For surgical wounds (i.e., cleaned and then stapled or sutured wounds) for planned healing by first intention (without the prior regrowth of missing tissue, i.e., healing immediately), the primary purpose of a dressing is to protect the wound from the environment [1, 3].

Traditional textile wound dressings are nearly always used since “modern” (moist) dressings offer no advantage and are much more expensive. Woven, knitted, braided and non-woven fabrics are used as textiles in direct contact with the wound.

Apart from synthetic fibres (PES - polyether sulfone, PP - polypropylene, PE - polyethylene), dressings usually consist mainly of biocompatible polysaccharides: cotton and viscose fibres in the form of, say, traditionally woven gauze compresses. These are very soft and supple, and are distinguished by their absorbency and air permeability.

Coatings can be used to effectively reduce their tendency to stick to the wound on direct contact.

In primary healing wounds (such as surgical wounds), the regenerative powers of the body are usually sufficient for the wound to heal rapidly (usually within about 10 to 14 days), assuming no adverse events. Such adverse events chiefly comprise (1) bacterial inflammation with initially localized increase in temperature and swelling [2], (2) haemorrhage or fluid secretion, with or without rupture of the suture (or staple line), and (3) swelling due to haemorrhage into tissue or fluid accumulation in peri-vulneral tissue.

Because it is not possible to see through dressings, they have to be changed regularly to monitor the condition of the wound, usually by visual inspection. From a medical standpoint, however, it would make far more sense for dressings to remain untouched for much longer – preferably until the stitches or staples are removed.

Instead of inspecting primary healing wounds, there are a number of physical and chemical wound parameters (e.g., temperature, moisture, pH, oxygen partial pressure, peri-vulneral blood flow, etc.) that have the potential to provide relevant, objective quantitative information about the condition of a wound. Using wound dressings with measuring capability could enable these variables to be gauged in order to monitor the wound.

Although there are various conceptual ways of integrating conventional sensors into wound dressings [4-7], a medical drawback they all have in common is that they impair the properties of dressings. Either the smoothness of the surface, the softness and the internal stiffness of dressings are deteriorated by sensors (which are more or less hard objects) positioned on a substrate or packed inside housing over a relatively large area, or the sensors only gather readings from small parts of the wound area.

If the wound dressings used are textile dressings, they need to be functionalized such that they can record selected variables continuously or sufficiently frequently as the wound heals without the need for complicated, physically disruptive sensors.

So far, smart textiles have been largely overlooked in connection with wound treatment (not to mention the care and monitoring of wounds), yet they seem to have great potential in this field. Smart textiles work on the principle of very thin, electrically conductive yarn flexibly integrated into or onto a textile by means of weaving, embroidery, sewing, warp knitting or weft knitting (Fig. 1).

When used as dressings, smart, functionalized textiles have a great advantage that they can still be soft and supple. They do not require hard, bulky, conventional sensors – foreign bodies that could cause further, highly undesirable injuries in or around the wound due to pressure, friction or skin intolerance, etc. Moreover, textile sensor structures can be produced easily and relatively inexpensively in almost any size. In addition, they are suitable for simultaneously gauging multiple physical variables, which is a decisive advantage over traditional sensors.

Fig. (1). Photographs of wound dressing textiles with sensor arrays. Left: Photograph of a textile with sensor array and clearly visible red embroidery upper thread. The sensor array consists of two insulated, very thin copper wires largely running in parallel to each other as double meanders, stitched onto the textile using asymmetric double lockstitch. The double meanders run here in ten loops towards the longer side, i.e., the length (L) of the sensor array. This array can be well stretched perpendicular to it (towards its width (B)). Right: Photograph of an area of textile embroidered with an insulated copper wire using asymmetric double lockstitch.

Adequate monitoring of the healing of surgical wounds with functionalized textile dressings requires at least the peri-vulneral detection of any:

(1) Increase in temperature – as an indicator of inflammation;

(2) Sudden increase in moisture/wetness – as an indicator of bleeding or seroma discharge from the wound; and possibly

(3) Elongation – as an indicator of an increase in volume caused by internal haemorrhage or seroma formation.

