A three-dimensional (3D) cast was used to capture intricate friction ridge patterns and achieve a precise replication of the finger’s anatomical shape. Xantopren, commonly used in forensic science for recovering microdetail from toolmark impressions, was selected due to its known capabilities of capturing detail down to 1 μm [18]. This choice was fitting considering the typical thickness of fingermark ridges range from 100 to 300 μm [19].

Casts produced consistently captured a high level of ridge detail. Additionally, the use of Xantopren in its initial liquid state facilitated thorough coverage of the finger, thereby minimizing the formation of air bubbles.

To ensure comprehensive coverage of all ridges within the cast, a medium was needed to facilitate the dispersion of conductive paint during application. For this purpose, Provil® was doped with conductive paint. Successful doping required an adjustment to the manufacturer’s guidance for the ratio of Provil® to activator. Specifically, a ratio of 2:1:1 for Provil®, activator and conductive paint was optimal for facilitating compound hardening, ensuring thorough dispersion of conductive paint throughout the cast.

Following mixing, the doped Provil® was placed into a disposable syringe and applied to the cast using a side-to-side motion ensuring a slight overlap during the application process. Curing times for the doped Provil® were longer compared to the undoped compound (< 60 min).

The conductive cast produced an accurate reproduction of friction ridges (Fig. 2). Despite minor instances of incomplete or absent detail in some areas, the overall quality of the reproduced ridges remained high. Consequently, these casts possess the potential to be utilized for identification purposes with minimal difficulty.

Conductive cast created using Provil® doped with conductive paint. Red circle highlights void areas created in the cast as a result of air pockets

However, the three-dimensional characteristics of the cast posed challenges during photographic documentation, particularly in capturing intricate details of the fingermark. These difficulties were exacerbated by factors such as colour and shininess of the cast, which further impeded visualization methods.

To improve visualization, the conductive cast was inked using the same technique employed in capturing suspect fingerprints, which entailed coating the conductive cast with black ink and delicately rolling it from side to side. Partial transfer of ridge detail was achieved (Fig. 3 (a)), although void areas were evident. While the conductive casts displayed a certain level of flexibility, it did not match that of human fingertips. Consequently, the presence of voids can be attributed to the reduced flexibility, which prevented the cast from achieving full contact with the surface of the paper.

To overcome this, a small amount of magnetic powder was applied to the conductive cast using a magnetic wand. The conductive cast was then rolled over a piece of JLar tape and affixed onto a transparent acetate sheet using a roller. Consistent with earlier observations, some voids were encountered (Fig. 3 (b)). The use of transparent materials (acetate and JLar) enabled the recovery of multiple lifts, facilitating the overlay of retrieved fingermarks (Fig. 3 (c)) for comparative analysis. This showed clear third level detail, where the fingermark would be easily identifiable.

Images showing the results of using a conductive cast to create impressions in (a) black ink; (b) impression after application of magnetic powder and performing a single lift; (c) overlay of two magnetic powder lifts

The conductive casts were tested using a range of devices, yielding varying results (Table 1). Notably, all devices equipped with ultrasonic scanners were successfully unlocked. Conversely, a mixed outcome was observed with capacitive scanners, and all attempts to unlock optical scanners using the conductive casts proved unsuccessful. With optical scanners, although the conductive cast was successfully detected by the sensor, indicating sufficient conductivity for sensor recognition, it did not enable successful unlocking. The mixed outcomes observed with capacitive scanners can be attributed to two primary factors. Firstly, the integration of the fingerprint sensor into the home button of the device caused a slight recess. This recess presented challenges when positioning the conductive cast to ensure close contact with the sensor. Secondly, unsuccessful attempts with capacitive scanners were associated with casts exhibiting poor friction ridge projection (Fig. 4 (d – e)). The projection of friction ridges is influenced by various factors including age, handwashing habits, and occupational activities [20]. Furthermore, a decrease in collagen production as individuals age results in a reduction of friction ridge projection [20, 21].

Conductive casts created from participants showing (a - c) good friction ridge projection and (d – e) poor friction ridge projection

In forensic practice, fingermarks are typically categorized into three primary types: latent, patent, and plastic. While latent and patent fingermarks are characterized by a two-dimensional nature, plastic fingermarks exhibit a three-dimensional structure. To evaluate the efficacy of the conductive fingermark method for the recovery and preservation of fingermarks, a series of exhibits commonly encountered in everyday life were investigated. Testing of fingermarks was conducted using the same donor on both capacitive (iPhone 8 plus) and ultrasonic (Samsung S24 Ultra) fingerprint scanners.

