FIELD OF THE INVENTION

The present invention relates to methods and apparatus for the volumetric fabrication of three-dimensional objects from pho- toresponsive materials. In particular, the present invention is related, but not restricted, to manufacturing systems wherein the optical distortion induced by the cylindrical wall of the container is compensated.

BACKGROUND

In tomography-based additive manufacturing methods, a volume of photoresponsive material (resin) comprised within a container, typically a cylindrical container, is irradiated from multiple angles with computed patterns of light in order to fabricate a three-dimensional object. The main advantage of this method is its very rapid manufacturing time (down to a few tens of se conds) compared to existing methods. For a detailed explanation of this method, reference is made to the publications in the next paragraph.

State-of-the-art tomographic printers usually include a bath of index-matching liquid surrounding the resin container (see e.g. US 2018/0326666 A1; "Volumetric additive manufacturing via tomo graphic reconstruction", B. E. Kelly, I. Bhattacharya, H. Hei- dari, M. Shusteff, C. M. Spadaccini, and H. K. Taylor; Science Vol. 363, Issue 6431, pp. 1075-1079 (08 Mar 2019); "Volumetric Bioprinting of Complex Living-Tissue Constructs within Seconds, "

P. N. Bernal, P. Delrot, D. Loterie, Y. Li, J. Malda, C. Moser, and R. Levato, Advanced Materials, 19 August 2019). The bath of index-matching liquid minimizes the lensing effect caused by the cylindrical shape of the container of photoresponsive material (e.g. US 2018/0326666 A1). Mitigating lensing effects is neces sary in order to use standard tomography algorithms for the cal- culation of the light patterns, such as the Radon transform.

However, using a bath of index-matching liquid is undesirable for multiple reasons. First, it is not always possible to find an index-matching liquid exactly matching the properties of the resin while simultaneously being safe to handle and convenient to clean up in case of spills.

Instead of using a bath of index-matching liquid, it is also possible to use a compensating lens as disclosed before (see WO 2019/043529 A1). This solution avoids the use of an index matching liquid, but the lens needs to be adapted to the optical properties of the container. For example, each time a different size or material is used for the container, the compensating lens should be replaced, too.

Consequently, there is a need for a method of tomographic addi tive manufacturing that can easily be adapted to different res- ins and containers without changes to the printing apparatus.

SUMMARY OF THE INVENTION

The present invention is related to a method of digitally com pensating distortions of rays of a light beam in tomography- based additive manufacturing, wherein said tomography-based ad- ditive manufacturing involves projecting light patterns from multiple angles into a container comprising photoresponsive ma terial, said method comprising the steps of: simulating the path of the light rays through the container and the photoresponsive material; digitally compensating the light projections based on the simulated path of the light rays, so as to obtain modified light projections.

The present invention is furthermore related to a method of pre paring an object in tomography-based additive manufacturing, wherein said tomography-based additive manufacturing involves projecting light patterns from multiple angles into a container comprising photoresponsive material, said method comprising the steps of: providing said container comprising photoresponsive materi al; carrying out a method of distortion compensation as defined above, so as to obtain modified light projections; projecting said modified light projections into the con tainer comprising photoresponsive material, thereby creating the object without distortions.

The present invention is furthermore related to an apparatus for digitally compensating distortions of rays of a light beam and of preparing an object in tomography-based additive manufactur ing, the apparatus comprising: a resin container for providing a resin to be polymerized, wherein said resin container is rotatable; a unit for providing a light beam to be projected into the resin container; a processing unit for performing a distortion compensation method, preferably as defined above, wherein said apparatus does not comprise a physical compensation component between the unit for providing said light beam and said container, such as a bath with index-matching liquid around said container or a lens. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood through the detailed de scription of non-limiting preferred embodiments and in reference to non-limiting drawings, wherein

Figure 1A is a perspective view of an embodiment of a volumetric additive manufacturing apparatus with a resin bath and an index matching liquid bath according to the prior art.

Figure IB is a top view of the apparatus of Figure 1A, where rays have been traced to illustrate the path of light projec tions in this embodiment.

