Extrusion-based 3D printing of biodegradable, osteogenic, paramagnetic, and porous FeMn-akermanite bone substitutes.

The development of biodegradable Fe-based bone implants has rapidly progressed in recent years. Most of the challenges encountered in developing such implants have been tackled individually or in combination using additive manufacturing technologies. Yet not all the challenges have been overcome. Herein, we present porous FeMn-akermanite composite scaffolds fabricated by extrusion-based 3D printing to address the unmet clinical needs associated with Fe-based biomaterials for bone regeneration, including low biodegradation rate, MRI-incompatibility, mechanical properties, and limited bioactivity. In this research, we developed inks containing Fe, 35 wt% Mn, and 20 or 30 vol% akermanite powder mixtures. 3D printing was optimized together with the debinding and sintering steps to obtain scaffolds with interconnected porosity of 69%. The Fe-matrix in the composites contained the γ-FeMn phase as well as nesosilicate phases. The former made the composites paramagnetic and, thus, MRI-friendly. The in vitro biodegradation rates of the composites with 20 and 30 vol% akermanite were respectively 0.24 and 0.27 mm/y, falling within the ideal range of biodegradation rates for bone substitution. The yield strengths of the porous composites stayed within the range of the values of the trabecular bone, despite in vitro biodegradation for 28 d. All the composite scaffolds favored the adhesion, proliferation, and osteogenic differentiation of preosteoblasts, as revealed by Runx2 assay. Moreover, osteopontin was detected in the extracellular matrix of cells on the scaffolds. Altogether, these results demonstrate the remarkable potential of these composites in fulfilling the requirements of porous biodegradable bone substitutes, motivating future in vivo research. STATEMENT OF SIGNIFICANCE: We developed FeMn-akermanite composite scaffolds by taking advantage of the multi-material capacity of extrusion-based 3D printing. Our results demonstrated that the FeMn-akermanite scaffolds showed an exceptional performance in fulfilling all the requirements for bone substitution in vitro, i.e., a sufficient biodegradation rate, having mechanical properties in the range of trabecular bone even after 4 weeks biodegradation, paramagnetic, cytocompatible and most importantly osteogenic. Our results encourage further research on Fe-based bone implants in in vivo.

Additive manufacturing of bioactive and biodegradable porous iron-akermanite composites for bone regeneration.

Advanced additive manufacturing techniques have been recently used to tackle the two fundamental challenges of biodegradable Fe-based bone-substituting materials, namely low rate of biodegradation and insufficient bioactivity. While additively manufactured porous iron has been somewhat successful in addressing the first challenge, the limited bioactivity of these biomaterials hinder their progress towards clinical application. Herein, we used extrusion-based 3D printing for additive manufacturing of iron-matrix composites containing silicate-based bioceramic particles (akermanite), thereby addressing both of the abovementioned challenges. We developed inks that carried iron and 5, 10, 15, or 20 vol% of akermanite powder mixtures for the 3D printing process and optimized the debinding and sintering steps to produce geometrically-ordered iron-akermanite composites with an open porosity of 69-71%. The composite scaffolds preserved the designed geometry and the original α-Fe and akermanite phases. The in vitro biodegradation rates of the composites were improved as much as 2.6 times the biodegradation rate of geometrically identical pure iron. The yield strengths and elastic moduli of the scaffolds remained within the range of the mechanical properties of the cancellous bone, even after 28 days of biodegradation. The composite scaffolds (10-20 vol% akermanite) demonstrated improved MC3T3-E1 cell adhesion and higher levels of cell proliferation. The cellular secretion of collagen type-1 and the alkaline phosphatase activity on the composite scaffolds (10-20 vol% akermanite) were, respectively higher than and comparable to Ti6Al4V in osteogenic medium. Taken together, these results clearly show the potential of 3D printed porous iron-akermanite composites for further development as promising bone substitutes. STATEMENT OF SIGNIFICANCE: Porous iron matrix composites containing akermanite particles were produced by means of multi-material additive manufacturing to address the two fundamental challenges associated with biodegradable iron-based biomaterials, namely very low rate of biodegradation and insufficient bioactivity. Our porous iron-akermanite composites exhibited enhanced biodegradability and superior bioactivity compared to porous monolithic iron scaffolds. The murine bone cells proliferated on the composite scaffolds, and secreted the collagen type-1 matrix that stimulated bony-like mineralization. The results show the exceptional potential of the developed porous iron-based composite scaffolds for application as bone substitutes.

