On the design of a DEA-based device to pot entially assist lower leg disorders: an analytical and FEM investigation accounting for nonlinearities of the leg and device deformations

Background

One of the recommended treatments for disorders associated with the lower extremity venous insufficiency is the application of external mechanical compression. Compression stockings and elastic bandages are widely used for the purpose of compression therapy and are usually designed to exert a specified value or range of compression on the leg. However, the leg deforms under external compression, which can lead to undesirable variations in the amount of compression applied by the compression bandages. In this paper, the use of an active compression bandage (ACB), whose compression can be regulated through an electrical signal, is investigated. The ACB is based on the use of dielectric elastomer actuators. This paper specifically investigates, via both analytical and non-linear numerical simulations, the potential pressure the ACB can apply when the compliancy of the human leg is taken into account. The work underpins the need to account for the compressibility of the leg when designing compression garments for lower extremity venous insufficiency.

Methods

A mathematical model is used to simulate the volumetric change of a calf when compressed. Suitable parameters for this calf model are selected from the literature where the calf, from ankle to knee, is divided into six different regions. An analytical electromechanical model of the ACB, which considers its compliancy as a function of its pre-stretch and electricity applied, is used to predict the ACB’s behavior. Based on these calf and ACB analytical models, a simulation is performed to investigate the interaction between the ACB and the human calf with and without an electrical stimulus applied to the ACB. This simulation is validated by non-linear analysis performed using a software based on the finite element method (FEM). In all simulations, the ACB’s elastomer is stretched to a value in the range between 140 and 220 % of its initial length.

Results

Using data from the literature, the human calf model, which is examined in this work, has different compliancy in its different regions. For example, when a 28.5 mmHg (3.8 kPa) of external compression is applied to the entire calf, the ankle shows a 3.7 % of volume change whereas the knee region undergoes a 2.7 % of volume change. The paper presents the actual pressure in the different regions of the calf for different values of the ACB’s stretch ratio when it is either electrically activated or not activated, and when compliancy of the leg is either considered or not considered. For example, results of the performed simulation show that about 10 % variation in compression in the ankle region is expected when the ACB initially applies 6 kPa and the compressibility of the calf is first considered and then not considered. Such a variation reduces to 5 % when the initial pressure applied by the ACB reduced by half.

Conclusions

Comparison with non-linear FEM simulations show that the analytical models used in this work can closely estimate interaction between an active compression bandage and a human calf. In addition, compliancy of the leg should not be neglected when either designing a compression band or predicting the compressive force it can exert. The methodology proposed in this work can be extended to other types of elastic compression bandages and garments for biomedical applications.

Electronic supplementary material

The online version of this article (doi:10.1186/s12938-015-0088-3) contains supplementary material, which is available to authorized users.

An active compression bandage based on shape memory alloys: a preliminary investigation

BACKGROUND

Disorders associated with excessive swelling of the lower extremities are common. They can be associated with pain, varicose veins, reduced blood pressure when standing and may cause syncope or fainting. The common physical remedy to these disorders is the use of compression stockings and pneumatic compression leg massagers, which both attempt to limit blood pooling and capillary filtration in the lower limbs. However, compression stockings provide a constant pressure, and their efficiency has been challenged according to some recent studies. Air compression leg massagers on the other hand, restricts patient mobility. In this work we therefore present an innovative active compression bandage based on the use of a smart materials technology that could produce intermittent active pressure to mitigate the symptoms of lower extremity disorders.

METHODS

An active compression bandage (ACB), actuated by shape memory alloy (SMA) wires, was designed and prototyped. The ACB was wrapped around a calf model to apply an initial pressure comparable to the one exerted by commercial compression stockings. The ACB was controlled to apply different values of compression. A data acquisition board and a LabVIEW program were used to acquire both the pressure data exerted by the ACB and the electrical current required to actuate the SMA wires. An analytical model of the ACB based on a SMA constitutive model was developed. An optimizer was implemented to identify optimal parameters of the model to best estimate the performance of the ACB.

RESULTS

The maximum increase in pressure due to the SMA wires activation was 40.8% higher than the initially applied pressure to the calf model. The analytical model of the ACB estimated the behaviour of the ACB with less than 0.32 mmHg difference with the experimental results.

CONCLUSIONS

The prototyped ACB was able to apply an initial compression comparable to the one applied by commercial compression stockings. Activation of the ACB resulted in an increase of compression up to 9.06 mmHg. Comparison between analytical and experimental results showed the analytical model was suitable to predict the behaviour of the ACB.

Deep vein thrombosis and Caesarean section

Deep vein thrombosis (DVT) presents a significant risk for women requiring Caesarean section. Venous thromboembolism (VTE) remains the leading direct cause of maternal death in the UK and is the second most common cause of maternal death worldwide, accounting for 11% of maternal deaths (RCOG 2009). This article uses a reflective cycle to explore the role of the preoperative practitioner in the prevention of DVT for women undergoing elective Caesarean section.

Use and wear of anti-embolism stockings: a clinical audit of surgical patients

The objective of this study was to determine if anti-embolism stockings are fitted and worn according to evidence-based guidelines in surgical patients. The methodology is that an observational study was undertaken during January and February, 2010. An audit tool was constructed for the recording of information on a variety of parameters relating to the correct fitting and wearing of anti-embolism stockings. The results show that eighty surgical patients were audited with a mean age of 66 years. Of these, 72.5% were ambulatory; 38.75% of patients had incorrect presentation of their stockings; 29% were fitted with the incorrect size for limb measurement and 70% of patients reported not receiving any information regarding the reason for anti-embolism stocking use. Written information was not received by 100% of participants. For 82.5% of participants deep vein thrombosis (DVT) risk was not recorded. The garment size fitted (and/or limb measurement) was not recorded for 100% of patients. Daily routine skin inspection was noted for 29%. The findings of this study showed that anti-embolism stockings were not fitted and worn according to evidence-based guidelines. The author recommends a new recording tool be developed to ensure consistency of assessment and documentation regarding risk assessment, contraindications identification, and education and monitoring of patients wearing anti-embolism stockings.

Graduated compression stocking and intermittent pneumatic compression device length selection

PURPOSE

The purpose of this project was to incorporate current evidence into length selection for graduated compression stockings (GCS) and intermittent pneumatic compression (IPC) devices in order to evaluate and modify current practice.

RATIONALE

Staff nurses relate concerns regarding the fit and complications related to poorly fitting thigh length GCS and IPC devices. Current evidence supports using knee length over thigh length.

DESCRIPTION OF THE PROJECT

With the use of Melnyk and Fineout-Overholt's model of evidence-based practice and Norwood's consultation model, current practice was evaluated and practice changes were implemented based on results of a literature review.

OUTCOME

Knee length will be selected instead of thigh length in general and urology surgery patients. The department of obstetrics and gynecology has taken the practice change under consideration.

CONCLUSION

Knee-length GCS and IPC devices are as effective as thigh-length GCS and IPC devices, are more comfortable for patients, are easier for staff and patients to use, pose less risk of injury to patients, and are less expensive. Knee-length GCS and IPC devices may safely be used for general, gynecology, and urology surgery patients and are preferable to thigh-length GCS and IPC devices.

IMPLICATIONS

Venous thromboembolism prophylaxis is an important component of surgical care; proper length selection of GCS and IPC devices should be based on current evidence. This project outlines a method to change practice regarding length selection of GCS and IPC devices.