The diabetic foot is a function of many complications, which are typically further amplified with the progression of associated diabetic nephropathy and peripheral vascular disease. This triad compromises the health of the lower limbs, often leading to limb loss [8]. It is therefore critical for clinical teams to continuously observe changes in the lower limbs/feet to allow for quick intervention and prevent the progression of complications. Accordingly, the novel preventative and management strategies discussed in this work focus on the important parameters critical for the identification of inflammation, infection, and poor wound healing. These include wound-related parameters, such as variations in the skin temperature and moisture level, reduced blood flow to the extremities, lack of hair growth, as well as any signs of cuts, blisters, or calluses [8]. Physical and chemical parameters include plantar pressure, magnitude and distribution, shear forces, temperature, humidity, microcirculation, and pH levels. Clinically, monitoring these parameters provides critical insights into foot health and skin barrier function, enabling early detection of complications and facilitating timely and targeted interventions to prevent ulcer formations and promote wound healing. The holistic integration of these multifaceted parameters into foot wearables technologies can provide means for prevention and mitigation by continuously monitoring the dynamic variations, particularly in high-risk regions as depicted in Fig. 2(a).The regions of interest (ROI) identified in most studies focused on the foot’s hallux, metatarsal heads, midfoot, and heel as shown in Fig. 2(b). These areas are typically subjected to high pressure and frequent shear forces which produce normal and shear stresses on the underlying tissue, hence rendering it susceptible to ulcerations. Moreover, it is important to note that the hallux, metatarsal heads, and heel are bony prominences further accentuating the risk of ulceration [23, 24].

(a) The triad of Infection, Peripheral Vascular Disease (PVD), and Diabetic Peripheral Neuropathy (DPN) and their related attributes, (b) ROI are more susceptible to ulcerations. They include the hallux, metatarsal heads, midfoot, and heel

Several physical parameters are associated with the onset and progression of foot ulceration, including plantar pressure magnitude and distribution, shear forces, temperature, and humidity. These attributes collectively provide important insight into the underlying foot condition, and variations in their respective quantitative values can reveal potential foot complications, including inflammation and infection of subcutaneous tissue.

Traditionally, plantar pressure has been considered among the key critical risk factors correlated to foot health and ulcer formation. High repetitive normal strain on the foot can result in injuries to the underlying tissue, leading to the development of ulcerations [23]. Consistent application of excessive pressure aggravates the condition of the injury and can greatly affect the gait of patients [24]. Additionally, the decreased sensation caused by peripheral neuropathy limits the patient’s perception of the external environment and ulcer development.

Another equally important, but less explored factor due to assessment challenges is shear stress. The ground reaction forces (GRF) repetitively exerted on the lower limbs during weight-bearing activities, such as walking, result in cyclic loading in both normal and shear directions. These repetitive stresses have been shown to cause microtrauma, thereby accelerating the breakdown of tissue [25]. Accordingly, extensive research efforts have been devoted to the development of smart wearables and sensor systems for monitoring plantar and more recently shear pressure dynamics.

These modalities include capacitive [26,27,28,29,30,31,32,33,34,35], inductive [36,37,38], resistive [39,40,41,42,43,44,45,46,47,48], piezoresistive [49,50,51,52], piezoelectric [53], and more recently fiber optic-based sensing mechanisms [54,55,56,57,58,59,60,61]. Each type of sensing technology presents advantages and drawbacks as elaborated next. In capacitive sensing systems, pressure is quantified by changes in the gap separating two conductive plates. This gap contains a dielectric material which changes in thickness when the pressure is applied resulting in capacitance variation.

In the system developed by Manikandan et al. [27], capacitive sensors were integrated into an insole to measure the foot’s plantar pressure. A pressure distribution color mapping approach was employed to indicate areas of elevated pressure. In a more recent study by Luna-Perejón et al. [28], capacitive sensors were developed using a polydimethylsiloxane (PDMS) composite which served as a dielectric separating the capacitive plates as shown in Fig. 3(a). The PDMS layer also acted as a shock absorber due to its elastic deformation characteristics. The sensors were then integrated into a shoe to collect plantar pressure data in 12 locations for each foot, providing a suitable assessment modality. The main limitation reported in this study was the electric field generated by the body inducing disturbance in the collected values. On the other hand, these authors recommended the application of multiplication operations to partially remove the disturbance while retaining important gait information. Tang et al. [26] also employed a capacitive sensing mechanism to investigate both normal pressure and shear stress as critical factors associated with ulcer formation. This work offered an innovative sensorized insole capable of normal pressure and shear assessment.