Figure 2A is a perspective view of an embodiment of a volumetric additive manufacturing apparatus according to the present inven tion without compensating elements. Figure 2B is a top view of the apparatus of Figure 2A, where rays have been traced to illustrate the path of light projec tions in this embodiment. The path of rays has been extended be yond the container, in order to illustrate that the rays do not all intersect in the same point. Figure 2C is a perspective view of the apparatus of Figure 2A, comparing the actual path of a ray versus the path expected in parallel-beam projection algorithms.

Figure 3 is an illustration of the ray configuration required to apply the fan beam algorithm. DETAILED DESCRIPTION

In tomographic volumetric additive manufacturing, a volume of photoresponsive material is illuminated from many directions with patterns of light. These patterns of light are computed with an algorithm similar to that used in X-ray computed tomog raphy, also known as medical CT scanners. These algorithms are known to the skilled person. An apparatus for tomographic addi tive manufacturing is described in detail in e.g. WO 2019/043529 A1 or US 2018/0326666 A1.

Up to now, all tomography-based volumetric additive manufactur ing systems have used a physical compensation method for the distortion caused by the cylindrical shape of the container of photoresponsive material, for example a bath of index-matching liquid (as illustrated in Figure 1A, and as described in e.g. US 2018/0326666 A1) or a compensation lens. These compensating ele ments mitigate the lensing effect caused by the cylindrical shape of the container of photoresponsive material, and allow light rays to travel straight through the photoresponsive mate rial (as illustrated in Figure IB). Straight rays are needed to use the standard parallel-beam tomography algorithms for the calculation of light patterns (e.g. the Radon transform and its inverse) .

This configuration is illustrated in Figure 1A, where a light beam 11 first enters an index-matching liquid bath 12 and then the container 13 with photoresponsive material. The container 13 is fixed to a rotation mount (platform) 14 in order to irradiate the photoresponsive material with the light patterns 11 from various angles, thereby fabricating the object 15.

As shown in the top-view section in Figure IB, the light beam 11 (here represented as individual rays) passes with a negligible distortion through the index-matching bath 12 and resin contain er 13 in this configuration. The ray trajectories simulated in Figure IB assume a resin and index-matching liquid with refrac tive index 1.53, and fused silica containers with refractive in- dex 1.47.

Unfortunately, compensating elements such as an index-matching liquid bath or compensation lenses must be matched to the geome try and materials used for each particular container of photore- sponsive material. This means that physical changes are needed to be made to the printing apparatus when the container, or the photoresponsive material, or both of them change. Additionally, index-matching liquids can be cumbersome to handle and compensa tion lenses require precise alignment.

Figure 2A illustrates a volumetric printing apparatus according to the present invention where no compensating elements are in cluded. Here, the light beam 21 directly enters the container 22 of photoresponsive material. This container is attached to a ro tation platform 23 as before in the apparatus shown in Fig. 1A, in order to irradiate the resin from various angles and produce the object 24.

Figure 2B illustrates the path of light rays through such an ap paratus, using a top-view section of the apparatus of Figure 2A. The rays of the light beam 21 now change direction as they enter the container 22 of photoresponsive medium. By virtually extend ing these rays outside of the container, one can observe that the rays do not all come to focus in a single point. The focus instead depends on the lateral offset of each ray before enter ing the container, as exemplified by the rays intersecting in points 251, 252 and 253. In summary, without using compensating elements, different light rays have different directions and the rays do not all intersect into the same point. This means that the light patterns can neither be calculated by a parallel-beam tomography algorithm (as in Figure IB), neither by a fan-beam tomography algorithm as this requires rays that converge into a point (as illustrated in Figure 3), which are the algorithms typically known to the person skilled in the art.

According to the present invention, there is disclosed a method of digitally compensating distortions of rays of a light beam in tomography-based additive manufacturing, wherein said tomogra phy-based additive manufacturing involves projecting light pat terns from multiple angles into a container comprising photore- sponsive material, said method comprising the steps of: simulating the path of the light rays through the con tainer and the photoresponsive material. digitally compensating the light projections based on the simulated path of the light rays, so as to obtain modified light projections.