These variables can all be measured with textile sensors [8, 9].

2.3.1. Temperature Measurement

In a current-carrying conductor, because the ohmic resistance R is essentially a function of temperature-dependent changes in electrical conductivity, it can be used as a measure of temperature. The correlation between resistance and temperature over a temperature range of a few degrees Celsius can be expressed in the following approximation involving the temperature coefficient α₀ , a reference temperature T₀ , and the reference resistance R₀ at a temperature T₀ [10]:

The sensitivity (change in measurand) ΔR/ΔT (dR/dT) is:

The change in ohmic resistance R due to temperature T is constant and rises with increasing reference resistance for a given material.

2.3.2. Moisture Determination

If two insulated, current-carrying conductors run parallel to each other (at least in part, e.g., as a meander or spiral) in a textile, in functional terms they form a planar (two-dimensional) capacitor with a capacitance C. The dielectric of this capacitor is predominantly formed by the textile, air and/or moisture between the conductors. In a dressing, an increase in particular in the amount of water due to the secretion of tissue fluid or blood leads to a significant change in the relative dielectric (ε_r,dry ≈ ε_r,textile ≈ ε_r,air ≈ 1, and ε_r,moisture = ε_r,water ≈ 80), altering the capacitance C.

Two wires with radius r running parallel to each other with spacing serve as a very simple model to estimate the determinant correlation which can be expected. At low frequencies, using the relative permittivity ε_r of the insulator as well as the electric field constant ε₀ (= 8.85 . 10^–12 As/Vm), the capacitance constant C* (capacitance per length of conductor) can be estimated as follows [10]:

The sensitivity ΔC^*/Δε_r (dC^*/dε_r) is:

The change in capacitance C due to a change in relative permittivity ε_r in this model is constant; it is correlated to the ratio of a to r. The capacitance values rise with increasing permittivity.

The correlation with the spacing between the conductors ΔC^*/Δa (dC^*/da) can be estimated as follows:

The change in capacitance C due to a change in geometry, for example, by increasing the spacing a, is not constant but negatively reciprocal (inverse) to a. The capacitance values decline as the distance increases compared to the change in capacitance C due to a change in permittivity.

2.3.3. Determination of Elongation

If, for example, only one meandering insulated electrical conductor in the textile is considered, in functional terms, it forms a planar (two-dimensional) coil with an inductance L. If the geometry is changed – if for instance the spacing increases due to the elongation or upwards bulge (third dimension) of the dressing – this inductance changes.

The very simple two-wire model introduced above can also be used to estimate the expected determinant correlation in connection with inductance. At low frequencies, given the relative permeability µ_r of the insulator (≈ 1) as well as the magnetic field constant µ₀ (= 4π . 10^–7 Vs/Am), the inductance constant L* (inductance per length of conductor) can be estimated as follows [10]:

The correlation ΔL^*/Δa (dL^*/da) can be estimated as:

The change in inductance L due to elongation (change in geometry) is reciprocally proportional to the spacing a without a elongation.

Liquids (wound fluid or blood) do not affect the relative permeability µ_r and therefore have no influence on inductance either.

consisted of a textile backing (‘white/heavy/elastic cotton jersey, type SW 45542-5003’, 95% cotton, 5% elastane, with 30% reversible stretch) and ‘CU-ETP 99.95%’ electrical conductors made of insulated enamelled copper wire (Elektrisola Dr. Gerd Schildbach, Reichshof / D). The textile sensors were contract-manufactured by a university textile institute as double meanders by means of asymmetric double lockstitch (pick-up embroidery with a seam thread as the upper thread, electrical conductors as the lower thread) using a ‘JF 0111-500’ programmable embroidery machine (ZSK Stick-Maschinen, Krefeld / D).

The parameters of the sample dressings are (1) the diameter of the electrical conductors (d), (2) the distance between an electrical conductor and the next (parallel) conductor (a), (3) the spacing of the embroidery seams (upper thread loops) along a conductor (b), and (4) the length and width of the double meander sensor array (L, B).

