fluids into and from the body, respectively. To date, there is no introducer dedicated to spinal stem cell deliveries. Our group proposed a dedicated introducer for spinal stem cell therapies, which consists of a needle and a hub (a base part of an introducer for gripping and handling). We focused onthe weighting property method (WPM) to select four polymer candidates for the hub:polystyrene, polycarbonate, polypropylene, and polyethylene. Prior to WPM, the mechanical integrity of the hub candidates was analyzed using the finite element method to determine the von Mises Stress (vMS) values. Accordingly, the vMS for each case and the material was compared with the respective tensile strength (TS), thereby vMS:TS ratio (MTR).MTR was included as one of the properties in WPM, in addition to prices, critical radiation doses, hardness, water absorption, and isopropyl alcohol resistance.According to the WPM calculation, polycarbonate received the highest score, thereby recommended for further introducer fabrications.

Finite element method introducer; Polymer; Spine; Weighting property method

    Stem cells are currently used extensively in many fields, including medicine. Reportedly, human embryonic stem cell-derived cardiac progenitors were used for severe heart failure treatment in a clinical case (Menasché et al., 2015). The mesenchymal stem cell (MSC), the most studied cell product, was also used for the treatment of many lung diseases, such as chronic obstructive pulmonary disease, acute respiratory distress syndrome,  and  idiopathic  pulmonary  fibrosis (Geiger et al., 2017).  Moreover,  due to  the development of regenerative medicine, stem cell transplantation has become a promising method for spinal diseases and injuries. Several in vivo studies and clinical trials demonstrated that using stem cells as a therapeutic tool induced improvement of motor functions and neurological conditions (Silvestro et al., 2020).

Stem cell therapies at the spinal cord include spine fusions, disc degenerations, and spinal cord injuries. MSCs from bone marrows, adipose tissues, periosteum, and skeletal muscles could differentiate into osteoblasts. Genetically modified MSCs reportedly induced spinal fusion in mice through BMP-2 expression, a growth factor related to spinal fusion. The degenerative disc disease is a result of an alteration in the biochemical composition, which affects the morphological condition of the spinal disc. The potential of MSCs to differentiate into various cell lineages was utilized by researchers to build chondrocyte-like cells expressing the nucleus pulposus-like phenotype (Schroeder et al., 2015). In several studies, the use of bone marrow MSCs on subjects with spinal cord injury showed improved motor functions and quality of life (Silvestro et al., 2020).

There are two current methods to deliver stem cells into the spinal cord: systemic and direct delivery methods. The systemic delivery methods consist of intravascular and intrathecal infusion, while the direct delivery method is performed by direct intraparenchymal injection. The systemic delivery methods require stem cells to migrate to the pathological areas, unlike the direct delivery method. The direct intraparenchymal injection requires laminectomy and opening of the duramater, which are usually performed without any stabilization. This method is considered high-risk due to several factors, such as the unreliable targeting to the spinal cord, the unsteady needle that can move and break the white matter tracts, and the uncontrolled injection rate that might promote reflux due to the elevated intraparenchymal pressure. However, direct injection is preferred for stem cell deliveries because it was more accurate and had a high success rate of cell engraftment. The advanced injection system could also be improved by advanced imaging-guided techniques, such as the integration with magnetic resonance imaging (MRI) to improve the targeting capability and reduce procedural errors. To enhance the efficacy of this method, the existing injection devices should be modified appropriately to make them compatible with the MRI (Donnelly et al., 2012).

To perform this method, using an introducer to guide a syringe needle to deliver and draw the stem cells was needed. However, there is no existing introducer dedicated to spinal stem cell delivery. The existing introducers for spinal therapies are mainly for anesthesia. Our group proposed a dedicated introducer for spinal stem cell therapies, which consists of a needle and a hub (a base part of an introducer for gripping and handling). As the needle dimensions are standardized, we focused on the weighting property method (WPM) to select four polymer candidates for the hub: polystyrene (PS), polycarbonate (PC), polypropylene (PP), and polyethylene (PE). Prior to WPM, the mechanical integrity of the hub candidates was analyzed using the finite element method (FEM) to determine the von Mises Stress (vMS) values. Accordingly, the vMS for each case and the material was compared with the respective tensile strength (TS), thereby vMS:TS ratio (MTR). MTR was included as one of the properties in WPM, in addition to prices, critical radiation doses (CRD), hardness, water absorption (WA), and isopropyl alcohol (IPA) resistance.

As the early step of material selection, FEM was successfully performed to compare four polymers: PS, PC, PP, and PE as candidates for hub materials of a spinal stem cell introducer. The result proved that the introducers made of AISI SS304 with PC-, PE-, PP-, and PS-hub are mechanically strong to survive the equivalent forces of accidental fall and grip of surgeons. The translated vMS, MTR, was categorized into three values, following the three separate cases, and used as the three most important properties in WPM. The properties following the three MTR values were prices, CDR, hardness, WA, and IPA resistance. WPM results indicated that PC received the highest score, thus recommended as a hub material.

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