The plasticine was manually kneaded for approximately two minutes until it reached body temperature, after which it was shaped into a ball. A fingermark was deposited onto the surface, resulting in the capture of ridge detail (Fig. 5 (a)). Following this, the conductive casting procedure was applied, allowing for the capture of minutiae characteristics; however, these were insufficient to provide a coincident sequence. This limitation can be attributed to the contrast and colouration of the conductive cast (Fig. 5 (b)). Minor voids were observed due to surface imperfections on the plasticine which transferred to the conductive cast ((Fig. 5 (b’)). Despite these imperfections, the conductive cast effectively facilitated the unlocking of both capacitive and ultrasonic scanners tested.

Image showing (a) fingermark deposited in plasticine and (b) conductive cast recovered from plasticine where voids present in the conductive cast (b’)

The modelling dough exhibited greater malleability compared to plasticine, requiring minimal kneading. The material was shaped into a small ball prior to fingermark application. The properties of modelling dough changed upon exposure to air; therefore observations were made under various conditions: (1) fingermarks deposited immediately after kneading (Fig. 6 (a)), (2) fingermark deposited immediately after kneading and left for 24 h under ambient conditions (Fig. 6 (b)), (3) fingermarks deposited after the material was left under ambient conditions for 30 min (Fig. 6 (c)) and (4) fingermarks deposited after the material was left under ambient conditions for 3 h (Fig. 6 (d)). High-quality fingermarks were obtained when the impressions were made promptly after kneading. However, a decline in impression quality was observed (Fig.’6 (b - d)) as environmental exposure time increased. This is attributed to the loss of moisture which caused shrinkage and decreased the malleability of the material. This loss in moisture coincided with the appearance of dots on the surface of the modelling dough.

Fingermarks deposited onto modelling dough when (a) fresh, (b) dried after deposition (24 h), (c) left 30 min before deposition and (d) left 3 h before deposition. Images (a’ – d’) show conductive casts recovered from modelling dough when (a’) fresh, (b’) dried after deposition (24 h), (c’) left 30 min before deposition and (d’) left 3 h before deposition

After the fingermarks were deposited and allowed to age, the conductive cast recovery technique was applied. The results (Fig. 6a’–b’) showed that a good level of ridge detail was obtained from each exhibit. All conductive casts exhibited discernible ridge flow patterns, with some minutiae characteristics visibly present.

However, the samples subjected to ageing under ambient conditions before fingermark deposition (Fig. 6 (c’-d’)) exhibited significantly less detail compared to the fresh samples. Subsequent testing showed that conductive casts obtained when the fingermark was applied immediately after kneading (Fig. 6 (a’-b’)) successfully unlocked the devices tested. Conversely, those in which the fingermark was applied after exposure to ambient conditions (Fig. 6 (c’-d’)) were unsuccessful in unlocking the devices tested.

Unscented white candle wax was melted and then allowed to cool. Once this began solidifying, indicated by a change in colour from clear to white, a fingermark was deposited onto the wax surface. The wax was then allowed to fully solidify.

The wax captured an excellent level of ridge detail, as shown in Fig. 7 (a). The conductive cast recovery technique was applied to this exhibit which resulted in casts retaining a comparable level of ridge detail Fig. 7 (b). Conductive casts obtained from wax exhibited excellent clarity and minutiae details, facilitating the establishment of a coincident sequence. The conductive cast obtained effectively facilitated the unlocking of both capacitive and ultrasonic scanners tested.

Image showing (a) fingermark deposition in wax where ridge detail has been enhanced using black magnetic powder and (b) conductive cast recovered from wax

Anti-climb paint, a petroleum gel-based substance, is formulated to retain its wet and slippery properties indefinitely upon application to a surface [22]. However, tackiness reduces over time [22], therefore the anti-climb paint was applied to an acetate sheet and left for several hours, prior to fingermark deposition, after which primary and secondary fingermarks were deposited. Due to the non-drying nature of anti-climb paint, the recovery technique developed by Davatwal et al. [22] was employed (Fig. 8 (a-b)).

The primary transfer revealed a good level of detail (Fig. 8 (a)), with some third level characteristics clearly visible. However, a notably higher level of detail was obtained for the secondary transfer (Fig. 8 (b)). The conductive casting technique was applied to both primary and secondary transfers. The conductive casts (Fig. 8 (a’ – b’)) showed a good level of ridge detail, with the secondary transfer showing a higher level of detail.

Fingermarks deposited in anti-climb paint and developed using cyanoacrylate ester fuming (CEF). Image (a) shows the developed fingermark from primary transfer, and image (b) from secondary transfer. Images (a’) and (b’) show the conductive casts recovered from CEF for the primary and secondary transfers, respectively

When tested on capacitive and ultrasonic fingerprint scanners, the conductive cast obtained from the primary transfer failed to unlock the devices. In contrast, the conductive cast obtained from the secondary transfer successfully unlocked the devices tested. These results align with previous findings, highlighting the critical role of the quality of ridge detail in the success of this technique.