The following were measured by way of example:

• Sample type for moisture measurement: d = 0.071 mm, a = 1 mm, b = 1 mm, L ∙ B = 80 mm ∙ 40 mm
• Sample type for elongation: d = 0.071 mm, a = 0.5 mm, b = 3 mm, L ∙ B = 40 mm ∙ 30 mm

A ‘Precision Impedance Analyzer WK 6500/5 B’ (Wayne Kerr Electronics, Chichester / UK) with gold-plated terminals in 4-wire technology was used to measure ohmic resistance R, electrical capacitance C, and electrical inductance L.

The temperature was measured using Pt100 sensors in 4-wire technology together with a Pt100 PC-based data logger measuring system ‘PT-104’ (Pico Technology, obtained from PSE Priggen Special Electronics, Steinfurt / D)

Distilled water was added manually with a piston-stroke pipette (‘Series 2000 Reference variable 0.5 to 10 μL’, Eppendorf / D). The pipetting tolerances (according to DIN 12650) are 1.9% for 5 μL and 1.2% for 10 μL.

To measure sensor dressings, suitable measuring stands are required that simulate the properties of the human skin to be measured by the wound dressing, i.e., technical skin models.

We did not find any commercially available technical models of the body surface with the main properties of the skin specific to the measured variables (above all, constant temperature in certain areas but varying locally as well as surface and material properties) for systematic investigations. Similarly, no descriptions of assemblies by other researchers for comparable purposes were found that could have served as a model.

4.1.2. Description of the Technical Skin Models

4.1.2.1. Technical Skin Temperature Model

In Fig. (2), the liquid (water) of a circulating thermostat (‘F 32-HL’, Julabo, Seelbach / D) flows through a U-shaped channel (diameter: 10 mm) in an aluminium block (B ∙ L ∙ H = 100 mm ∙ 100 mm ∙ 50 mm; flow rate: about 700 mL/min). The aluminium block is surrounded on all sides and at the bottom by porous foam insulation about 18 mm thick.

After a while, an equilibrium is reached between the heat supplied from the circulating water and the heat emitted into the environment on all surfaces, including the upper one. On the top of the block is a replaceable plate (L ∙ W ∙ H = 100 mm . 100 mm . 10 mm) made of black POM (polyoxy methylene, polyacetal), a poor heat conductor that has good mechanical working properties (heat transfer coefficient: λ = 0.31 - 0.37 W/(K·m)), with or without a slot (opening, L ∙ W ∙ H = 50 mm ∙ 5 mm ∙ 10 mm), where a cuboid of aluminium (Al) of the same size, acting as an excellent heat conductor (λ = 236 W/(K·m)), promotes heat transfer from the central aluminium block to the surface, thus simulating a warmer (‘inflamed’) skin suture (here: 50 mm ∙ 5 mm). To maximize thermal conductivity, the gap between the aluminium block and the aluminium cuboid is filled with about 2 drops of fine machine oil.

The theoretically different temperatures on the surfaces of the materials mentioned also depend somewhat on their location (due to the slightly different distance from the flowing hot water).

As close as possible below the surface (central distance: about 2 mm) there are some boreholes (diameter: about 3.1 mm) parallel to the surface for Pt100 temperature probes (diameter: about 3.0 mm) to measure the surface temperature. Again, good thermal conductivity in these holes is ensured by fine machine oil (about 1 drop in each hole).

4.1.2.2. Technical Skin Moisture Model

Human tissue (especially subcutaneous tissue and muscles) comprises more than 60% water. To simulate this, the surface on which the sample dressings lie during measurement should also consist mainly of water. A piece of plastic film (fish pond film, thickness: 1 mm) made of EPDM or PVC was chosen (in the absence of practicable alternatives) as a boundary layer (quasi as a skin substitute) above the water and stretched over a small basin (internal dimensions: 170 mm . 90 mm . 50 mm). The temperature-controlled water of a circulation thermostat (circulation thermostat ‘F 32-HL’, Julabo, Seelbach / D) flowed through the basin (flow rate: about 700 mL/min).