Various sensing mechanisms used in wearables for the management of diabetic foot (a) Scheme of main layers that make up the proposed plantar insole capacitive PDMS [28] CC-BY license, (b) Pressure and shear monitoring using three-coil induction force sensor [36] ©2015 IEEE, (c) Prototype system proposed to monitor and analyze gait by looking specifically at the foot force and the plantar flexion using FSR and FBS sensors [39] CC-BY license

Several other studies employed capacitive sensing methods to analyze pressure distribution patterns through commercially available systems, such as Pedar and Pliance [62, 63]. Raspovic et al. [29] used the Pedar system to validate an affordable, easy-to-use DF pressure relief shoe by evaluating its efficacy in redistributing the pressure away from areas susceptible to ulceration. Similarly, in the study conducted by Begg et al. [30, 31], both plantar and shear pressure were quantified and monitored for patients who had already developed ulcers. In these cases, immediate care is critical to reduce the pressure on the ulcerated sites using a total contact cast (TCC), an effective intervention method commonly employed for pressure reduction. Using capacitive measuring systems (Pedar and Pliance), Begg et al. monitored the pressure between the foot and shoe and measured the contact area, pressure, as well as the maximum forces between the TCC and the anterodistal and posterolateral-distal regions subjected to the highest loads, respectively. Withers et al. [32] also employed a Pedar system to assess the offloading effects of a removable cast walker (RCW). In this study, the authors investigated the impact of plantar pressure, reporting on the efficacy of pressure reduction for diabetes-related foot ulcerations. Van Netten et al. [33] adapted the capacitive Pedar system to investigate plantar pressure for indoor vs. outdoor customized footwear while also studying the adherence of high-risk patients to the prescribed shoes. This study found that both indoor and outdoor footwear provided similar offloading effects. On the other hand, indoor custom-made footwear resulted in significantly higher patient adherence. In another work by Zwaferink et al. [34], the Pedar system was employed for the development of customizable footwear aimed at managing high plantar pressure. High-risk areas were evaluated to recommend important modifications which can be applied to effectively reduce the peak pressure and optimize the footwear. Notably, multiple studies have shown the significance of manufacturing patient-specific footwear customized to the shape, size, and plantar pressure distribution of the patient’s foot to reduce elevated pressure. Forogh et al. [35] conducted a comprehensive study to test the reliability of the Pedar system between different days. While the authors confirmed its accuracy and reliability, they demonstrated that with increased neuropathy, the system’s reliability tends to decrease in between-day tests.

Inductive sensing relies on electromagnetic fields for the detection of surrounding conductive objects. The movement of these objects causes variations in the electromagnetic induction proportional to the applied changes in forces, allowing the measurement of plantar pressure [36]. Du et al. [36] developed a low-cost inductive sensor to enable the simultaneous measurement of normal forces applied on the plantar surface, as well as the two-axis shear forces as shown in Fig. 3(b). The sensor’s small size rendered it practical, as it could be easily embedded into footwear for continuous real-time monitoring. In another study conducted by Wang et al. [37], a shear load inductive plantar sensing (SLIPS) system was developed and integrated into an insole to measure both plantar and shear pressures across the foot. The results of this study verified that the SLIPS system operated efficiently under dynamic gait loading. Further studies also reported the design of tri-axis force sensors which concurrently quantify both normal and shear loads on the plantar surface. Wang et al. [38] investigated the performance of such sensor for different configurations using a computational finite element model, concluding that using four-square coils with the maximum number of turns yielded the optimal sensor configuration.

Resistive sensors are also commonly employed in measuring foot pressure. These leverage the principle where mechanical forces exerted on these sensors causes the conductive particles to move closer, hence resulting in a decrease in the electrical resistance proportional to the applied force. The use of this measurement technique provides a low-cost dynamic approach for measuring the ground reaction forces (GRFs). In a study conducted by Anas [39], a resistive-based prototype system was proposed to monitor and analyze gait by assessing the GRFs as shown in Fig. 3(c). The system is comprised of a Force-Sensitive Resistor (FSR) and a Flexible Bend Sensor (FBS) to measure the foot forces and flexion, respectively. On the other hand, while these sensors are small in size and cost-effective, they tend to exhibit low accuracy and resolution [39]. It is also essential to highlight the importance of FSR sensor placement and its impact on accuracy, as well as the selection of suitable materials which can greatly affect the measured forces. FSR sensors were also used within feedback systems to ensure that the measured peak plantar pressure is maintained within a set threshold, thereby facilitating the customization of therapeutic footwear and enabling patient-specific adjustment [40]. Likewise, Bose et al. [41] reported the development of a smart insole incorporating 11 FSR sensors within silicone inserts designed to measure plantar pressure. The study demonstrated the insole’s efficacy in offloading plantar pressure, highlighting its potential in preventing ulcer progression.