The digital compensation can for example be carried out by resampling :

The light projections are calculated at every angle using a parallel-beam tomography algorithm, thereby neglecting any distortion incurred by the light beam in the actual printing apparatus .

Parallel-beam tomography algorithms are known in the art (see e.g. A.H. Delaney; Y. Bresler; A fast and accurate Fourier algorithm for iterative parallel-beam tomography, IEEE Trans actions on Image Processing, Volume 5, Issue 5, May 1996, 740-753) .

This yields a three-dimensional dataset containing two- dimensional light projections at several angles. This dataset can be represented mathematically as I _pa raiiei(x, y, Q), where x and y represent the two spatial coordinates and Q is the an gle of each projection. The position and orientation of the rays assumed by the parallel-beam algorithm are compared to the position and ori entation of the rays obtained by simulating the light propa gation through the container and photoresponsive material. This is illustrated in Figure 2C: the path of a light ray 271 is simulated until the midplane 26 of the container 22 of photoresponsive material, and is compared to the path 272 that the ray would have taken without deviation. This yields a coordinate mapping between the ray positions and angles as sumed by the parallel-beam algorithm (x, y, Q), and the simu lated ray positions and angles which we describe as x', y', q'. The coordinate mapping can be expressed mathematically as a function:

(x, y, q) (x’, y’, Q')·

The light projections calculated with the parallel-beam al gorithm are then resampled (for example by linear interpola tion) using the simulated coordinates:

1compensated Iparallel( r r Q )

Using said distortion compensation method, it is possible to generate different objects in a tomography-based additive manu facturing method in a time- and cost efficient manner, without any need for physical modification of components of the appa ratus used for performing this method.

In detail, the present invention is also related to a method of preparing an object in tomography-based additive manufacturing, wherein said tomography-based additive manufacturing involves projecting light patterns from multiple angles into a container comprising photoresponsive material, said method comprising the steps of: providing said container comprising photoresponsive materi al; carrying out a method of distortion compensation according to any of claims 1 to 3, so as to obtain modified light pro- jections; projecting said modified light projections into the con tainer comprising photoresponsive material, thereby creating the object without distortions.

A method of preparing an object in tomography-based additive manufacturing is known from the art, e.g. from WO 2019/043529 A1 or US 2018/0326666 A1. However, the method of the present inven tion is characterized by the fact that the modified light pro jections obtained from the distortion compensation method de scribed above are not projected through a physical compensation component, such as a bath with an index-matching liquid or a lens.

The present invention is also related to an apparatus for digi tally compensating distortions of rays of a light beam and of preparing an object in tomography-based additive manufacturing, the apparatus comprising: a resin container for providing a resin to be polymerized, wherein said resin container is rotatable; a unit for providing a light beam to be projected into the resin container; a processing unit for performing a distortion compensation method, preferably as defined above, wherein said apparatus does not comprise a physical compensation component between the unit for providing said light beam and said container, such as a bath with index-matching liquid around said container or a lens. An apparatus of preparing an object in tomography-based additive manufacturing is known from the art, e.g. from WO 2019/043529 A1 or US 2018/0326666 A1. However, the apparatus of the present in vention is characterized by the fact that there is no physical compensation component, such as a bath with an index-matching liquid or a lens.

Since the apparatus of the present invention does not comprise a physical compensation component between the unit for providing said light beam and said container, such as a bath with index- matching liquid around said container or a lens, but rather dig itally compensates any distortion of the rays of a light beam, the apparatus of the present invention does not have to be adapted to different resins and containers by changes to the printing apparatus, such as exchanging an index-matching liquid or a compensating lens.

According to a preferred embodiment, said resin container is at tached to a rotation platform. By this, irradiation of the pho- toresponsive material with projected light patterns from multi ple angles is carried out by rotating the container comprising the photoresponsive material relatively to the unit for provid ing said light beam.