Poly(2-ethyl-2-oxazoline) coating of additively manufactured biodegradable porous iron.

Additively manufacturing of porous iron offers a unique opportunity to increase its biodegradation rate by taking advantage of arbitrarily complex porous structures. Nevertheless, achieving the required biodegradation profile remains challenging due to the natural passivation of iron that decrease the biodegradation rate. Moreover, the biocompatibility of iron is reported to be limited. Here, we address both challenges by applying poly(2-ethyl-2-oxazoline) coating to extrusion-based 3D printed porous iron. We characterized the specimens by performing in vitro biodegradation, electrochemical measurements, time-dependent mechanical tests, and in vitro cytocompatibility assays. The coated porous iron exhibited a biodegradation rate that was 2.6× higher than that of non-coated counterpart and maintained the bone-mimicking mechanical properties throughout biodegradation. Despite the formation of dense biodegradation products, the coating ensured a relatively stable biodegradation (i.e., 17% reduction in the degradation rate between days 14 and 28) as compared to that of non-coated specimens (i.e., 43% drop). Furthermore, the coating could be identified even after biodegradation, demonstrating the longevity of the coating. Finally, the coated specimens significantly increased the viability and supported the attachment and growth of preosteoblasts. Our results demonstrate the great potential of poly(2-ethyl-2-oxazoline) coating for addressing the multiple challenges associated with the clinical adoption of porous iron.

Extrusion-based 3D printed magnesium scaffolds with multifunctional MgF2 and MgF2-CaP coatings.

Additively manufactured (AM) biodegradable magnesium (Mg) scaffolds with precisely controlled and fully interconnected porous structures offer unprecedented potential as temporary bone substitutes and for bone regeneration in critical-sized bone defects. However, current attempts to apply AM techniques, mainly powder bed fusion AM, for the preparation of Mg scaffolds, have encountered some crucial difficulties related to safety in AM operations and severe oxidation during AM processes. To avoid these difficulties, extrusion-based 3D printing has been recently developed to prepare porous Mg scaffolds with highly interconnected structures. However, limited bioactivity and a too high rate of biodegradation remain the major challenges that need to be addressed. Here, we present a new generation of extrusion-based 3D printed porous Mg scaffolds that are coated with MgF2 and MgF2-CaP to improve their corrosion resistance and biocompatibility, thereby bringing the AM scaffolds closer to meeting the clinical requirements for bone substitutes. The mechanical properties, in vitro biodegradation behavior, electrochemical response, and biocompatibility of the 3D printed Mg scaffolds with a macroporosity of 55% and a strut density of 92% were evaluated. Furthermore, comparisons were made between the bare scaffolds and the scaffolds with coatings. The coating not only covered the struts but also infiltrated the struts through micropores, resulting in decreases in both macro- and micro-porosity. The bare Mg scaffolds exhibited poor corrosion resistance due to the highly interconnected porous structure, while the MgF2-CaP coatings remarkably improved the corrosion resistance, lowering the biodegradation rate of the scaffolds down to 0.2 mm y-1. The compressive mechanical properties of the bare and coated Mg scaffolds before and during in vitro immersion tests for up to 7 days were both in the range of the values reported for the trabecular bone. Moreover, direct culture of MC3T3-E1 preosteoblasts on the coated Mg scaffolds confirmed their good biocompatibility. Overall, this study clearly demonstrated the great potential of MgF2-CaP coated porous Mg prepared by extrusion-based 3D printing for further development as a bone substitute.

Extrusion-based 3D printing of ex situ-alloyed highly biodegradable MRI-friendly porous iron-manganese scaffolds.

