Investigating the Performance Characteristics of Auxetic Foams in Neuropathy Treatment Applications

1. Introduction

Peripheral neuropathy encompasses disorders of the nerves outside the brain and spinal cord [1]. Affecting over 20 million people, this condition arises from metabolic, autoimmune, and hereditary factors, as well as trauma, infection, or radiation [1,2,3,4]. Nerve damage often results in impaired balance and coordination, which manifests as increased Ground Reaction Forces (GRFs) during locomotion [1,2,3,4].

GRFs are critical metrics in orthotic design, categorized into vertical, mediolateral, and anterior-posterior components. Vertical GRF is the primary focus for clinicians as it represents the largest force component during gait and is essential for musculoskeletal injury analysis, gait abnormality detection, and the evaluation of prosthetic and orthotic interventions [5,6,7,8,9,10,11,12,13,14,15,16,17,18].

Current orthotic designs primarily target diabetes-related neuropathy, the most prevalent subtype [19,20,21,22,23,24]. Consequently, existing rehabilitative orthoses often overlook other etiologies (e.g., oncology or trauma-related neuropathy) and fail to utilize novel materials that might better mitigate balance and coordination deficits. A comprehensive understanding of specific pathologies and foot types is necessary to inform effective rehabilitative strategies [25,26]. Given the complexity of neuropathy, management must include a detailed evaluation of etiology-specific characteristics, including GRFs [26].

Orthotics alleviate neuropathic pain by providing cushioning and reducing pressure points [27,28,29]. While custom-made orthotics are generally preferred [29,30,31], auxetic foams offer a novel alternative. Characterized by a negative Poisson’s ratio—meaning they expand when stretched and contract when compressed [31] (Figure 1) —these materials provide superior shape conformity, energy absorption, and indentation resistance [32,33,34,35,36,37,38,39]. These unique properties suggest that auxetic foams may be more effective than traditional materials at reducing GRFs across heterogeneous neuropathic subtypes.

2. Materials and Methods

Participants

Six participants were recruited from NASA (Table 1). The cohort included five individuals with neuropathy attributed to diabetes (Subjects A & C), arthritis (Subject B), cancer (Subject D), and personal injury (Subject F), plus one healthy control (Subject E). Subject F utilized leg braces for additional stability. Inclusion criteria mandated a minimum age of 40 years to reflect the higher epidemiological prevalence of peripheral neuropathy in older populations. With the exception of the healthy control, all participants were required to have a confirmed diagnosis of neuropathy. All participants provided informed consent as approved by NASA’s Institutional Review Board.

Auxetic Foam and Insole Fabrication

The auxetic material was manufactured at FAMU-FSU via tri-axial compression of conventional polyurethane (PU) foam under elevated temperatures. This process induces buckling of the reticular cell ribs to create a re-entrant structure, which is then fixed into place [38,39]. The material formulation involved reactive foaming and phase separation of urethane, alcohol, and water( Figure 1b).

Figure 1. b. Illustration of typical polyurethane foaming reaction and phase-separated morphology.

Figure 1. b. Illustration of typical polyurethane foaming reaction and phase-separated morphology.

To ensure proper fit, insoles were extracted from standard OTC brands (Eduropro, Comfort Gel, and Athletic Works) to serve as templates. These templates were traced onto the auxetic foam and cut to size, ensuring compatibility with the participants' existing footwear (sizes 6–13) (Figure 2).

Procedures

Baseline data, including age, weight, stride length, and shoe size, were recorded upon arrival(Table 1). Participants performed six walking trials at a self-selected pace across four force plates (Figure 3). Each trial consisted of walking four steps, turning, and returning to the starting position to ensure data collection for both the left and right feet. Testing was conducted under three conditions:

1. Barefoot (with socks).

2. OTC Insoles (non-auxetic).

3. Auxetic Foam Insoles.

Subject F performed additional trials with and without leg braces to isolate the impact of the auxetic material on their specific orthotic requirements. In each cycle, the participants’ peak vertical GRFS were evaluated (Tables A 2–5), and their gait cycles were simultaneously observed using motion capture (MARVUE Video Camera Camcorder 2D device). Figure 3(b) illustrates a participant in the barefoot condition. Data from the fourth force plate were excluded from the final analysis due to inconsistent foot strikes; consequently, peak vertical GRFs from the remaining three plates were utilized.

Figure 3. b.Subject walking and representative measurement of GRF in the barefoot(with socks) condition.

Figure 3. b.Subject walking and representative measurement of GRF in the barefoot(with socks) condition.

Measures and Analysis

Peak vertical GRFs and qualitative comfort assessments were compared between auxetic foam and OTC insoles. Due to the small sample size, normality was not assumed; therefore, this feasibility study utilized 3 × 5 (insole × neuropathy subtype) Kruskal-Wallis tests. To account for the two subjects with diabetic neuropathy (A and C) and the use of leg braces by Subject F, four separate statistical iterations were performed(Tables A 6–9). These groupings allowed for a granular analysis of how auxetic material impacts GRFs across specific neuropathic subtypes. The independent variables were insole condition and neuropathic subtype, while peak vertical GRF served as the dependent variable. Data were numerically coded for group identification. The alpha level was set at p<0.05, with a post-hoc Dunn’s test applied to identify specific pairwise differences. Additionally, mean vertical GRFs were calculated per condition to quantify the percentage increase or decrease associated with the auxetic material.