In a study by Ferber et al. [42], the commercially available pressure-sensing array, Orpyx [43], was integrated into an insole to provide real-time feedback. This is of notable significance for neuropathy patients who exhibit limited sensation in the lower limbs towards ulcer and limb loss prevention. Similarly, Matijevich et al. [44] employed Orpyx technology to monitor plantar pressure and temperature, while simultaneously tracking patient step count and adherence. Another study that utilized the Orpyx intelligent insole added an alerting system that notifies the patient of increased pressure. The results showed that alerting patients led to a 71% reduction in ulcers. Additionally, this research team found that continuous feedback reduced the plantar pressure by inducing a neuro-learning response where users were trained to maintain posture that led to pressure variability on the plantar surface of the foot [45, 46]. Further studies employed resistive sensor-based systems, such as the Fscan (Tekscan Inc.,USA) [64] to study the impact of footwear on plantar and shear loading during gait at various walking speed [47, 48].

In contrast to the previously discussed sensing methods, piezoresistive sensors are devices fabricated from materials that exhibit changes in electrical resistance in response to applied mechanical stress due to the piezoresistive effect. This technology was adopted in the development of the WalkinSense, a portable plantar pressure measuring system proposed by Healy et al. [49]. Perrier et al. [50] also used piezoresistive sensing, where a smart diabetic sock was custom-designed to accurately measure plantar pressure in real-life conditions. This customizable, wireless, yet washable prototype has the additional capability to send notifications to the user/clinician to raise alerts in case of ulceration risk. The integration of a warning system in wearables can significantly mitigate the drastic progression of ulcer formation by prompting immediate action to control and minimize risks. Tiwari et al. [51] developed a cost-effective insole composed of a polyester/nylon blend fabric, designed to measure plantar pressure in individuals with diabetes. The insoles encompassed 31 piezoresistive sensors, each capable of detecting pressures up to 960 kPa. The design emphasizes flexibility and durability, ensuring both user comfort and accurate pressure sensing during daily activities. Similarly, Wang et al. [52] developed a novel low-cost flexible sensing system that provided continuous long-term monitoring on a daily basis. The insole included a high-sensitivity pressure sensor array that tracked and transmitted the data wirelessly, hence displaying the changes in real-time through a mobile application.

Similar to piezoresistive sensors, piezoelectric sensors also leverage the properties of specific materials to generate electricity in response to mechanical stress. On the other hand, they produce electrical charges instead of resistive variations when deformed. Klimiec et al. [53] reported on the development of a plantar pressure diagnostic system to analyze the distribution of foot pressure during. According to the authors, this design offered many advantages, including reliability, durability, and user-friendliness.

Optical Fiber Sensors (OFS) present a unique sensing mechanism capable of quantifying different parameters with associated DF pathology. Fiber Bragg Grating sensors (FBGs), acting as a wavelength selective filter, consist of an optical fiber segment with periodic variations of the refractive index fiber core. These variations form the grating structure, which reflects specific wavelengths of light (Bragg wavelength). When the fiber is exposed to light, wavelengths that do not abide by the Bragg condition are transmitted, while those that meet the Bragg condition are reflected. Therefore, when an external stimulus, such as strain/ pressure or temperature, is induced in the structure containing the Bragg sensor, the Bragg wavelength shifts triggering a change in the reflected wavelength peak. This shift can be monitored and analyzed to quantify the interrelated stimulus [54, 55]. Figure 4(a) demonstrates the working mechanism of FBGs.

(a) The sensing mechanism of FBG sensors [55] ©2019 IEEE, (b) A diagram showing FBG sensors integrated into a shoe insole [55] ©2019 IEEE

Numerous studies have explored the use of FBGs in monitoring plantar pressure, as demonstrated by Suresh et al. [