Additively manufactured biodegradable porous iron has been only very recently demonstrated. Two major limitations of such a biomaterial are very low biodegradability and incompatibility with magnetic resonance imaging (MRI). Here, we present a novel biomaterial that resolves both of those limitations. We used extrusion-based 3D printing to fabricate ex situ-alloyed biodegradable iron-manganese scaffolds that are non-ferromagnetic and exhibit enhanced rates of biodegradation. We developed ink formulations containing iron and 25, 30, or 35 wt% manganese powders, and debinding and sintering process to achieve Fe-Mn scaffolds with 69% porosity. The Fe25Mn scaffolds had the ε-martensite and γ-austenite phases, while the Fe30Mn and Fe35Mn scaffolds had only the γ-austenite phase. All iron-manganese alloys exhibited weakly paramagnetic behavior, confirming their potential to be used as MRI-friendly bone substitutes. The in vitro biodegradation rates of the scaffolds were very much enhanced (i.e., 4.0 to 4.6 times higher than that of porous iron), with the Fe35Mn alloy exhibiting the highest rate of biodegradation (i.e., 0.23 mm/y). While the elastic moduli and yield strengths of the scaffolds decreased over 28 days of in vitro biodegradation, those values remained in the range of cancellous bone. The culture of preosteoblasts on the porous iron-manganese scaffolds revealed that cells could develop filopodia on the scaffolds, but their viability was reduced by the effect of biodegradation. Altogether, this research marks a major breakthrough and demonstrates the great prospects of multi-material extrusion-based 3D printing to further address the remaining issues of porous iron-based materials and, eventually, develop ideal bone substitutes. STATEMENT OF SIGNIFICANCE: 3D printed porous iron biomaterials for bone substitution still encounter limitations, such as the slow biodegradation and magnetic resonance imaging incompatibility. Aiming to solve the two fundamental issues of iron, we present ex-situ alloyed porous iron-manganese scaffolds fabricated by means of multi-material extrusion-based 3D printing. Our porous iron-manganese possessed enhanced biodegradability, non-ferromagnetic property, and bone-mimicking mechanical property throughout the in vitro biodegradation period. The results demonstrated a great prospect of multi-material extrusion-based 3D printing to further address the remaining challenges of porous iron-based biomaterials to be an ideal biodegradable bone substitutes.

Extrusion-based 3D printed biodegradable porous iron.

Extrusion-based 3D printing followed by debinding and sintering is a powerful approach that allows for the fabrication of porous scaffolds from materials (or material combinations) that are otherwise very challenging to process using other additive manufacturing techniques. Iron is one of the materials that have been recently shown to be amenable to processing using this approach. Indeed, a fully interconnected porous design has the potential of resolving the fundamental issue regarding bulk iron, namely a very low rate of biodegradation. However, no extensive evaluation of the biodegradation behavior and properties of porous iron scaffolds made by extrusion-based 3D printing has been reported. Therefore, the in vitro biodegradation behavior, electrochemical response, evolution of mechanical properties along with biodegradation, and responses of an osteoblastic cell line to the 3D printed iron scaffolds were studied. An ink formulation, as well as matching 3D printing, debinding and sintering conditions, was developed to create iron scaffolds with a porosity of 67%, a pore interconnectivity of 96%, and a strut density of 89% after sintering. X-ray diffracometry confirmed the presence of the α-iron phase in the scaffolds without any residuals from the rest of the ink. Owing to the presence of geometrically designed macropores and random micropores in the struts, the in vitro corrosion rate of the scaffolds was much improved as compared to the bulk counterpart, with 7% mass loss after 28 days. The mechanical properties of the scaffolds remained in the range of those of trabecular bone despite 28 days of in vitro biodegradation. The direct culture of MC3T3-E1 preosteoblasts on the scaffolds led to a substantial reduction in living cell count, caused by a high concentration of iron ions, as revealed by the indirect assays. On the other hand, the ability of the cells to spread and form filopodia indicated the cytocompatibility of the corrosion products. Taken together, this study shows the great potential of extrusion-based 3D printed porous iron to be further developed as a biodegradable bone substituting biomaterial.

Additively manufactured biodegradable porous zinc.