3. Results

Statistically significant differences in vertical GRFs were observed across neuropathic subtypes and insole conditions ( p=0.0001; Tables A 6–9), confirming that both the etiology and the orthotic material significantly influence force distribution. An interaction effect between insole type and neuropathy was also identified. Specifically, auxetic foam reduced vertical GRFs in Subject A (diabetes), Subject D (arthritis), and Subject F (injury, without braces) compared to OTC insoles (Figure 4). Conversely, slight increases in GRFs were noted in Subjects B (cancer), C (diabetes), E (control), and F (injury, with braces) (Figure 5). These increases were attributed to external factors: swift walking cadences in Subjects E and C, the absence of assistive walking sticks for Subject B, and the mechanical influence of leg braces for Subject F. Despite varied GRF data, motion capture confirmed that all participants exhibited improved gait cycles when using the auxetic material.

4. Discussion

This study assessed the efficacy of auxetic foams—multi-phase cellular materials with a negative Poisson’s ratio—in attenuating vertical GRFs compared to standard OTC insoles. This research challenges traditional orthotic design paradigms by applying systems engineering principles to address diverse neuropathic subtypes beyond diabetes. A robust engineering system must account for the synergy between its subsystems; here, the pathological variables of neuropathy and the mechanical properties of rehabilitative orthoses are treated as inseparable components of the patient’s ambulatory system.

The synthesized foam features a re-entrant structure that integrates hard domains for structural integrity with soft domains for elasticity (Figure 1b). Unlike conventional materials, auxetics expand under tension, providing superior shock absorption and pressure relief. The data confirmed that significant performance differences exist across heterogeneous neuropathic conditions. While GRF reductions in Subjects A, D, and F highlight the material's potential for pressure attenuation, instances of increased GRFs in other subjects were likely driven by confounding variables, such as increased gait velocity or the absence of standard mobility aids, rather than material failure. Ultimately, the auxetic foam enhanced gait symmetry for all participants, suggesting benefits beyond simple force reduction that include improved proprioceptive control and overall system stability.

Qualitative Findings

Qualitatively, the auxetic foam insoles significantly enhanced gait by providing superior support and facilitating a more natural heel-to-toe weight transfer.

• Subject A (Diabetic Neuropathy): Transitioned from an antalgic, "twisted" walk with frequent pauses to a fluid, natural gait (Figure 6).

• Subject B (Cancer-related Neuropathy): Showed a reduction in a pre-existing limp, though pauses remained due to the absence of their usual walking sticks.

• Subject C (Diabetic Neuropathy): Exhibited improved weight distribution, though a swifter walking pace resulted in higher recorded forces.

• Subject D (Arthritic Neuropathy): Corrected a steppage gait; instead of landing on the toes, the subject achieved a midfoot-to-heel strike, resulting in a visible improvement in coordination and reduced GRFs (Figure 7).

• Subject E (Control): Displayed greater stability and control despite an increase in force due to a naturally rapid cadence.

• Subject F (Injury-related Neuropathy): Showed a reduction in steppage gait characteristics, gaining more stability from the material's conforming properties.

Limitations and Future Research

This study represents an initial feasibility investigation; thus, the auxetic foam material remains exploratory. In addition to the small sample size, the findings have several limitations. The participant cohort provided only a narrow representation of the broader neuropathic population. Future research should include a larger, more diverse population that varies in age, duration of symptoms, and the specific etiologies of cancer, diabetes, or injury that led to neuropathy. Recruitment challenges due to participants' occupational demands were also noted.

Furthermore, discerning the precise root causes of neuropathy proved challenging when participants presented with potential dual etiologies (e.g., Subject D attributed their condition primarily to arthritis but also had a history of cancer). Such confounding variables complicate the ability to definitively pinpoint the neuropathic cause.

Future iterations of this research should focus on the specificities of these variables (e.g., type 1 vs. type 2 diabetes, type of cancer, nature of injury) during the assessment. Additionally, incorporating more kinematic assessments would allow for an analysis of the auxetic foam’s impact on targeted muscle groups. These steps are necessary to inform the improved design of orthoses for diverse neuropathic conditions.

5. Conclusions

The auxetic foam material proved to be an effective aid in reducing peak vertical GRFs in a subset of neuropathic participants and improving gait cycles across various neuropathic subtypes compared to standard OTC insoles. While the effectiveness varied among subjects, the auxetic foam consistently enhanced comfort and stability for all participants. Overall, the study demonstrated both kinematic and kinetic improvements, warranting further investigation into this promising orthotic material.

Author Contributions:. Conceptualization, L.B..; Methodology, L.B., C.Z., E.J. and S.P.; Software, L.B. and C.Z..; Validation, L.B., C.Z., E.J., and S.P.; Formal Analysis, L.B.; Investigation, L.B, C.Z., E.J., and S.P..; Resources, L.B., C.Z, and E.J..; Data Curation, L.B.; Writing – Original Draft Preparation, L.B. and C.Z. .; Writing – Review & Editing, L.B., C.Z., E.J., and S.P..; Supervision, L.B., C.Z., E.J., and S.P; Project Administration, L.B.