A Pt100 temperature probe was inserted through a small hole in the basin wall with its tip directly beneath the centre (distance: max. 1 mm) of the sample dressings lying on the foil. Schematic representations and photographs of the model are contained in Fig. (3). Water was added manually (by pipetting) with a piston-operated pipette.

4.1.2.3. Technical Elongation Model

Determining the electrical inductance and capacitance of planar (or nearly planar) arrays of electrically insulated conductors is practically independent of temperature in the area to be investigated, and the presence of potentially disruptive materials e.g., magnetizable metals) nearby can be avoided when designing measuring stands. Given this, a simple but evidently sufficient way to measure elongation is to hang up a sample dressing from one end and stretch it by suspending weights from the other end.

To hold the sample dressings in place, clamps that were as small and light as possible made mainly from light POM (mass: about 18 g) were used; see Fig. (4), top left. The elongation model with a sample dressing held between two such clamps and suspended for measurement is shown in Fig. (4), right.

Sample wound dressings were suspended from a laboratory stand with clamps on both sides and connected to an LCR meter to measure electrical capacitance or inductance. The terminals of the LCR meter were attached to the samples at a suitable height to avoid unwanted tensile forces (Fig. 4, right).

Fig. (2). Photographs and diagram of the temperature model. Left: The photograph shows the temperature model with foam insulation and a sample dressing (80 mm ∙ 25 mm) covered with two plexiglass plates on the measuring surface. On the right (‘east’) and in front (‘south’), the metal shafts of two Pt100 temperature measuring probes are visible. Their measuring tips are located in boreholes (‘south’) precisely below the centre of the measuring surface (measuring point T1) or (‘east’) about 5 mm to the right (measuring point T2). Centre: The photograph shows the temperature model (without the foam insulation), the aluminium cuboid in the central slot of the POM plate, three boreholes on the right (‘east’) of the POM plate (the middle one leading to measuring point T2), and another borehole on the right (‘east’) in the aluminium block for temperature probes. Right: Diagram of the surface of the temperature model (with slot filled with an aluminium cuboid) and projection of the locations of the reference or measurement temperatures T1 and T2 measured (centre of the temperature probes) about 2 mm below the surface.

Fig. (3). Diagrams (top) and photographs (bottom) of the moisture model. Top left: top view diagram. Top right: cross section diagram. Bottom: Top view and right side view photographs. The inlet of the circulating water is at the bottom left (front), the outlet (overflow) is at the top right (rear). On the right of the model is an open pipe connection, which serves to continuously equalize the pressure of the gas space above the circulating water with the environment. On the surface of the plastic film is a sample dressing (ready for moistening with drops of water).

Fig. (4). Photographs: Top left: Individual clamp for sample dressings Bottom left: Sample dressing with two clamps fitted. Right: The elongation model with a suspended sample dressing stretched by a 100 g weight

A ruler was suspended centrally in the area of the sensor structures in front of the sample dressings in order to determine the elongation of the available section e (as well as the elongated section e + Δe stretched by a certain amount depending on the load) of the width, i.e., the distance (difference in value) between the clamps. To avoid parallax error, the distances on the ruler were always read from an exactly perpendicular angle. Electrical inductances and capacitances were each measured after a few seconds (to allow the entire array to mechanically “come to rest” after manipulation).

When a sample wound dressing is suspended “without” mass loading (m = 0), the distance e between the clamps (e(m) = e(m = 0) ≡ e₀ ) is the portion of the width (B) of the sample that can be stretched laterally between the clamps when weights are applied, since it is not in the area of clamping between the jaws of the clamps. This distance e is:

where

m    Mass of the weights attached (weight force F_G = m · 9.81 m/s²)

e₀     Distance e without suspended mass (m = 0)

Δe  Elongation, i.e., increase of the distance e by a suspended mass

The relative elongation is:

The weights for elongation (50 or 100 g) had a measured relative deviation from the nominal mass of < 0.18%.