Additively manufacturing (AM) opens up the possibility for biodegradable metals to possess uniquely combined characteristics that are desired for bone substitution, including bone-mimicking mechanical properties, topologically ordered porous structure, pore interconnectivity and biodegradability. Zinc is considered to be one of the promising biomaterials with respect to biodegradation rate and biocompatibility. However, no information regarding the biodegradability and biocompatibility of topologically ordered AM porous zinc is yet available. Here, we applied powder bed fusion to fabricate porous zinc with a topologically ordered diamond structure. An integrative study was conducted on the static and dynamic biodegradation behavior (in vitro, up to 4 weeks), evolution of mechanical properties with increasing immersion time, electrochemical performance, and biocompatibility of the AM porous zinc. The specimens lost 7.8% of their weight after 4 weeks of dynamic immersion in a revised simulated body fluid. The mechanisms of biodegradation were site-dependent and differed from the top of the specimens to the bottom. During the whole in vitro immersion time of 4 weeks, the elastic modulus values of the AM porous zinc (E = 700-1000 MPa) even increased and remained within the scope of those of cancellous bone. Indirect cytotoxicity revealed good cellular activity up to 72 h according to ISO 10,993-5 and -12. Live-dead staining confirmed good viability of MG-63 cells cultured on the surface of the AM porous zinc. These important findings could open up unprecedented opportunities for the development of multifunctional bone substituting materials that will enable reconstruction and regeneration of critical-size load-bearing bone defects. STATEMENT OF SIGNIFICANCE: No information regarding the biodegradability and biocompatibility of topologically ordered AM porous zinc is available. We applied selective laser melting to fabricate topologically ordered porous zinc and conducted a comprehensive study on the biodegradation behavior, electrochemical performance, time-dependent mechanical properties, and biocompatibility of the scaffolds. The specimens lost 7.8% of their weight after4 weeks dynamic biodegradation while their mechanical properties surprisingly increased after 4 weeks. Indirect cytotoxicity revealed good cellular activity up to 72 h. Intimate contact between MG-63 cells and the scaffolds was also observed. These important findings could open up unprecedented opportunities for the development of multifunctional bone substituting materials that mimic bone properties and enable full regeneration of critical-size load-bearing bony defects.

Additively manufactured biodegradable porous iron.

Additively manufactured (AM) topologically ordered porous metallic biomaterials with the proper biodegradation profile offer a unique combination of properties ideal for bone regeneration. These include a fully interconnected porous structure, bone-mimicking mechanical properties, and the possibility of fully regenerating bony defects. Most of such biomaterials are, however, based on magnesium and, thus, degrade too fast. Here, we present the first report on topologically ordered porous iron made by Direct Metal Printing (DMP). The topological design was based on a repetitive diamond unit cell. We conducted a comprehensive study on the in vitro biodegradation behavior (up to 28 days), electrochemical performance, time-dependent mechanical properties, and biocompatibility of the scaffolds. The mechanical properties of AM porous iron (E = 1600-1800 MPa) were still within the range of the values reported for trabecular bone after 28 days of biodegradation. Electrochemical tests showed up to ≈12 times higher rates of biodegradation for AM porous iron as compared to that of cold-rolled (CR) iron, while only 3.1% of weight loss was measured after 4 weeks of immersion tests. The biodegradation mechanisms were found to be topology-dependent and different between the periphery and central parts of the scaffolds. While direct contact between MG-63 cells and scaffolds revealed substantial and almost instant cytotoxicity in static cell culture, as compared to Ti-6Al-4V, the cytocompatibility according to ISO 10993 was reasonable in in vitro assays for up to 72 h. This study shows how DMP could be used to increase the surface area and decrease the grain sizes of topologically ordered porous metallic biomaterials made from metals that are usually considered to degrade too slowly (e.g., iron), opening up many new opportunities for the development of biodegradable metallic biomaterials. STATEMENT OF SIGNIFICANCE: Biodegradation in general and proper biodegradation profile in particular are perhaps the most important requirements that additively manufactured (AM) topologically ordered porous metallic biomaterials should offer in order to become the ideal biomaterial for bone regeneration. Currently, most biodegradable metallic biomaterials are based on magnesium, which degrade fast with gas generation. Here, we present the first report on topologically ordered porous iron made by Direct Metal Printing (DMP). We also conducted a comprehensive study on the biodegradation behavior, electrochemical performance, biocompatibility, and the time evolution of the mechanical properties of the implants. We show that these implants possess bone-mimicking mechanical properties, accelerated degradation rate, and reasonable cytocompatibility, opening up many new opportunities for the development of iron-based biodegradable materials.

Biodegradation and mechanical behavior of an advanced bioceramic-containing Mg matrix composite synthesized through in-situ solid-state oxidation.

Recent studies have shown great potential of Mg matrix composites for biodegradable orthopedic devices. However, the poor structural integrity of these composites, which results in excessive localized corrosion and premature mechanical failure, has hindered their widespread applications. In this research, an in-situ Powder Metallurgy (PM) method was used to fabricate a novel biodegradable Mg-bredigite composite and to achieve enhanced chemical interfacial locking between the constituents by triggering a solid-state thermochemical reaction between Mg and bredigite particles. The reaction resulted in a highly densified and integrated microstructure, which prevented corrosion pits from propagating when the composite was immersed in a physiological solution. In addition, chemical interlocking between the constituents prohibited interparticle fracture and subsequent surface delamination during compression testing, enabling the composite to withstand larger plastic deformation before mechanical failure. Furthermore, the composite was proven to be biocompatible and capable of maintaining its ultimate compressive strength in the strength range of cortical bone after 25-day immersion in DMEM. The research provided the necessary information to guide further research towards the development of a next generation of biodegradable Mg matrix composites with enhanced chemical interlocking.

Additively manufactured biodegradable porous magnesium.

An ideal bone substituting material should be bone-mimicking in terms of mechanical properties, present a precisely controlled and fully interconnected porous structure, and degrade in the human body to allow for full regeneration of large bony defects. However, simultaneously satisfying all these three requirements has so far been highly challenging. Here we present topologically ordered porous magnesium (WE43) scaffolds based on the diamond unit cell that were fabricated by selective laser melting (SLM) and satisfy all the requirements. We studied the in vitro biodegradation behavior (up to 4 weeks), mechanical properties and biocompatibility of the developed scaffolds. The mechanical properties of the AM porous WE43 (E = 700-800 MPa) scaffolds were found to fall into the range of the values reported for trabecular bone even after 4 weeks of biodegradation. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), electrochemical tests and µCT revealed a unique biodegradation mechanism that started with uniform corrosion, followed by localized corrosion, particularly in the center of the scaffolds. Biocompatibility tests performed up to 72 h showed level 0 cytotoxicity (according to ISO 10993-5 and -12), except for one time point (i.e., 24 h). Intimate contact between cells (MG-63) and the scaffolds was also observed in SEM images. The study shows for the first time that AM of porous Mg may provide distinct possibilities to adjust biodegradation profile through topological design and open up unprecedented opportunities to develop multifunctional bone substituting materials that mimic bone properties and enable full regeneration of critical-size load-bearing bony defects. STATEMENT OF SIGNIFICANCE: The ideal biomaterials for bone tissue regeneration should be bone-mimicking in terms of mechanical properties, present a fully interconnected porous structure, and exhibit a specific biodegradation behavior to enable full regeneration of bony defects. Recent advances in additive manufacturing have resulted in biomaterials that satisfy the first two requirements but simultaneously satisfying the third requirement has proven challenging so far. Here we present additively manufactured porous magnesium structures that have the potential to satisfy all above-mentioned requirements. Even after 4 weeks of biodegradation, the mechanical properties of the porous structures were found to be within those reported for native bone. Moreover, our comprehensive electrochemical, mechanical, topological, and biological study revealed a unique biodegradation behavior and the limited cytotoxicity of the developed biomaterials.

Biodegradation of ballast tank coating investigated by impedance spectroscopy and microscopy.

This research paper addresses the biodegradation process for ballast tank coatings in marine environments. As part of this new approach, a commercially available ballast tank coating was exposed to bacteria obtained from a culture collection and to a natural bacterial community isolated from a real ballast tank. The natural community was chosen to explore the interaction of natural biofilms with the coating, an aspect, which is not covered in standard procedures. It is shown that biological activity significantly affects the coating properties. Micro-cracks and holes have been identified using AFM. Acidic bacteria generated holes with 0.2-0.9 µm in depth and 4-9 µm in width. Whereas the natural community additionally caused cracks of 2-8 µm in depth and 1 µm in length. The overall effect of this degradation was examined using the EIS technique. However, the bacterial affected coatings (exposed to acid producing bacteria and a natural community) show a decrease in corrosion resistance. Impedance IZI values decreased over time from 1.18 × 10(9) to 1.87 × 10(7) Ω for acidic bacteria and from 1.71 × 10(9) to 2.24 × 10(7) Ω for the natural community, indicating a clear loss in coating resistance over time. It is also revealed that the coating corrosion resistance declines after 40 days of exposure for the natural community, leading to the formation of blisters. Bacterial settling could be linked to some specific biofilm patterns affecting different types of coating attack. It can be concluded that it is necessary to include natural communities in coating degradation studies to identify possible degradation mechanisms and the severity of the attack over time.

The imaging quality of angiodynography in the ilio-femoral tract.

The imaging quality of angiodynography (Quantum; Philips) in the ilio-femoral tract was compared with uniplanar angiography. One hundred ilio-femoral tracts were investigated with both translumbar conventional angiography and angiodynography. The ilio-femoral tract was divided into five parts; proximal and distal common iliac, proximal and distal external iliac and common femoral. Stenosis was scored from 0-24, 25-49, 50-74, 75-99 and 100% and the lengths of those of 25% or more were measured in centimeters. Because the thickness of the subcutaneous fat layer decreases the depth range (normally 11.5 cm) of the 5.0 MHz transducer it was also measured in centimetres. The results showed that a significantly larger number of vessels were not seen (especially the common iliac) with an increasing thickness of the subcutaneous fat. The results based on the real-time imaging quality alone of angiodynography showed a reasonable accuracy of 83.9% when detecting stenoses graded from 0-24, 25-49, 50-74, 75-99 up to 100%. The clinically more important discrimination between a haemodynamically important stenosis of more than 50% showed a sensitivity of 88% and specificity of 98% on visual information alone. By integrating the peak velocity measurements to the results, this sensitivity rose to 95% and the specificity became 99%. The correlation in length was within a range of 1 cm in 94% and within 1-2 cm in 6% of the stenoses. In conclusion, angiodynography is a reliable technique for investigating the ilio-femoral tract.

Slow potential correlates of frontal function, psychosis, and negative symptoms.

The contingent negative variation (CNV) was used to study cortical activation in frontal and central areas in psychotic and nonpsychotic patients. Psychotic patients showed a higher frontal relative to central CNV amplitude than nonpsychotic patients, a finding that was correlated with performance on a test of frontal function. Negative symptom ratings (physical anhedonia and social closeness) were correlated with relatively low frontal versus central CNV amplitude, but only in psychotic patients.

Dynamic imaging frequency analysis in clinical electroencephalography.

A new way of processing and presenting multichannel EEG recordings is introduced, called dynamic imaging frequency analysis (DIFA). Firstly power spectral analysis is applied to 14 EEG signals simultaneously. The cross-power spectra or transfer functions of 13 EEG signals are calculated in relation to the 14th EEG signal, thus obtaining not only the average amplitude but also the relative phase of diffusely occurring frequencies. The information per electrode obtained in this way can be presented in a numerical form, for quantification and statistical evaluation. Also, the information from 14 electrodes per significant frequency can be presented as a dynamic image display (frequency animation) yielding immediate information on the average amplitude and relative phase of the average activity per frequency of the EEG record. Application of the DIFA technique to several dozen patients demonstrates the value of phase information in EEG analysis.

An application of mathematical models in posturography.

The measurement of the horizontal displacements (HD) of the centre of mass of the human body when standing still, is often performed by means of a platform. The resulting measured HD as a function of time, both in the left-right or sinister-dexter and the posterior-anterior direction, are known as stabilograms. Such stabilograms are being more widely used to obtain the so called measured statokinesigram, similar to a Lissajous-figure, by eliminating time from the sinister-dexter stabilogram and the posterior-anterior stabilogram. It is shown that in using mathematical models of the standing person to obtained the stabilograms, the resulting statokinesigram is very different from the measured statokinesigram. It is also shown that the quantity line-integral calculated from a statokinesigram is much smaller when determined from a statokinesigram corrected by means of the mathematical models.

Human cerebral potentials evoked by moving dynamic random dot stereograms.

In 11 normal healthy human subjects an evoked potential was elicited by moving dynamic random dot stereograms. The random dots were generated by a minicomputer. An average of each of 8 EEG channels of the subjects tested was made. The maximum of the cerebral evoked potentials thus found was localized in the central and parietal region. No response earlier than 130--150 msec after the stimulus could be proved. The influence of fixation, the number of dots provided, an interocular interstimulus interval in the presentation of the dots, and lense accommodation movements on the evoked stereoptic potentials was investigated and discussed. An interocular interstimulus interval (left eye leading) in the presentation of the dots caused an increase in latency of the response much longer than the imposed interstimulus interval itself. It was shown that no accommodation was needed to perceive the depth impression, and to evoke the cerebral response with random dot stereograms. There are indications of an asymmetry between the two hemispheres in the handling of depth perception after 250 msec. The potential distribution of the evoked potentials strongly suggests that they are not generated in the occipital region.